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Carbon isotopic heterogeneity and limited oxygen isotopic homogeneity in the cretaceous Cucamonga terrane, southeastern San Gabriel Mountains, California

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Carbon isotopic heterogeneity and limited oxygen isotopic homogeneity in the cretaceous Cucamonga terrane, southeastern San Gabriel Mountains, California

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CARBON ISOTOPIC HETEROGENEITY AND LIMITED OXYGEN ISOTOPIC
HOMOGENEITY IN THE CRETACEOUS CUCAMONGA TERRANE,
SOUTHEASTERN SAN GABRIEL MOUNTAINS, CALIFORNIA
by
Eric William Hovanitz
A Dissertation Presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
Doctor of Philosophy
(Geoloc ’ cal Sciences)
August 1998
Copyright 1998 Eric W illiam H ovanitz
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LTNrVERSITV OF SOUTHERN CALIFORNIA
THE GRADUATE SCHOOL
UNIVERSITY PARK'
LOS ANGELES. CALIFORNIA 90007
This dissertation, w ritten by
Eric WilliamHovanitz
under the direction of /Lis. Dissertation
Committee, and approved by ail its members,
has been presented to and accepted by The
Graduate School, in partial fulfillment of re
quirements for the degree of
DOCTOR OF PHILOSOPHY
Dean o f Graduate Studies
Date
DISSERTATION COMMITTEE
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A cknow ledgem ents
I w ish to thank my advisor, Jean M orrison, for exceptional support,
encouragem ent, and helpful discussions during the production of this
dissertation. Jean's num erous reviews, suggestions and the ever present
"open door" have been invaluable. Num erous discussions, advice and
support from Lawford Anderson is also greatfully acknowledged. I also thank
m y other committee members, Dr s. Greg Davis, Perry Ehlig and Bill Weber
for their support and encouragement. Perry Ehlig has always been a source of
great knowledge of the San Gabriel Mountains.
This research was made possible by generous financial help from (1)
N ational Science Foundation, (2) Geological Society of America Grant to E.
H ovanitz, (3) ARCO, (4) the Foss Foundation, (5) the University of Southern
California D epartm ent of Earth Sciences G raduate Student Research Fund,
and (6) the Departm ent of Earth Sciences and the G raduate School at the
U niversity of Southern California.
Nami Kitchen and Yue Jin Chang provided invaluable stable isotope
laboratory assistance and Nami and I had num erous disscussions of granulite
rocks and caldte-graphite thermom etry. Geologic discussions and
encouragem ent from fellow graduate students and friends Dave Mayo,
Gustavo M urrillo, Chris Hill, Julie Francis, Chris Carlson and Dave Bowman
are sincerely appreciated. I will remember you always.
W ithout the help and encouragem ent of m y family this project could
not have been finished. I first w ish to thank Karen and Hans for their
gracious understanding of my frequent absence from their lives during this
project. My mother, Barbara Hovanitz, was always understanding in helping
out whenever needed. My other family, Elmer and Alice Nielson were always
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ready to help w ith w hatever was needed during the ever-present emergency
situations. W ithout all of your help and encouragem ent, I could not have
finished. Thanks to you all.
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ERIC WILLIAM HOVANITZ JEAN MORRISON
CARBON ISOTOPIC HETEROGENEITY AND LIMITED OXYGEN ISOTOPIC
HOMOGENEITY IN THE CRETACEOUS CUCAMONGA TERRANE,
SOUTHEASTERN SAN GABRIEL MOUNTAINS, CALIFORNIA
The Cretaceous Cucamonga granulite terrane, located in the southeast San
Gabriel M ountains, consists of mafic and felsic gneisses, m etapelite, marble
and calc-silicate lithologies that have been intruded by m inor mafic and felsic
intrustions. The Cretaceous m etam orphic age is unusual in contrast to m ost
exposed granulite terranes that are Precambrian. Terrane-scale caldte 51 3 C
values average 0.8 ± 3.7%o and vary from +8.7 to -9.8%o. The range in 51 3 C
values throughout the terrane is -3 0 % o . Outcrop- and cm-scale 51 3 C values
are also heterogeneous. Heterogeneous 8l3C values suggests that granulite
form ation w asn't accomplished by pervasive C02*flooding.
At the outcrop scale, there is oxygen isotope evidence for localized H 2 0 -
rich fluid flow. Thirty-nine core drilled samples from three adjacent outcrops
w ithin a span of -1 0 m have homogeneous caldte (16.1 ± 0.4%o),
dinopyroxene (13.6 ± 0.4%o), garnet (14.0 ± 0.4%o) and biotite (13.7 ± 0.3 % o )
5lsO values across all metam orphosed lithologies that w ould be expected to
have different protolith values. This is interpreted to result from a local but
lithology crossing H2 0-rich fluid flow during post-peak m etam orphic cooling
at tem peratures of 720-750°C. This hydrous fluid-flow requires that marble
layers were permeable at granulite conditions.
A t the terrane-scale caldte S 1 8O s m o w values average 19.3 ± 2.8 % o w ith a
range of 14.5 to 24.7%o. Heterogeneous 51 3 C and 5I 8 0 values suggest the
Cucamonga terrane retains near sedim entary isotopic values little affected by
m etam orphism .
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C aldte-graphite carbon isotopic tem peratures from the core drilled
outcrops record peak metamorphic tem peratures of > 850°C. Locally pervasive
fluid flow at 720-750°C did not significantly reset the caldte-graphite
therm om eter, which implies the caldte-graphite therm om eter is resistant to
resetting by retrograde hydrous m etam orphism . Peak m etam orphic
tem peratures >850°C would require fluid-absent m elting in appropriate
lithologies w hich w ould partition H2 0 into the m elt phase. The m elts m ust
have been efficiently extracted to higher crustal levels leaving a granulite
restite w ithout migm atite.
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Table of Contents
Page
A ck n o w led g m en ts.............................................................................. ii
Abstract ..................................................................................................................... iii
List of F ig u res.......................................................................................................... viii
List of Tables............................................................................................................... ix
C hapter 1. CARBON IOTOPIC SYSTEMATICS OF THE
CRETACEOUS CUCAMONGA GRANULITE TERRANE.................1
A bstract............................................................................................2
Introduction.................................................................................... 3
Geological Setting.......................................................................... 5
Analytical P rocedure.....................................................................8
Results ........................................................................................... 9
Petrology.................................................................................... 9
Carbon isotopie system atics..................................................12
Rayleigh decarbonation effects on 81 3 C .............................. 14
Cucamonga marble v e in s .....................................................15
C aldte-graphite isotopic therm om etry.............................. 16
D iscu ssio n .................................................................................... 19
C ondusions................................................................................... 21
R eferences.................................................................................... 25
Figure C aptions............................................................................. 31
C hapter 2. HIGH TEMPERATURE FLUID FLOW AND LOCAL
ENHANCEDMARBLE PERMEABILITY IN THE CRETACEOUS
CUCAMONGA GRAUNLITE TERRANE,
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V
SOUTHERN CALIFORNIA.............................................................. 44
A bstract......................................................................................... 45
In tro d u c tio n ................................................................................46
Geological setting ...................................................................... 47
Analytical m eth o d s.................................................................... 50
Results ......................................................................................... 53
Petrology ................................................................................53
Terrane-scale carbonates........................................................54
Out-crop-scale carbonates and silic a te s..............................57
D iscu ssio n ................................................................................... 60
Stable isotope sytematics .....................................................60
Fluid characteristics.............................................................. 64
Fluid com position...........................................................64
Fluid source....................................................................... 65
Fluid flow tim ing.............................................................. 68
High tem perature carbonate perm eability........................ 68
C aldte-graphite therm om eter sta b ility ..............................69
C ondusions...................................................................................70
R eferences....................................................................................74
Figure C aptions.............................................................................85
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List of Figures
Page
Chapter 1
Figure
1. Location of the Cucamonga terrane in southern C a lifo rn ia.............36
2. Map of the Cucamonga and adjacent San Antonio te rra n e...............37
3. Histogram comparison of carbonate d!3C analyses from
Cucamonga (western), Deer (central), and San Sevaine Canyon
(eastern) regions of the Cucamonga terrane ......................................38
4. Carbon isotopic compositions of caldte and graphite for the
core drilled traverses in Deer C anyon................................................. 39
5. Com parison of the range and average isotopic values from
other carbonates........................................................................................40
6. Co-existing 51 3 Ccai vs. 51 3 Ccr data from the San Sevaine Canyon,
Deer & Day Canyon and Cucamonga C anyon....................................41
7. Diagram showing the changes in 51 3 Ccai and coexisting 5I3 Ccr if
isotopic equilibrium is attained from starting conditions of
51 3 Cgt = 2 % o and 51 3 CC a i = -25 % o at the solid sq u a re .......................... 42
8. Range in dl3C values for different lithologies throughout the
Cucamonga te rra n e ................................................................................43
Chapter 2
Figure
1. Location of the Cucamonga terrane in southern C a lifo rn ia.............87
2. Simplified map of the Cucamonga terrane and adjacent San
A ntonio te rra n e ......................................................................................88
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vii
3. Coupled caldte 51 3 C and 5lsO plot of Deer Canyon core drill
samples, hand sam ple outcrops in Deer and Day Canyon
(centra) area, and other hand sam ple outcrops in the
Cucamonga te rra n e .................................................................................89
4. Coupled caldte 81 3 C and SlsO plot com paring the isotopic
composition of calc-silicate (<95% caldte) rocks and marbles
(>95% cald te).............................................................................................90
5. Histogram com parison of 51 8 0 values from different portions
of the Cucamonga te rra n e ..................................................................... 91
6. Histogram com parison of 51 8 0 values from the Cucamonga
terrane, relatively unm etam orphosed w orld-w ide carbonates,
the Esplanade Range, the Adirondacks, and Sri L an k a.................... 92
7. Plot of caldte, graphite and dinopyroxene isotopic trends
across the three core drilled trav erses..................................................93
8. Plot of 19 co-existing caldte-diopside pairs from Deer C an y o n .........94
9. Plot of 4 co-existing caldte-phlogopite pairs from the Deer
Canyon core-drilled trav erse................................................................. 95
10. Relationship of Xco2 and Log/02 which shows that a hydrous
fluid in contact w ith graphite m ust have a carbon-bearing
com ponent like CO2 or C H *................................................................... 96
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V l l l
Abstract
The Cretaceous Cucamonga granulite terrane, located in the southeast
San Gabriel M ountains, consists of mafic and felsic gneisses, metapelite,
m arble and calc-silicate lithologies that have been intruded by m inor mafic
and felsic veins and dikes. M etam orphism occurred in the lower crust
associated w ith the Cretaceous C ordilleran magma tic arc. The Cretaceous
m etam orphic age is unusual in contrast to m ost exposed granulite terranes
that are Precambrian. Terrane-scale caldte values average 0.8 ± 3.7% o and
vary from +8.7 to -9.8%o. The range in 81 3 C values throughout the terrane is
~30%o. Outcrop- and cm-scale 51 3 C are also heterogeneous. Four unusually
high 51 3 C values (up to 8.7%o) are difficult to attain by metam orphic processes
and probably represent Neoproterozoic depositional values.
Terrane-scale caldte 5 18O s m o w values are heterogeneous w ith an
average value of 19.3 ± 2.8%o and a range of 14.5 to 24.7%o. Decarbonation or
devolatilization reactions and isotopic equilibration between co-existing
caldte and silicate minerals or caldte and graphite do not significantly affect
oxygen or carbon isotopic heterogeneity.
At the outcrop scale there is evidence for localized H 2 0-rich fluid flow.
Thirty-nine core drilled sam ples from three adjacent outcrops w ithin a span
of -1 0 m have homogeneous caldte, clinopyroxene, garnet and biotite 5lsO
values across mafic and alum inous gneiss, felsic granulite, m arble and calc-
silicate lithologies that w ould be expected to have different protolith isotopic
values. This is the result of locally pervasive H 2 0-rich fluid flow during post
peak m etam orphic cooling at tem peratures of 720-750°C. Pervasive hydrous
fluid-flow requires that 60 cm thick m arble layers were perm eable at granulite
conditions where m arble is generally thought to be impermeable.
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Even after allowance is m ade for carbonate ^ -d e p le tin g processes, the
heterogeneity of the carbon isotopic data is inconsistent w ith the hypothesis
of granulite facies m etam orphism being accomplished by pervasive
(fluid/rock ~ 0.1-03) infiltration of a C0 2 -rich fluid or magma of mantle
origin.
C aldte-graphite carbon isotopic tem peratures from the core drilled
outcrops record peak metamorphic tem peratures of > 850°C. Locally pervasive
fluid flow that m ust have contained a carbon com ponent (C02 or CH*;
Xh? q <0.9) at 720-750°C did not significantly reset die caldte-graphite
therm om eter, which implies the caldte-graphite therm om eter is resistant to
resetting by retrograde water-bearing but also carbon-bearing fluid
m etam orphism . Peak metamorphic tem peratures >850°C suggests that fluid-
absent (dehydration) m elting occured in favorable lithologies that partitioned
H2 0 into the m elt phase which was then extracted to higher crustal levels to
produce the Cucamonga granulite restite.
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1
C hapter 1
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2
ABSTRACT
Sixty-five carbon isotopic compositions of caldte and graphite have been
com bined w ith existing data for the Cretaceous Cucamonga granulite terrane
of southern California. Terrane-wide carbon isotopic com positions in marble
are heterogeneous w ith an average value of -0.13 % o and a range of 18.4%o.
Well crystallized graphite of several distinct textures is also heterogeneous
w ith an average 81 3 C of -8.7%o and a range of 22.9%o. Carbon isotopic
heterogeneity of 3.1 % o in graphites separated by < 1 cm suggests low carbon
mobility. We interpret terrane-scale, outcrop-scale and cm-scale carbon
isotopic heterogeneity to dem onstrate the lack of pervasive infiltration of a
C 0 2 -rich fluid w hich im plies that dehydration of previously hydrous
sedim entary m ineralogies was not produced by C 0 2 -flooding.
C aldte-graphite carbon isotopic therm om etry indicates a m etam orphic
tem perature in excess of c. 850°C in the central region of the terrane and c. 800
°C in the eastern portion of the terrane. Tem perature results from the
w estern area show a c. 500°C isotherm and weak evidence for a c. 600°C
isotherm . The Cucam onga Canyon carbonates have been m ylonitically
deform ed that has likely caused retrograde re-setting. The c. 50°C difference in
peak-m etam orphic tem peratures is consistent w ith cation exchange
therm om etry results and suggests that the central portion of the Cucamonga
terrane either attained higher tem peratures or that the terrane interior retains
higher m etam orphic tem peratures than either end of the terrane.
Tem peratures of c. 800-850°C are probably suffident to have produced
fluid-absent partial m elting in most Cucamonga lithologies, so we believe
that the nearly anhydrous granulite fades mineralogies characteristic of
granulite fades w ere accomplished by w ater extraction into a melt phase. Melt
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3
extraction m ust have been efficient due to the lack of m igm atites w ithin the
Cucamonga terrane. Magmas produced by these partial m elts w ould likely
have contributed to magmatism at higher crustai levels and could be related
to Late Cretaceous magmatism in the nearby Placerita-San Antonio and San
Gabriel terranes that were at higher crustai levels during anatexis.
The lack of high tem perature carbon isotopic equilibrium betw een co
existing caldte and graphite in some sam ples containing granoblastic textures
is likely due to post peak-m etam orphic brittle and mylonitic deform ation
possibly w ith accompanying m inor retrograde channelized fluid-flow (i.e.
veins) that have been disguised by low er tem perature caldte annealing
following peak m etam orphism .
INTRODUCTION
Accurate knowledge of the tem perature, pressure and fluid
characteristics of the lower to m iddle crust are crudally im portant, yet
inadequately understood in assessing their role in magma tic arc melt
generation and the nature of granulite metam orphism. Much evidence
suggests the lower to m iddle crust in a subduction zone setting is a likely
source region for m any higher level plutons (e.g., Wilson, 1993) and is where
significant magma modification occurs by assimilation and contam ination.
Formation of anhydrous granulites produced by the dehydration of more
hydrous m etam orphic grade lithologies is contentious, but also occurs in the
lower to m iddle crust.
The extent to which these processes occur and are related is dependent upon
an explicit understanding of the tem perature, pressure, fluid com positions
(e.g. H2 0 , C 0 2 and CH4) and the characteristics of fluid flow in the low er to
m iddle crust. Partial m elting relationships are highly inter-dependent upon
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the tem perature and the am ount of w ater present, w ith fluid-present melting
occurring at significantly lower tem peratures (c. 750° C) than fluid-absent
melting (c. 800-850° C). Accurate high tem perature determ inations are
hindered by the retrograde re-equilibration (e.g. Frost & Chacko, 1989; Spear &
Florence, 1992; Florence & Spear, 1993) of cation exchange thermometers
which may be incapable of peak metamorphic tem perature retention above c.
700° C, the tem perature where the potential for partial m elting is highly
interdependent on the nature of fluids present.
The mechanism of granulite dehydration is also dependent on the nature
of fluid com position and fluid flow in the lower to m iddle crust. The
formation of dehydrated lithologies from hydrous ones can occur during the
extraction of m elt (Fyfe, 1973), the pervasive infiltration of large am ounts of
C 0 2 -fluid (Newton, 1980), or a C 0 2 -rich magma (Frost & Frost, 1987). Since
granulite formation occurs at tem perature, pressure and fluid conditions that
overlap m any anatectic melting relationships, accurate tem perature
determ ination and the characterization of fluids in the lower to m iddle crust
are crucial.
Because of the long time required to unroof deep buried rocks in a cratonal
setting, most granulites that have been studied are Precambrian in age and
were buried at lower crustai depths (e.g. Adirondacks, Bohlen et al., 1985,
Valley et al., 1990; Rauer Group of Antarctica, Buick et al., 1994; and southern
India, Newton et al, 1980, Santosh & Wada, 1993) for periods of up to 150 Ma
(Bohlen, 1991). Lengthy residence time in the lower crust for Precambrian
granulites have often allowed m ultiple m etam orphic events to occur
(Bohlen, 1991; Yoshida & Santosh, 1994) that have possibly obscured peak
m etam orphic tem perature, pressure and fluid conditions.
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5
G ranulites from an active plate m argin where there is evidence of
rapid cooling and unroofing may provide an ideal location to study the role
of the lower to m iddle crust in anatexis and granulite form ation. The
Cucamonga granulite terrane of southern California is therefore a good
candidate to study the potential of anatexis during granulite m etam orphism.
The Cucamonga granulites have a m id to late Cretaceous m etam orphic age,
were located in a developing magmatic arc at variable distances above the
Laram ide subducting plate, and rapidly uplifted following m etam orphism
(May & W alker, 1989; Barth & May, 1992).
GEOLOGIC SETTING
The Cucamonga granulite terrane is located in the southeastern portion of
the San Gabriel M ountains and is one of four crystalline basem ent complexes
that com prise the central Transverse Ranges of southern California. In
addition to the Cucamonga terrane, the San Gabriel M ountains contain the
San Gabriel, Placerita-San Antonio and Pelona Schist terranes. Figure 1 shows
the location of the Cucamonga terrane and the relationship between the
different terranes.
The Cucamonga terrane is small, being only c. 18 km long by c. 3 km w ide and
is composed of variably deform ed m eta-sedimentary rocks that were intruded
by several types and generations of Mid to Late Cretaceous igneous intrusions
(Hsii, 1955; Barth, 1989; May & W alker, 1989; Barth & May, 1992). The
intrusive magmatic rocks are sim ilar to those in the adjacent Placerita-San
A ntonio terrane (May & W alker, 1989). Granulite facies m etam orphism was
in progress by c. 108 Ma and continued to at least c. 88 Ma (May & Walker,
1989) based on the deform ation of hypersthene-bearing intrusive rocks.
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6
Discordant zircon U-Pb upper intercepts of 1.6 to 1.7 Ga from mafic and
pegm atitic intrusions are sim ilar to upper intercept ages from the San
Antonio terrane and suggest an affinity w ith the Proterozoic crystalline rocks
of the San Gabriel terrane (May and W alker, 1989). W hile the Cucamonga
terrane is entirely fault bounded (Figure 2), and has no definitely recognized
protolith or correlative rocks (Dibblee, 1982; May & W alker, 1989), similarities
in m etam orphic grade, sedim entary protoliths and tectonic position w ithin
the evolving Cretaceous magmatic arc led Ehlig (1981) to suggest that the
sedim entary protoliths of the Cucamonga and Placerita-San Antonio terranes
were likely the sedim entary cover over Proterozoic gneissic crystalline
basem ent of the San Gabriel terrane. May and W alker (1989) argue that
lithologic differences between the Placerita-San Antonio and Cucamonga
terranes likely preclude a correlation w ithout a significant facies change
which is not unreasonable in a transcurrent magm atic arc environm ent
w here a minimum offset of 20 km exists (May & W alker, 1989).
Following Late Cretaceous vertical and horizontal juxtaposition of the San
Gabriel, Placerita-San Antonio and Cucamonga terranes (May & Walker,
1989), the Late Cretaceous-Early Cenozoic subduction zone angle decreased
and consequendy tectonically eroded the lower continental crust (Cross &
Pilger, 1978; Dickinson & Snyder, 1978). The m etam orphosed, continentally-
derived, sedim entary cover over the subducting plate rem ains as the
m inim um 3500 m thick Pelona Schist terrane (Ehlig, 1981; Dibblee, 1982;
Jacobson, 1983). Peak m etam orphic tem perature and pressure conditions in
the Pelona Schist are estim ated to be c. 420-500 °C (Graham & England, 1976;
Jacobson, 1995) at 9-10 kb (Graham & Powell, 1984; Jacobson, 1995) during
latest Cretaceous to early Paleocene.
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7
Following underthrusting and em placem ent of the Pelona Schist terrane
between c. 58-61 Ma (Conrad & Davis, 1977; Miller & Morton, 1980; Jacobson et
al., 1988) the now juxtaposed four terranes were quickly uplifted so that
Paleocene sedim ents overlie Proterozoic gneisses in the western and
northern San Gabriel M ountains (Sage, 1973). Evidence for rapid uplift and
unroofing of the Cucamonga terrane includes the presence of "Black Belt
M ylonite" clasts w ithin upper Miocene sedim ents of the Puente and San Jose
Hills south of the San Gabriel M ountains (W oodford et al., 1946; Alf, 1948).
Much of the Cucamonga terrane shows the effects of intense mylonitic and
brittle deformation (Figure 2, May & Walker, 1989; Barth & May, 1992).
Cucamonga Canyon carbonates (western portion of the terrane) sam pled as
p art of this study are typically mylonitically deformed. Samples collected from
Deer or Day Canyons (central part of the terrane) exhibit little mylonitic and
brittle deformation which is the reason this area was chosen for detailed
sampling. The San Sevaine Canyon (eastern region of the terrane) marbles
and calc-silicate lithologies generally have granoblastic textures and do not
exhibit the mylonitic deform ation that is characteristic of the interlayered
silicate lithologies (May & Walker, 1989; Barth & May, 1992).
The age of the Cucamonga terrane sedim entary protolith is poorly
constrained. On the basis of lithologic sim ilarities w ith rocks of southeastern
California, the age may be Paleozoic (Ehlig, 1981; Powell, 1982; May & Walker,
1989), though there is some suggestion that the terrane maybe in part
Precambrian (Dibblee, 1982). A single carbonate ^ ^ S r analysis suggests that at
least one of the Cucamonga terrane carbonates does not fall on the
Phanerozoic seawater curve, (Barth, personal communication) which
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8
suggests that some of the terrane may span the Proterozoic-Phanerozoic
boundary.
ANALYTICAL PROCEDURES
Samples were collected from Cucamonga, Deer, Day and San Sevaine Canyon
areas of the Cucamonga terrane (Figure 2). In Deer Canyon, three separate,
small scale traverses were made using a core drill over a span of c. 10 m at c.
10 cm intervals. The cores contained num erous diverse lithologies that
included m arble and calc-silicate, aluminous gneiss, mafic gneiss, quarto-
feldspathic gneiss, and quartz granulite lithologies. A total of 44 samples were
obtained from 43 cores, w ith one core intersecting m arble and mafic gneiss
along a m inor fold.
Approximately 3 cm3 sections of caldte were rem oved from the carbonate
cores using a core splitter and further reduced to individual grains using a
hardened steel "atom smasher". From each crushed sam ple, caldte and
graphite were removed by hand-picking. To remove potential adhering
caldte, graphite was rinsed in 6.2 N HC1. For core drilled samples, the average
volume of caldte and graphite pairs is less than 1 cm3 . H and samples had
higher average volumes that ranged from 1 to 15 cm3 depending on the
graphite concentration and partide size in the sample. Megascopic graphite
was acquired by hand-picking, while microscopic graphite was obtained by
dilute HC1 dissolution of carbonate to obtain suffident graphite for isotopic
analysis.
C aldte 51 3 C values were determined using 10-15 mg of pow der and the
conventional phosphoric ad d extraction m ethod of McCrea (1950) at 25 °C.
Phosphoric a d d w ith a specific gravity of 1.925 (at 60 °F) was used. Graphite
81 3 C values were determ ined by two methods: (1) c. 1 m g aliquots were
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9
combusted w ith > 30 mg CuO at 900 °C for at least 10 hours in seeded 8 mm
diam eter quartz tubing or (2) in an evacuated quartz tube w ith > 30 mg CuO at
1100 °C for 15 minutes.
Purified C0 2 -gas was analyzed on a VG PRISM m ulti-collector gas ratio mass
spectrom eter. Isotope values are reported in standard per m il notation.
Values of 81 3 C are reported relative to PDB. D uring the course of sample
analysis, 26 analyses were m ade of the USC-CC1 internal caldte standard
which had an average 81 3 C value of 3.05 ± 0.03 % o . O ur laboratory USC-CC1
standard has an average 81 3 C value of 3.02 ± 0.09%o and was calibrated against
NBS-18 and NBS-19. NBS-21 spectrographic graphite w as analyzed 36 times
dining the course of graphite analysis w ith an average 81 3 C value of -28.05 ±
0.06 % o . The excepted S1 3 C value for NBS-21 is -28.1 % o . Duplicate caldte 81 3 C
analysis varied by < 0.2%o and duplicate graphite 81 3 C analysis by up to 0.4 % o
in a few samples. M oderate isotopic variation in a few duplicate graphite
analyses is attributed to small scale graphite isotopic heterogeneity because of
low NBS-21 standard variation (<0.16%o) in repeat analyses.
X-ray diffraction analyses were perform ed on 12 random ly selected graphite
sam ples using a com puterized Rigaku pow der diffraction spectrom eter w ith
graphite monochronometer. Scans w ere m ade using copper K a radiation, a
scan range from 25 to 28° 20, and a data sam pling interval of 0.01° 2© at 1° 20
per m inute. Accuracy of 20 alignm ent w as determ ined using quartz dioi =
26.67° and the quartz quintuplet.
RESULTS
Petrology
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10
The Cucamonga terrane is lithologically and chemically heterogeneous at the
terrane, outcrop and thin section scale (Barth et al. 1992). O ur m apping has
show n that the Cucamonga Canyon to Day Canyon (western to central)
portion of the terrane consists predom inantly of mafic gneisses w ith minor
alum inous and felsic gneisses containing intercalated < 1% carbonate-bearing
lithologies (Figure 2). The eastern San Sevaine Canyon region consists
predom inantly of felsic gneiss w ith volum etrically less mafic and alum inous
gneisses and contains c. 3-5% carbonates. Carbonate-bearing lithologies are
subdivided into marbles that contain > 95% caldte and calc-silicates that
contain < 95% caldte. The small size and disrupted nature of the Cucamonga
terrane m eans the relationship between different carbonate outcrops is
unknow n and different outcrops could represent the same carbonate layer.
Alf (1948), Hsii (1955), May & Walker (1989) and Barth & May (1992) have
exam ined m any of the granulites of the Cucamonga terrane. Barth & May
(1992) and Barth et al. (1992) divided the rocks of the Cucamonga terrane into
five prim ary types: (1) mafic gneisses that are compositionally sim ilar to basalt
or basaltic ash, (2) alum inous gneisses w ith a pelitic protolith, (3) felsic
gneisses that are com positionally sim ilar to an im pure sandstone and (4)
m arble and calc-silicate that have an im pure limestone protolith. The four
m eta-sedim entary rock types are concordantly to discordantly intruded by
small mafic and felsic pegm atitic bodies and a biotite granite emplaced at near
peak m etam orphic conditions (May & Walker, 1989; Barth & May, 1992).
There is no evidence of m igm atization anywhere w ithin the Cucamonga
terrane (Hsii, 1955; Barth, 1989) despite prolific m igm atization in the adjacent
San Antonio terrane m etam orphosed at upper am phibolite facies (Hsii, 1955;
Ehlig, 1981; May & Walker, 1989; Barth and May, 1992).
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11
Most of the Cucamonga carbonates contain granoblastic textures locally
modified by weak to strong brittle deform ation fabrics. N early pervasive
deform ation fabrics are observed in many carbonate sam ples from
Cucamonga Canyon. Carbonates throughout the terrane contain variable
quantities of caldte, dolomite, diopside, feldspar, scapolite, phlogopite,
forsterite, sphene, orthopyroxene, garnet and pink spinel as prim ary minerals
and trace am ounts of trem olite and chlorite as texturally secondary minerals.
Foliation in the Cucamonga terrane is well developed and generally sub
parallel to east-west trending, nearly vertical lithologic layering (May &
W alker, 1989; Barth & May, 1992). Compositional layering varies in thickness
from c. 1 mm calc-silicate layers w ithin marble, up to 3-5 m thick marble plus
calc-silicate layers. M apping by May & Walker (1989), Barth & May (1992), and
our own work has show n that layering of m eta-sedim entary lithologies in the
Cucamonga terrane has been transposed and disrupted by both mylonitic and
brittle shear deform ation so that lithologic layering typically can only be
traced along strike from a few cm for thin layers to 100 m for 3-5 m thick
layers.
Most carbonate/calc-silicate lithologies are graphitic and contain from < 0.01
to c. 5% graphite. A few samples of mafic gneiss contain up to 5% graphite
and one sam ple contains 20% graphite. Most graphite is wholly intergranular
in marble, calc-silicates and silicate lithologies. A sm all portion of graphite
from the mafic gneiss is intercalated w ith biotite and a m inor quantity is
completely enclosed w ithin pyroxene or garnet (i.e., arm ored). Graphite
arm ored by silicates frequently have depleted 51 3 C values compared to
graphite enclosed w ithin caldte (Wada & Suzuki, 1983) because of the
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difficulty of isotopic diffusion through silicate minerals. Only unarm ored
graphite samples were analyzed as part of this study.
Morrison & Barth (1993) noted three distinct graphite textures in their
Cucamonga terrane marble and calc-silicate samples. These included flake,
trails and dissem inated grains. Flake texture was characterized by high
reflectance, individual graphite flakes; trail graphite was linear trails of fine
grained graphite, and dissem inated graphite consisted of very fine
dissem inated graphite. Further examination of Cucamonga graphite textures
suggests that trail texture may be the result of disaggregation due to shear
deformation and the very fine grained disseminated graphite as the result of
extreme disintegration of the graphite, perhaps due to the weak nature of the
doooi basal cleavage that provides the lubricating quality of graphite
Carbon isotope systematics
A total of 49 carbonate and 53 graphite carbon isotopic analyses were
conducted for this study. The results are shown in Table 1. Figure 3 shows the
carbonate carbon isotopic com position distribution from Cucamonga Canyon,
Deer and Day Canyons, and San Sevaine Canyon area. The Deer and Day
Canyon area has a range and average isotopic composition of -1 .2 ± 2.5 % o and
a range in isotopic values of 8.0%o. Cucamonga Canyon area carbonate 51 3 C
values are -1.1 ± 2.9%o w ith a range of 10.7%o. These two areas are isotopically
the same w ithin analytical uncertainty w ith sim ilar average and range
isotopic compositions. Carbon isotopic values in San Sevaine Canyon area
average 1.1 ± 3.9% o and a range in isotopic values of 18.5%o. However, 4 of the
13
San Sevaine Canyon sam ples have unusually enriched 8 C values > 8 % o and
one sample has an unusually low isotopic composition of -9.8 % o (Morrison
& Barth, 1993). If these 5 unusual isotopic values, which are discussed further
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13
below, are excluded then the average isotopic value in San Sevaine Canyon is
0.4 ± 2.2 % o w ith a range of 9.9%o. Thus, the San Sevaine Canyon area is
enriched by a minimum of c. 2% o over die Cucamonga Canyon, Deer Canyon
and Day Canyon areas, bu t does have a sim ilar range in 51 3 C values.
C aldte and graphite carbon isotopic compositions from the three core drilled
traverses in Deer Canyon are shown in Figure 4. The carbon isotopic
com positions from the three traverses is heterogeneous at all scales w ith a
range in 8 13C q i of c. 10.4%o and 51 3 Ccr of c. 11.1 % o . The carbon isotopic
com position of graphite hosted by silicate is c 3.2%o lower than nearby
graphite that is in a m arble matrix. In Traverse #1, the 51 3 Ccai values vary by
c. 7.2% o and 51 3C cr varies by c. 11.2%o. Core drilled sample 95CB-10 (third
from left in Figure 4) intersected a m inor fold and sam pled both calc-silicate
and mafic gneiss lithologies. G raphite analyzed from marble had 8 C of
-633%o while graphite hosted by silicate mineralogies less than 1 cm away
had a 5 1 3 C of -9.45%o, a variation of c. 3.1 % o .
Figure 5 shows that the range of caldte carbon isotopic values throughout die
Cucamonga terrane is the nearly the same as that determ ined for relatively
unm etam orphosed carbonates (0.9 ± 2.6%o, Veizer & Hoefs, 1976), the upper
am phibolite to granulite Adirondacks (3.6 ± 4.0%o, Valley & O 'N eil, 1984), die
Esplanade Range of British Columbia (0.2 ± 2.2% o , Ghent & O 'Neil, 1985), and
the granulites of the H ighland Complex of southern Sri Lanka (0.0 ± 1.8 % o ,
Hoffbauer & Spiering, 1994). Variations in carbon iso topic com position can be
due to (1) original sedim entary isotopic values (e.g., Valley & O 'N eil, 1984;
Tucker, 1990b), (2) the effects of diagenesis, particularly during dolom itization
(Veizer, 1992; Tucker, 1990a), (3) isotopic depletion during m etam orphic
decarbonation reactions (Valley & O 'Neil, 1984; Valley, 1986), and (4)
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14
channelized infiltration of carbon-rich fluids like CO2 (e.g., Newton, 1986;
H arris & Bickle, 1989; Raith & Srikantappa, 1993).
Because the stratigraphic relationship between the different carbonate
outcrops is poorly constrained and the terrane is narrow parallel to lithologic
layering, it is not unreasonable to presum e that some of the analyzed
carbonate outcrops were originally from the same sedim entary bed that was
disrupted during dynam ic metamorphism. If some of the carbonates are the
sam e, then these carbonates w ould be expected to have sim ilar isotopic
signatures. The lack of a recognized protolith for the Cucamonga terrane and
the variable dolom ite content means it is im possible to evaluate original
sedim entary 81 3 C values as a source of original carbon isotopic heterogeneity.
H owever, the potential effects of Rayleigh decarbonation and channelized
1 ^
C0 2-fluid flow on 8 C values may be made.
Rayleigh decarbonation effects on 8 C
Carbon isotopic depletion in a rock will result from high tem perature
m etam orphic decarbonation reactions. This may produce carbon isotopic
variation in a rock which was once isotopically homogeneous but was
m ineralogically heterogeneous. Variations in silicate m ineral concentration
affects the degree of decarbonation progress that results in isotopic depletion.
In order to quantify the potential isotopic depletion caused decarbonation
reactions such as:
dolomite + quartz = diopside + CO2;
CaMg(C03)2 + 2Si02 = CaMgSi20 6 + ICOi', (1)
w ould have on post m etam orphic carbon isotopic composition using the
m ethod of Valley & O'Neil (1984) and Valley (1986). Rayleigh volatilization is
used since it provides the largest theoretical fractionation, a fractionation
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15
factor of 1.0022 ( < x 1 3 CcQ2 -rock) and variable F-values (mole fraction of carbonate
remaining). Theoretical 81 3 C depletions are show n as a function of F-value in
Table 2. C aldte containing diopside could theoretically be depleted in 1 3 C by
up to 0.1 l%o. Calc-silicate rocks of the Cucamonga terrane have variable
caldte com positions that range from 5 to 95 % w ith m ost samples containing
5-30% silicate mineralogies. In the case of 20 % diopside, a theoretical
depletion of c. 0.8 % o is possible and in the rare case of a calc-silicate rock with
only c. 5% caldte and c. 95%, a maximum potential 1 3 C depletion of = 6.6 % o is
possible.
Cucamonga m arble veins
Several types and generations of veins are sparsely present throughout the
Cucamonga terrane marbles (Morrison & Barth, 1993) that potentially could
have affected the carbon isotopic composition of either the carbonates or
graphite if the fluids were of suffident quantity. In the Cucamonga Canyon
area, a cross-cutting brittle fracture zone in marble (93CB-lb) contains c. 40%
graphite in contrast to the marble c. 0.3 m to the side of the fracture that
contains c. 1% graphite (93CB-la). It is petrographically undear if this zone
represents predpitation of graphite from carbon-bearing fluids during brittle
shearing or if carbonate was preferentially rem oved. However, the carbon
isotopic compositions are very different (81 3 Cgr = -11.2%o and 81 3 Ccai = -5.47%o
for the former; 81 3 Cgr = -5.73% o and S1 3 Ccai = 0.46% o for the latter). While the
carbon isotopic compositions from the two samples are quite different, the
Acai-Gr values are sim ilar (5.7%o vs. 6.2%o).
In Deer Canyon one of the core drilled sam ples (95CB-15a) contains a small
cross-cutting graphite vein. This vein is < 0.01 mm in thickness but is quite
obvious due to the contrast of black, very fine grained graphite in a white
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16
marble. Marble on either side of the graphite vein uncharacteristically
contains no other graphite. This may be due to a fluid that infiltrated either
side of the vein and could have oxidized any pre-existing graphite, if present,
followed by a change in fluid chemistry that caused late graphite vein
precipitation. The caldte carbon iso topic composition of this sam ple and an
adjacent sam ple (95CB-16a) are unusual in com parison w ith other nearby
sam ples and is discussed in detail below. Several sam ples in the San Sevaine
Canyon area contain veins that are distinguished by the presence of
recrystallized caldte and a lack of graphite w ithin the zone of re-crystallized
caldte.
C aldte-graphite isotopic therm om etry
Peak m etam orphic tem perature estim ates may determ ined using the
presence of co-existing caldte and graphite if isotopic equilibrium is attained
(Valley & O'Neil, 1984; Dunn & Valley, 1992; M orrison & Barth, 1993; Kitchen
& Valley, 1995). To evaluate the conversion of carbonaceous m aterial to
graphite, eighteen random samples of graphite from carbonate, calc-silicate
and gneissic hosts were analyzed by X-ray powder diffraction analysis. All 18
samples produced sharp, high intensity peaks corresponding to dooo2 of 3.35 to
3.36 A w ithout any trace of larger d-spadng peaks that w ould be suggestive of
incom plete conversion of organic carbon com pounds to graphite. D -spadngs
of 3.35 to 3.36 A indicates well crystallized, fully ordered graphite which is
necessary for application of the caldte-graphite isotopic therm om eter (Landis,
1971; M orrison & Barth, 1993) because equilibration of carbon isotopes
between co-existing caldte and graphite prim arily occurs during the
transform ation of disordered, am orphous sedim entary organic carbon to well
ordered, well crystallized graphite (Landis, 1971; Grew, 1974; Valley & O'Neil,
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17
1981; Buseck & H uang, 1985; M orrison & Barth, 1993; Scheele & Hoefs, 1992;
Kitchen & Valley, 1995).
Ideally, caldte-graphite isotopic com positions should retain peak
m etam orphic tem peratures due to a high carbon diffusion rate in caldte,
which is by far the largest carbon reservoir in a m arble and is therefore
unlikely to be significantly affected on mass balance basis by minor
infiltration of isotopically distinct carbon, coupled w ith a slow diffusion rate
in graphite, representing a m uch sm aller carbon reservoir (Thrower & Mayer,
1978; Kitchen & Valley, 1995).
Figure 6 shows co-existing caldte and graphite 51 3 C values from the different
geographic regions of the Cucamonga terrane. Six sam ples from Cucamonga
Canyon (Figure 6a) yield m etam orphic tem peratures that range from c. 504 ±
24°C (n=3) to c. 695 ± 58°C (n=3).
Twenty-six co-existing caldte-graphite pairs from Deer Canyon and one pair
from Day Canyon from the central portion of the terrane are plotted in Figure
6b. Twenty-two of the pairs plot along a high tem perature isopleth (Acai-Gr =
81 3 Ccai - 51 3 Ccr) of 2.69 ± 0.37%o. The isotopic range in values is 8.8%o for
graphite and 7.5 % o for caldte. A range of this m agnitude in isotopic values
along an isopleth is good evidence that isotopic equilibrium was attained in
these samples. Acai-Gr of c. 2.69 % o corresponds to an extrapolated tem perature
of c. 887 ± 87° C, which exceeds the 850° C tem perature calibration of Kitchen
& Valley (1995). However, one of the sam ples contains a late-stage graphite
vein and has an unusually low Acai-Gr/ ° ne sam ple is adjacent to the vein and
two of the sam ples have unusually doudy caldte in thin section. If these four
samples are exduded, then Acai-Gr is c- 2.71 ± 0.37%o which corresponds to an
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18
extrapolated tem perature of c. 880 ± 87 °C. Five sam ples from Deer Canyon
have calculated tem peratures that group near 485 ± 32° C.
Fifteen co-existing caldte-graphite isotopic values from San Sevaine Canyon
area are shown in Figure 6c. Nine sam ples plot along an isopleth that
averages 3.11 ± 0.22 % o , and yields a tem perature of 797 ± 36 °C. Four samples
plot away from the c. 800° C isotherm , two plot at 492°C and two plot at 562°C.
Thus, it appears from the high tem perature results, that the Deer & Day
Canyon (central region) of the Cucamonga terrane retains peak metamorphic
tem peratures that are at least 50° C higher than those recorded in the San
Sevaine Canyon (eastern region) and c. 100° C higher than the highest
tem peratures recorded in the Cucamonga Canyon area. While the difference
in m etam orphic tem perature is w ithin the standard deviation between the
calculated tem peratures of different regions (695 ± 58 °C; 880 ± 87° C; 797 ±
36°C; w est to east), this tem perature difference is sim ilar to the c. 40 °C
difference Barth & May (1992) have shown using cation-exchange
therm om etry.
In addition to variations in tem perature and am ount of mylonitic
deform ation in the w estern, central and eastern portions of the terrane,
graphite textures are also different. G raphite textures from Cucamonga
Canyon are either dissem inated or trail graphite textures. G raphite from Deer
Canyon and Day Canyon is entirely high reflectance flake texture w ith crystal
sizes that range from c. 0.5 mm to 5 mm in size. The eastern samples indude
all three textures induding flake, trail and dissem inated graphite. All of the
sam ples that plot aw ay from the 797 ± 36° C isotherm are either trail or
dissem inated textures, although som e trail and dissem inated graphite
sam ples do plot on the 797° C isotherm .
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19
DISCUSSION
Based on caldte-graphite therm om etry, the eastern portion of the Cucamonga
terrane retains a peak metam orphic tem perature of c. 800 °C, the middle
portion of the terrane retains a peak metam orphic tem perature of > 850° C
and the eastern portion of the terrane, a tem perature of c. 750° C. These may
reflect real therm al variations or variable retrograde resetting. It is interesting
to note that these results, w ith the inner portion of the terrane yielding a
tem perature c. 50° C higher than either end, are consistent w ith Barth &
M ay's (1992) cation exchange thermom etry which suggests c. 40° C higher
tem peratures in the central portion of the terrane. Two independent
estim ates of elevated peak-m etam orphic tem perature in the central region
suggests this may be a real phenom enon. A higher retained tem perature
could be either that the central region was metamorphosed at a higher
tem perature than either end, or that the San Sevaine Canyon region and
likely the Cucamonga Canyon area have been affected by retrograde an d /o r
m ylonitic metam orphic affects (e.g., van der Pluijm & Carlson, 1989).
At tem peratures of c. 850 °C and pressures of c. 8 kb (Barth & May, 1993)
w idespread fluid-present m elting of m ost Cucamonga lithologies would be
expected irrespective of the lack of well constrained melting tem peratures
(e.g., H uang & Wyllie, 1974; Thompson & Algor, 1977; Bohlen et al., 1983;
M ontana & Brearley, 1989; Peterson & Newton, 1989; Vielzeuf & Clemens,
1992; M orrison & Barth, 1993). The presence w ithin the terrane of many
lithologies that should have melted at c. 850 °C and the lack of migmatites
that w ould be expected during extensive m eta-sedimentary anatexis suggests
that w idespread fluid-present m elting did not occur. However, if the
Cucamonga terrane was already largely dehydrated and fluid-absent, then
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20
peak m etam orphic tem peratures >850 °C at 8 kb are sufficient for fluid-absent
muscovite- or biotite-dehydration m elting reactions in m etapelites to
commence (Thompson & Algor, 1977; Le Breton & Thompson, 1988; Vielzeuf
& Montel, 1994) that could have resulted in 10-15% pelite melting (Vielzeuf &
Montel, 1994). D ehydration m elt volum e is extrem ely sensitive to small
differences in tem perature, pressure and bulk chem istry present in the
Cucamonga terrane (Vielzeuf & M ontel, 1994).
In addition to a high tem perature grouping of c. 750-850° C, all three regions
sam pled as part of this study have a cluster of sam ples that have tem peratures
c. 500° C. It is unlikely that the c. 500° C found in all three sampled regions is
due to incom plete isotopic equilibration because of the range in isotopic
values w hile having isotopic values falling along an isopleth. The low
tem perature is inconsistent w ith the am phibolite retrograde metam orphism
associated w ith m ylonitic deform ation (May & W alker, 1989).
One explanation for the c. 500 °C m etam orphic tem peratures is that it is the
Cucamonga terrane response to regional therm al perturbation associated with
reset biotite and hornblende K-Ar age data caused by underthrusting of the
Pelona Schist terrane during flattening of the Paleocene subduction (Miller &
M orton, 1980). O ther possibilities are that the c. 500°C tem peratures represents
the loss of high tem perature equilibrium values during post metamorphic
events such as retrograde m ylonitization (van der Pluijm & Carlson, 1989;
May & W alker, 1989) or retrograde re-equilibration during infiltration of CO2-
rich fluids (e.g. M orrison & Valley, 1988, 1991; M orrison & Barth, 1993).
The c. 2% o difference in m arble carbon isotopic com position between the San
Sevaine Canyon region and other areas to the w est (Figure 3) cannot be due to
the effects of isotopic equilibration between co-existing caldte and graphite
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2 1
because the graphite contents of the two areas overlap and except for two
sam ples, there is insufficient graphite in the m ode to significantly alter
carbonate isotopic com position (Figure 7). The one sam ple that has been
affected by isotopic equilibration has a 51 3 C of -9.8%o and is from a calc-silicate
that contains c. 5% graphite and only 20 % caldte (Morrison & Barth, 1993).
M uch of the isotopic difference is due to the presence of four unusually
enriched m arble isotopic com positions above 8 % o (Figure 3). Excluding these
four high 51 3 C values lowers the average 81 3 C value to 0.4 ± 2.2 % o which is
m uch d oser to the isotopic values from other parts of the terrane and the
w orld-w ide unm etam orphosed carbonate values (Veizer & Hoefs, 1976).
Enriched isotopic com positions above c. 3 % o are difficult to obtain by
m etam orphic processes and are therefore pre-m etam orphic in origin.
Restricted basin lim estone deposition (DeGiovani et al, 1974), m ethane
production during diagenesis (M urata, et al., 1967) and high burial rates of
isotopically light biological carbon are methods of obtaining isotopically
enriched carbon isotopic signatures (Wickham & Peters, 1993). High
values have been observed in other parts of the N orth American Cordillera
(e.g., W ickham & Peters, 1993; Ghent & O'Neil, 1985).
Isotopic equilibration betw een co-existing caldte and graphite will prim arily
change graphite 51 3 C values from their pre-m etam orphic values since
carbonates typically have isotopic compositions near 0% o and graphite
derived from sedim entary organic material usually has isotopic values near
-2 8 % o (M orrison & Barth, 1993).
CONCLUSIONS
The C u c a m o n g a terrane contains an c. 23% o variation in 51 3 C cr values at the
terrane scale, an c. 11 % o isotopic variation at die outcrop scale and an c. 3.1 % o
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22
variation at the cm scale. C aldte 5 C values show a sim ilar pattern, w ith c.
18.4%o variation at the terrane scale and c. 11.0%> isotopic variation a t die
outcrop scale. Isotopic variation of this extent means that the Cucamonga
terrane is heterogeneous w ith regard to carbon isotopes. M etamorphic
decarbonation reactions and isotopic equilibration of relatively enriched
carbonate w ith depleted graphite will cause a shift towards lighter carbon
isotopic com positions from sedim entary values, but the resulting isotopic
change is not significant enough to account for the range in isotopic
44
com positions. The 8 C values likely represent original sedim entary values
w ith m inor isotopic depletion caused by decarbonation and isotopic
equilibration between two isotopically different reservoirs.
The degree of carbon isotopic heterogeneity is sim ilar to that seen in
unm etam orphosed carbonate rocks and marbles that were m etam orphosed in
the absence of a fluid phase (Bohlen et al., 1987; Valley et al., 1990; Buick et al.,
1994; Newton et al., 1980). Carbon isotopic heterogeneity dem onstrates that
the Cucamonga terrane retains carbon isotopic values that are likely near
original sedim entary values. Isotopic heterogeneity predudes the possibility
the terrane was infiltrated by a pervasive CCh-rich fluid. We condude,
therefore, that dehydration of the Cucamonga terrane metasedim ents to
anhydrous granulite mineralogies was not accomplished by C0 2 -flooding
(e.g., N ew ton et al., 1980).
Calculated caldte-graphite carbon isotopic tem peratures from isotopically
equilibrated San Sevaine Canyon sam ples are c. 800 °C. Temperatures
calculated from equilibrated samples in the central Deer Canyon area are > 850
°C. Barth & May (1992) determ ined c. 40 °C higher tem peratures in die Deer
Canyon area using cation exchange geothermometry. While the tem perature
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23
differences between Deer Canyon and the other portions of the terrane are
w ithin the statistical variation, two independent estim ates of similar
tem perature variation suggest that the higher tem perature are probably real.
The age of the protolith may be constrained the presence of 4 81 3 C values that
average 8.3%o from the San Sevaine Canyon area. High 8l3C values have been
shown to be suggestive of neo-Proterozoic age marble in other locations in
the Cordillera. This combined, w ith a single y / 8 6 Sr value that does not fall on
the Phanerozoic seawater curve (personal communication, Barth), suggests
that the age of the Cucamonga terrane sedim entary protoliths may span the
late Proterozoic to Phanerozoic, however, other explanations for enriched
carbon isotopic are possible.
D ehydration of hydrous am phibolite grade rocks to anhydrous granulites
during metamorphism may be accomplished by (1) extraction of H2 0 into the
m elt phase followed by removal of the melt (Fyfe, 1973), (2) reduction of H 2 0 -
activity by m antle-derived C 0 2 -flooding (Newton et al., 1980), dehydration
accomplished during passage of a C 02 -rich magma (Frost & Frost, 1987), or (3)
the metam orphism of rocks that are already dehydrated (Valley & O'Neil,
1984). Because the Cucamonga terrane is composed of sedim entary
lithologies, it is unlikely that the terrane was in a dehydrated state. The lack of
any suggestion that the Cucamonga terrane contains hom ogenized carbon
isotope values requires that the Cucamonga terrane was not infiltrated by
pervasive flow of a C0 2 -rich fluid or a C 0 2 -rich magma that could have
caused the formation of anhydrous granulite mineralogies. Since
metamorphic temperatures exceeded 850° C, it is likely that dehydration-
m elting of mafic gneisses and alum inous lithologies occurred in the
Cucamonga terrane which suggests that dehydration to granulite conditions
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24
occurred due to the partitioning of H2O into the m elt phase and removal of
the generated m elts. The m elt phase produced was of necessity efficiently
extracted due the absence of migmatites, however, the presence of minor
mafic to felsic intrusions m ay be evidence of m elt production (Barth & May,
1992, Thom pson & Connolly, 1992). Anatectic m elts derived from the m iddle
to low er crustal-level Cucamonga terrane therefore could have contributed to
the higher crustal level m agmatism observed in the Placerita-San Antonio
and San Gabriel terranes.
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25
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31
FIGURE CAPTIONS
Figure 1. Location of the Cucamonga terrane in southern California. The
relationships between the San Gabriel, Placerita-San A ntonio and Pelona
Schist terranes along w ith the num erous Cenozoic faults that have
disrupted these terranes are shown. Contacts between terranes are faults.
Blank areas are recent sedim ents or Cenozoic sedim entary rock units.
Figure 2. Map of the Cucamonga and adjacent San Antonio terrane (modified
from May and W alker, 1989; Barth et al, 1992). The shaded areas are
variably affected by mylonitic deform ation and retrograde m etam orphism
(May & Walker, 1989). W hite area in the central region of the Cucamonga
terrane is generally free of mylonitic fabrics and is relatively unaltered.
The contact between the Cucamonga terrane and San Antonio terrane is
the "Black M ylonite" (Alf, 1948).
Figure 3. Histogram com parison of carbonate 51 3 C analyses from Cucamonga
(western), Deer (central), and San Sevaine Canyon (eastern) regions of the
Cucamonga terrane. The range and average of carbon isotopic values for
Cucamonga and Deer Canyon areas are virtually identical. The San
Sevaine Canyon area has about the same range but is enriched by c. 2 % o
relative to the Cucamonga and Deer Canyons. Isotopic enrichm ent is
partially due to unusually enriched isotopic values as discussed in the text.
Included in this histogram are 23 marble analyses from the data set of
Morrison and Barth (1993) and are shown by light shading.
Figure 4. Carbon isotopic compositions of caldte (squares) and graphite
(drdes) for the three core drilled traverses in Deer Canyon. Each one of the
three traverses represents a single outcrop and the three traverses
(outcrops) are w ithin 10 m of each other. The distance betw een cores is 10
cm. Isotopic fractionation between co-existing caldte-graphite pairs is
shown by the length of the connecting line for co-existing caldte-graphite
pairs. The third sample from the left of traverse #1 has two graphite
analyses, one for m arble hosted graphite and the other for gneissic hosted
graphite. The significance of this is discussed in the text. A13C c ai-gr = 2.7, the
average value for non-altered samples, is shown by shading for scale. This
corresponds to a calculated tem perature in excess of 850°C using the
calibration of Kitchen and Valley (1995). It should be noted that silicate
hosted graphite 81 3 C values are usually depleted relative to nearby
graphite hosted by caldte which suggests that graphite-carbon co-existing
w ith isotopically heavier caldte-carbon was isotopically equilibrated. These
results show that the carbon isotopic composition is heterogeneous at the
outcrop scale.
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32
Figure 5. Com parison of the range and average values from other carbonates,
including w orld-w ide relatively unm etam orphosed carbonates (Veizer &
Hoefe, 1976), the Esplanade Range of British Columbia (Ghent & O 'Neil,
1985), the A dirondacks (Valley & O'Neil, 1984), and southern Sri Lanka
(Hoffbauer & Spiering, 1994). The range and average carbon isotopic
com position of these data sets is sim ilar despite differing m etam orphic
grade and location.
Figure 6. Co-existing 813C c a i versus 51 3 C cr plot of data from the San Sevaine
Canyon region, Deer & Day Canyon and Cucamonga Canyon. Isotherm s of
5 0 0 °C , 750 °C and 8 5 0 °C are shown for reference (Kitchen & Valley, 1995).
The average Acai-Gr equilibrium data from San Sevaine Canyon is = 3.1 % o
w hile that from Deer Canyon is =2.7%o; a calculated difference of at least
50°C. Forty degree higher peak-m etam orphic tem peratures in Deer
Canyon have also been noted by Barth & May (1992) using cation exchange
therm om etry. Samples plotted as open circles are from M orrison & Barth
(1993).
Figure 7. Diagram show ing the changes in 81 3 Ccai and coexisting 51 3 Ccr if
iso topic equilibrium is attained from starting conditions of 81 3Q >i = 2 % o
and 51 3 Ccr = -25%o at solid square. Tick marks show AcaJ-Gr = 2 ,3,4,10 and
20 % o along mass balance paths. In the case where the graphite content is c.
1% graphite, then the change in 51 3 Ccai is c. 1.9% o w hen Acai-Gr = 3%».
Alm ost all Cucamonga sam ples have < 0.1% graphite which corresponds
to negligible change in 81 3 Ccai- On the basis on mass balance isotopic
effects, therefore, there is little change in 51 3 Ccai at low Cucamonga
graphite contents.
Figure 8. Range in 81 3 C values for different lithologies throughout the
Cucamonga terrane. An overall range of c. 30 % o in isotopic values shows
considerable carbon isotopic heterogeneity at the terrane-scale that
dem onstrates the absence of a pervasive C0 2 -rich fluid.
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33
Table 1. Sample location, rock type, isotopic com position and mineralogy.
R o c k
S a m p le L o c a tio n T y p e 5 13C cai 5 13C Gr ACal_ Gr
9 3 C B -1 -1 C C M -5 .8 2
9 3 C B -1 -2 C C M -0 .0 6 -3 .6 7
9 3 C B -1 -3 C C M 0 .2 9 -3 .1 1
9 3 C B - l a C C M 0 .4 6 -5 .7 3
9 3 C B -lb C C M -5 .4 7 -1 1 .2
9 3 C B -2 a C C M 2 .5 4 -3 .1 7
9 3 C B -2 c C C M 0 .9 9 -3 .4 3
9 3 C B -4 C C FG -1 6 .5
9 3 C B -5 C C F G -1 9 .1
9 6 C B -1 1 C C C S 1.16
9 6 C B - l lb C C C S -2 .1 7
9 6 C B -1 8 b C C M - 0 3 7
9 6 C B -lg C C M -4 .4 6
9 6 C B -2 0 C C M -4 .4 3
9 3 C B - l lc - l D a C C S -0 .6 5 -9 .0 5
9 3 C B -1 2 c D C M 4.11
9 3 C B -1 2 d D C M 3 .8 5
9 3 C B -1 3 a D C M - 0 3 1 -6 .2 7
9 3 C B -1 3 b D C M 2 3 6 -4 .4 7
9 3 C B -1 4 a D C C S -4 .3 4 -6 .8 7
9 3 C B -1 4 b D C C S - 1 3 8
9 5 C B -0 8 D C FG -1 1 .0
9 5 C B -0 9 D C B S -1 0 .0
9 5 C B -1 0 G D C G -9 .4 5
9 5 C B -1 0 a D C C S -3 .1 8 -6 .3 3
9 5 C B - l l a D C C S -4 .7 4
9 5 C B -1 2 a D C M -1 .7 1 -4 .6 2
9 5 C B -1 3 a D C M G - 8 3 1
9 5 C B -1 4 a D C M 1 .14 0.15
9 5 C B -1 5 a D C M 4 .0 6 -1 .8 1
9 5 C B -1 6 a D C M G - 2 3 7 -2 .6 0
9 5 C B -1 6 b D C M G -2 .9 4 -5 .8 3
9 5 C B -1 7 D C FG -9 .0 0
9 5 C B -1 8 D C FG -9 .3 1
9 5 C B -1 9 D C F G -9 .1 2
9 5 C B -2 0 b D C M 0 .8 9 -2 .1 5
9 5 C B -2 3 a D C M - 2 3 7 - 2 3 7
9 5 C B -2 4 D C C S -3 .0 2 -5 .6 9
9 5 C B -2 8 b D C M -2 .4 4 - 5 3 4
9 5 C B -2 9 D C M -0 .3 9 -3 .5
9 5 C B -3 0 D C C S -1 .1 6 -3 .0 8
9 5 C B -3 1 D C C S -6 .3 4 -8 .6 8
9 5 C B -3 2 D C C S -7 .9 0
9 5 C B -3 3 D C C S -8 .2 0
M in e r a lo g y
C a l, C p x , G r
3 .6 C a l, C p x , G r
3 .4 C a l, C p x , G r
6 .2 C a l, C p x , G r
5 .7 C a l, C p x , S e p , G r
5 .7 C a l, C p x , G r
4 .4 C a l, C p x , P h i, G r
PI, Q tz , G r
PI, Q tz , G r
C a l, C p x , G r
C a l, C p x , G r
C a l, C p x , G r
Ceil, D i, G r, S e p
C a l, C p x , P I, G r, (T r)
9 .7 C a l, S e p , C p x , P I, G r
C a l, Phi, C p x , G r
C a l, S e p , P h i, C p x , G r
5 .8 C a l, C p x , S e p , G r
6 .8 C a l, C p x , G r
2 .5 C p x , C a l, P I, G r
Ceil, C p x , P I, G r
Q tz , G rt, C p x , B io, O x , G r, Z ir
B io, P I, C p x , G rt, Q tz , W o , O x , A p , Z ir, G r
3 .2 C p x , S e p , C a l, T tn , G r
C p x , P I, Q tz , T tn , O x , S e p , O r, G r, Z ir, Chi
2 .9 C a l, C p x , F o , G r
PI, C p x , B io , Q tz , T tn , O x , G r, (W o)
1.0 C a l, C p x , S e p , T tn , F s p , G r
5 .9 C a l, C p x , G r ( in v e in )
2 .9 C p x , P I, Q tz , C a l, T n t, G r, (Z ir, S rp )
2 .9 C p x , PI, Q tz , C a l, T n t, G r, (Z ir, Srp)
PI, Q tz , C a l, C p x , B io , T tn , Z ir, G r, (Chi)
Q tz , P I, C p x , O x , C a l, A p , T tn , G r, (Chi)
Q tz , P I, C p x , T tn , O x , G r, (Chi, Cal)
3 .0 C a l, C p x , G r
2 .8 C a l, B io, C p x , G r , (K sp )
2 .7 C a l, C p x , P I, P h i, T tn , A p , G r
3.1 C a l, C p x , G r
3.1 C a l, C p x , G r
1.9 S ep , C p x , P I, C a l, T tn , (Sill, G r)
2 .3 C p x , S q>, P I, C a l, T tn , (C rd , A p , Chi, G r)
C p x , S e p , Q tz , P L T tn , O x , G r, Hem
Q tz , C p x , P I, O x , Chi, G r
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34
9 5 C B -3 5 b D C M 0 .5 7 -0 .9 1 1 5 C a l, G r, C p x , P h i, A p
9 5 C B -3 6 D C M - 1 5 0 -4 .1 2 2 .6 C a l, C p x , S e p , O x , G r, A p
9 5 C B -3 7 D C M -3 .8 7 -5 .9 3 2.1 C a l, C p x , G r
9 5 C B -3 8 a D C C S -4 .6 3 -7 .0 8 2 5 C p x , C a l, G r
9 5 C B -3 9 a D C M -1 .4 9 -3 .7 2 22. C a l, G r, G r t, C p x , B io, A p , (Chi, Sqp)
9 5 C B -4 0 D C M -1 .0 8 -3 .9 4 2.9 C a l, F o, S r p , C h i, C p x , G r, G rt
9 5 C B -4 1 D C M G - 6 5 7 C p x , P I, B io , T tn , G r
9 5 C B -4 2 D C C S -4 .3 3 -6 .8 0 2 5 C p x , C a l, G r , P h i, A p , S e p
9 5 C B -4 3 D C C S - 4 5 5 - 7 5 2 3.0 C p x , C a l, O x , Chi, P h i, G r
9 5 C B -4 4 D C M G - 7 5 4 C p x , B io, P I, T tn O x , A p , G r, Z ir
9 5 C B -4 5 D C C S - 4 5 8 - 7 5 8 3 5 C p x , C a l, S e p , T tn , G r, Z ir
9 5 C B -4 6 D C C S 0.7 5 -5 .6 9 6 .4 C p x , C a l, S e p , G r t, P h i, A p , G r
9 5 C B -4 7 D C FG - 6 5 8 PI, C p x , q tz , G r, (Chi, T tn , Cal)
9 2 C B -2 S S C M 0.8 2 C a l, Q tz , P I, O x , C p x , G r
9 2 C B -3 a S S C C S 8.6 8 2 5 3 6.4 C a l, P I, C p x , Q tz , G r
9 2 C B -4 a S S C M 8 .0 7 2.4 0 5.7 C a l, C p x , P I, G r
9 2 C B -5 a S S C M 8.4 9 -7 .0 7 15.6 C a l, C p x , P h i, G r
9 2 C B -6 a S S C M - 1 5 1 - 4 5 5 2 .8 C a l, C p x , P I, P h i, G r
9 2 C B -7 a S S C M 2.11 C a l, C p x , P I, G r
9 2 C B -8 a S S C M 0 5 0 C a l, O l, G r
9 2 C B -9 a S S C M 2 .4 4 C a l, (C p x , G r)
Rock Types: M, marble; CS, calc-silicate; MG, mafic gneiss; FG, felsic granulite;
AG, alum inous gneiss; BS, biotite schist; G, gneiss. Locations: CC, Cucamonga
Canyon; DaC, Day Canyon; DC, Deer Canyon; SSC, San Sevaine Canyon.
Mineral abbreviations: Ap, apatite; Bio, biotite; Cal, caldte; Cpx,
dinopyroxene; Fo, forsterite; Gr, graphite; Grt, gam et; Ksp, potassium
feldspar; Ol, olivine; Ox, opaque oxide; Phi, phlogopite, PI, plagiodase; Qtz,
quartz; Rt, ruble; Srp, serpentine; Sep, scapolite; SU, sillim anite; Tr, tremolite;
Ttn, dtanite (sphene); Wo, wollastonite; Zir; zircon. M inerals in parentheses
are in trace am ounts; italics are texturally secondary. Location abbreviations:
CC, Cucamonga Canyon; DaC, Day Canyon, DC, Deer Canyon, SSC, San
Sevaine Canyon area.
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35
Table 2. Relationship between F-value and calculated hypothetical caldte 51 3 C
using the diopside forming reaction (1), the Rayleigh devolatilization
equation 5 } = 5y- lOOOf^3 ' 1 ^ !) from Valley (1986), a fractionation factor
(a 1 3 CC02-rock) = 1.0022 and an initial 51 3C of 6.00 % o .
F - v a lu e C a lc u la te d
5 13C c a l%»
C h a n g e in
5 13C o i % °
1.00 6 .00 0 .0 0
0 .95 5 .89 0.11
0 .90 5 .7 7 0 3 3
0 .85 5 .6 4 0 3 6
0 .80 5 5 1 0 .4 9
0 .75 5 3 7 0 .6 3
0 .70 5 3 2 0 .78
0 .65 5 .05 0 .95
0 .60 4 3 8 1.12
0 5 5 4 .6 9 1 3 1
0 5 0 4 .4 8 1 5 2
0 .45 4 3 4 1.76
0 .40 3 .99 2.01
0 3 5 3 .69 2 3 1
0 3 0 3 3 5 2 .65
0 3 5 2.95 3.05
0 3 0 2 .4 7 3 5 3
0.15 1 3 4 4.1 6
0 .10 0.95 5.05
0 .05 - 0 5 7 6 5 7
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36
A fadre
r ~ ^ ~ 1 - San Gabriel Terrane
— H - Placenta Terrane
- San Antonio Terrane
1773 - Cucamonga Terrane
I B - Pelona Schist Terrane
Figure 1.
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37
m ylonite zones R?
S a n A n to n io
Cucamonga Terrane
Figure 2.
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5
4
u * 2
a * 3
a
2 2
1
0
Cucamonga Canyon
(average = -1.2 ± 2.5 % o )
■ ■ ■
7
Deer Canyon
6
(average = -1.1 ± 2.9 % o )
5
4
3
2
1
0
.£ >
s
3
2
8
7
6
w 5
E 4
3
2 3
2
1
0
(average = +1.1 ± 3.9 % o ;
' +0.4 ± 2.2% o without
- extreme values)
San Sevaine
-10
10
Figure 3.
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39
513C
8
4
0
-4
-8
-12
-16
Traverse #1
m u
i-t-ttl I l-lltfl M 11
0 40 80 120
l!
U I
Traverse #2
II
l
j-p ca ld te
g r a p h ite
. . V Jul
Traverse #3
G n n m i i i i n i i i m
11 H 111 n 1-11 IJ I Mt i 111 1 f l 111 It
120 200 '40 80 0 40 80 120
D istan ce in cm
I Alum inous gneiss
1 Quartzfeldspathic gneiss
Litholoev: I Mafic gneiss
H Calc-silicate
0 Marble
1 Quartzite
Figure 4.
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40
Cucamonga
Terrane
World-Wide
U n m etm o rp h o sed
C arb o n ates
(ave.°09±Z6*.)
S J 20
Range
(ave.» 3.6 * 4.0%.)
Adirondacks
< a v e . » 0 . 2* Z 2 X . >
Sri Lanka
(ave.3 0.0
8 10
8i3C
Figure 5.
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41
Cucamonga Canyon
I ‘ T1 I 1 I I 1 I 1
8
4
-4
-8
-14 -10 -6 - 2 2
5 1 3 Ccr
Deer & Day Canyon
i i i — i* * i i * i— i
- 2 2 -14 -10 -6
S^Ccr
San Sevaine Canyon
8
4
5«C cal 0
-4
-8
-14 -10 - 6 - 2 2
S ^C cr
Figure 6.
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42
Caldte-Graphite
Mass Balance Trajectory
4
ILL2L 2
0
-2
-4
-6
* 8
■ 5 0 -25 -10 -30 -20 -15
5l3Cgr
Figure 7.
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43
Carbon Isotope Variation
Marble carbonate 81 3 C
Marble graphite 81 3 C
Mafic gneiss graphite 81 3 C
Calc-silicate graphite 81 3 C
Felsic gneiss graphite 81 3 C
Aluminous gneiss graphite 81 3 C
1 1
1 1
1
1
1 1 1 1
1 |
I 1
1 1 1
1 1 . . 1—1
1 1 1 1
1 1
1 1
1 1
1 1
1 1
1
1
1
1 1 1 1
1 \'~ T i 1
I 1 1 1
1 1
1 1
1 I
1 1
1 I
1 1
1
I
III!
1 1 1 1
1 1
1 1
I |
1
1 1 1 1 1
1 1
1 I
1 1
1 1
1
-i—
1
1 1 1 1
1 1 I 1
1 1
1 1
1 1
-2 4 -2 0 -1 6 -1 2 -8 -4 0 4 8 1 2
8 1 3 C
Figure 8.
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44
CHAPTER 2
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45
Abstract
Carbon and oxygen stable isotopic compositions of the Cretaceous
Cucamonga granulite terrane carbonates are heterogeneous with isotopic
signatures that are only minimally depleted from reasonable average
sedimentary carbonate values. Small isotopic depletions from average
unmetamorphosed values are likely due to granulite metamorphism
devolatilization reactions and isotopic exchange with isotopically light silicate
minerals. Isotopic heterogeneity demonstrates the lack of significant
pervasive H2 0 or C 0 2 fluid-flow throughout the terrane that would have
homogenized the isotopic values. Dehydration of the terrane to granulite
conditions was therefore not accomplished in the presence of a CQrbearing
fluid.
However, at the outcrop scale there is evidence for localized H2 0-rich fluid
flow. Thirty-nine core drilled samples from three adjacent outcrops within a
span of c. 10 m have homogeneous caldte, dinopyroxene, garnet and biotite
from mafic gneiss, quartzo-feldspathic granulite, marble, and aluminous
gneiss lithologies that would be expected to have had distinctive protolith
1 f t
5 O values. We interpret this to be the result of locally pervasive H2 0-rich
fluid flow during post-peak metamorphic cooling at granulite temperatures
of 720-750°C. At a temperature of c. 750°C, the oxygen isotopic composition of
the hydrous fluid must be c. 14%o, which constrains the source to either a
purely metamorphic fluid, such as devolatilization fluids, or to igneous fluids
derived from anatectic melts of metamorphic rocks, possibly the deeper
portions of the Cucamonga terrane itself.
These condusions require that the up to 20 cm thick carbonate layers be
permeable to hydrous fluid flow despite being at middle to lower crustal
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46
pressures where marble is commonly thought to be impermeable. Increased
marble permeability was likely due to syn-metamorphic ductile deformation
that enhanced fluid flow through carbonate.
Calcite-graphite carbon isotopic temperatures from the core drilled
outcrops record peak metamorphic temperatures of > 850°C. At granulite
temperatures, a hydrous fluid in contact with ubiquitous graphite must have
a small carbon-bearing component such as C 0 2 or CH4. Locally pervasive fluid
flow at 720-750°C even with a minor carbon-bearing component (Xrp? > 0.1)
did not significantly reset the calcite-graphite thermometer, which implies
the calcite-graphite thermometer is resistant to resetting by retrograde high
temperature hydrous fluid flow even containing a significant carbon-bearing
component.
INTRODUCTION
Processes operative in the middle to lower crust during granulite facies
metamorphism are poorly understood yet critically important to the
development of a coherent model of high grade metamorphism and arc
genesis in a subduction environment. A detailed understanding of
temperature, pressure, fluid compositions (e.g. H2 0 , C 0 2 and CH4) and the
nature of fluid flow in the lower to middle crust is required in order to assess
these processes. Conditions for partial melting are dependent upon bulk
composition, temperature, pressure and fluid characteristics during
metamorphism. If metamorphic temperatures reach c. 650° C and a hydrous
fluid phase is present, then partial melting is expected during prograde
metamorphism (Huang & Wyllie, 1974; Thompson & Algor, 1977). However,
if the lower to middle crust is substantially dehydrated prior to peak
metamorphism, then peak metamorphic temperatures c. 800-850° C are
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47
necessary for fluid-absent (dehydration) partial melting of minerals such as
biotite (Thompson & Algor, 1977; Le Breton & Thompson, 1988; Vielzeuf &
Montel, 1994). Partial melts derived from such lower to middle crustal
regions could represent a potential source for high-level granitic magmas.
The Cretaceous Cucamonga granulite terrane is an ideal area to examine
lower to middle crustal processes related to arc construction. Metamorphism
in the terrane is Cretaceous in age, unlike most exposed granulite terranes
that are Precambrian. The terrane is composed of mafic and felsic, ortho- and
para-gneisses, tonalite to dioritic intrusive rocks, and minor pegmatite dikes.
Discontinuous layering, bedding(?), folding and the presence of multiple
subparallel mylonite zones suggest a terrane tectonically disrupted at high
temperature and pressure. Considerable evidence of tonalitic plutonism and
the close proximity of the underthrust Pelona Schist, generally interpreted as
representing the sediment cover over the upper portion of the Laramide
subducting plate (Ehlig, 1981; Burchfiel & Davis, 1981; Jacobson, 1983a)
suggests an intimate association with Cretaceous subduction where magma tic
arc construction occurred (Barth & May, 1992; Morrison & Barth, 1993).
Therefore, an accurate understanding of the Cucamonga terrane's geologic
history during Cretaceous time will greatly improve our understanding of the
involvement of the middle to lower crust during magma tic arc construction.
GEOLOGIC SETTING
The Cucamonga terrane is a parau tochthonous block exposed in the
southeastern portion of the San Gabriel Mountains of southern California
(Figure 1). To the north of the Cucamonga terrane, and separated by a broad
mylonite zone (Alf, 1948), is the San Antonio portion (May & Walker, 1989)
of the Placerita-San Antonio terrane which is likely an amphibolite grade
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48
equivalent of the Cucamonga terrane (Ehlig, 1981). To the west, separated by a
sharp vertical discontinuity, is the eastern portion of the San Gabriel terrane,
composed of Proterozoic gneiss, anorthosite, and the numerous Mesozoic
intrusions (Ehlig, 1981; Barth, 1990). The easternmost portion of the terrane is
covered by recent sediments of Lytle Creek, and is bounded by the Lytle Creek
fault, a part of the San Andreas-San Jacinto fault system (Math et al., 1985).
Recent sediments also overlap the southern portion of the terrane, although
the structural boundary is the reverse Cucamonga fault, the eastern portion of
the active, Sierra Madre frontal fault system (Ehlig, 1981). Combined, the San
Gabriel, Placerita-San Antonio and Cucamonga terranes are referred to as the
amalgamated Tujunga terrane (Powell, 1982).
Structurally underlying the Tujunga terrane and separated by the Vincent
thrust fault and an inverted metamorphic gradient is the Late Cretaceous
Pelona Schist terrane (Ehlig, 1975, 1981). The Pelona Schist terrane represents
the continentally-derived metasedimentary cover overlying probable oceanic
crust that was being subducted at low angle during Late Cretaceous to Early
Paleocene time beneath the Tujunga terrane (Ehlig, 1981; Jacobson, 1983a). At
least 3.5 km and possibly as much as 10 km of schist was welded to the
overlying Tujunga terrane during this event (Ehlig, 1981; Jacobson, 1983a;
Frost & Okaya, 1986). The inferred oceanic basement for the Pelona Schist
terrane is not exposed (Ehlig, 1981) but may be related to meta-basalt (Ehlig,
1981).
Several sinistral mylonite zones are present within the Cucamonga
terrane. These have a northeast-southwest trend and were undergoing
deformation during amalgamation of the Tujunga terrane (Hsu, 1955; Ehlig,
1981, May & Walker, 1989).
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49
Paleocene marine sedimentary rocks unconformably overlie Precambrian
gneisses of the San Gabriel terrane (Sage, 1975). The first sedimentary clasts of
Pelona Schist appear in the early to middle Miocene which implies very rapid
uplift of the San Gabriel Mountain terranes from mid-crustal levels following
cessation of Pelona Schist terrane subduction (Ehlert, 1982). This structural
evidence is consistent with the high uplift rate implied by K-Ar data in upper
plate rocks (Miller & Morton, 1980), Fe/M g zoning in garnet and pyroxene by
Barth & May (1992), and 40A r/39Ar cooling ages (Jacobson, 1990).
The terranes of the San Gabriel Mountains have been displaced c. 240-300
km along the San Andreas fault from their pre-Miocene position east of the
Salton Sea where rock units correlative to the San Gabriel and Pelona Schist
terranes lie (Dillon & Ehlig, 1993). Possibly due to differences in uplift and
denudation, lithologies correlative with the Placerita-San Antonio and
Cucamonga terranes are not found east of the San Andreas Fault (Ehilg, 1981).
Except for the lack of orthoquartzite in the Cucamonga terrane (May &
Walker, 1989), the meta-sedimentary rocks appear to be correlative with the
Placerita-San Antonio terrane meta-sedimentary rocks (Ehlig, 1981; Powell,
1982). Basement to the Cucamonga terrane sedimentary protoliths are not
exposed, however, it is probably the San Gabriel terrane as discussed later.
The age of the Cucamonga terrane protoliths are unknown due to intense
Cretaceous deformation and metamorphism and the consequent lack of
fossils. They are however, presumed to be Paleozoic due to lithologic
similarities with Paleozoic rocks of southeastern California (Ehlig, 1981; May
& Walker, 1990; Powell, 1982). The terrane may be in part Precambrian
(personal communication, Barth, 1993) based on a carbonate Sr, analysis that
does not fall on the Phanerozoic seawater curve and the presence of four
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50
enriched 51 3 C (> 8% o ) values suggestive of Neoproterozoic carbonates
(Hovanitz & Morrison, in submission; Wickham & Peters, 1992).
Discordant zircon U-Pb age dates from May & Walker (1989) suggest that
granulite facies metamorphism was ongoing by c. 108 Ma and that plutonism
locally persisted in the Cucamonga terrane as late as c. 88 Ma. Late Cretaceous
plutonism in the previously juxtaposed San Antonio terrane persisted as late
as c. 75 Ma (May & Walker, 1989). Concordia upper intercepts of c. 1.7 Ga from
synmetamorphic intrusions suggest that either the basement for the
Cucamonga terrane was a San Gabriel-type terrane or the Cucamonga
protolith sediments contained San Gabriel terrane age zircons or both (May &
Walker, 1989).
ANALYTICAL METHODS
Two sampling techniques were used in the study of the Cucamonga
granulites. To assess the terrane-scale mechanism of granulite dehydration, 26
hand samples were collected from 19 carbonate outcrops at 14 locations across
the terrane (Figure 2). In order to determine the local potential of fluid-flow
in the Cucamonga terrane, 44 samples were collected by drilling 43 one inch
diameter cores at c. 10 cm intervals in Deer Canyon (Figure 2) along three
outcrop traverses all within c. 10 m of each other. Each of the cores was drilled
down-dip and parallel to the lithologic unit being sampled so that cores
sampled individual lithologic layers. Cores varied in length from 1 to 12 cm.
One of the cores intersected multiple lithologies due to a minor fold.
Hand sample carbonate analyses were made on splits of an average c. 15
cm3 of whole rock powder carefully selected to preclude any weathered
material. Core drilled samples were prepared by removing approximately 3
cm3 sections from the cores using a core splitter and further disaggregated by
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51
means of a hardened steel "atom smasher". From each crushed sample,
caldte, diopside, garnet and biotite were removed by hand-picking to
minimize compound grains. Silicate mineral separates were washed in 3.1 N
HC1 to remove any adhering caldte. The average volume between caldte and
silicate pairs is less than 3 cm . Due to fine grain size and compound grain
problems with many gneissic samples, c. 15 cm3 of sample was crushed to
hand-pick c. 5 mg of pure silicate mineral separate.
Optically transparent caldte was selected for analysis, except when only
altered caldte was present (discussed further below). Carbonate hosted
clinopyroxene (diopside) was round, transparent, light green grains similar to
that described by Sharp and Jenkin (1994). Depending on grain size, 1 to 5
grains were used per analysis. Gneiss and granulite clinopyroxene were
carefully selected to exdude compound minerals grains. Samples were
generally deavage fragments. Because of grain size variations between
different samples, 1 to 12 grains (0.8 to 1.8 mg) were used per analysis. It was
impossible to exdude all foreign material from the garnet samples, so garnet
analyses induded variable amounts of clinopyroxene, plagiodase, graphite,
and unidentified trace minerals. Foreign material did not exceed 10 - 15 % by
volume in the garnet. Single crystals of phlogopite were extracted from
carbonate by hand-picking before analysis with 1.2 to 2.2 mg (single grains)
being used. Clean biotite books were removed from gneisses and schists by
handpicking to exdude other silicates.
Caldte 81 3 C and 5lsO values were determined using 10-15 mg of powder
and the conventional phosphoric ad d extraction method of McCrea (1950) at
25° C. Phosphoric ad d density of 1.925 (at 60 °F) was used and the results were
recalculated using a phosphoric ad d fractionation factor of 1.01025 (Friedman
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52
and O'Neil, 1977). Graphite 51 3 C values were measured using c. 1 mg aliquots
combusted with c. 30 mg CuO at 900° C for at least 10 hours in sealed 8 mm
diameter quartz tubing.
Clinopyroxene, biotite and garnet oxygen isotopic compositions were
determined using the USC on-line 20 watt Melles Griot CO2 laser extraction
line following the method of Sharp (1990). Sample mass of c. 0.8-2.2 mg were
used in 2 mm diameter wells drilled in solid nickel plugs. Samples were
pretreated overnight with BrFs followed by laser-heating and BrFs
fluorination. Evolved oxygen was cryogenically purified, passed through a
small Hg-diffusion pum p to extract excess fluorine and converted to CO2
using a heated platinum-catalyzed graphite rod. Purified C C > 2-gas was
immediately run on a VG Isogas multi-collector mass spectrometer controlled
by SIRA software. Isotope values are reported in standard per mil notation.
51 3 C values are reported relative to V-PDB and 5lsO values are reported
relative to V-SMOW.
During the course of sample analysis, 26 analyses were made of the USC-
CC1 internal standard which had an average 81 3 C value of 3.05 ± 0.03%o and
an average 51 8 0 value of 30.16 ± 0.13%o. Our laboratory USC-CC1 standard has
been calibrated against NBS-19 (n=7) and has an average 51 3 C value of 3.02 ±
0.09%o and an average 5lsO value of 30.24 ± 0.18%o. Duplicate caldte 8lsO
analysis varied by less than 0.3 % o . Our laboratory standard for laser silicate
analysis is Gore Mountain GMG-2 (Valley et al., 1995) which was analyzed 18
times with an average 8lsO value of 5.69 % o and a daily predsion of better
than 0.21 % o This compares to our laboratory average isotopic value of 5.64 ±
0.24 % o . Valley et al, (1995) have determined the 81 8 0 value of GMG-2 to be
5.8%o. Daily duplicate diopside 8lsO analyses varied by less than 0.16% o .
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53
Whole rock X-ray diffraction mineral analyses were performed using a
Philips PW1840 powder diffraction spectrometer at California State
University, Los Angeles. Scans were made using copper Ka radiation, a scan
range from 4 to 45° 20, and a scan rate of 1° 2© per minute. Two-theta
alignment accuracy was determined using quartz dioi = 26.67° and the quartz
quintuplet.
RESULTS
Petrology
The Cucamonga terrane is lithologically, chemically and isotopically
heterogeneous at the terrane, outcrop and thin section scale (Barth et al. 1992;
Hovanitz & Morrison, in submission). Our mapping has shown that the
Cucamonga Canyon to Day Canyon (western to central) portion of the terrane
consists predominantly of mafic gneisses with minor aluminous and felsic
gneisses containing intercalated < 1% marble and calc-silicate lithologies
(Figure 2). The eastern San Sevaine Canyon region consists predominantly of
felsic gneiss with volumetrically less mafic and aluminous gneisses and
contains 3-5% marble and calc-silicate lithologies.
For the purposes of this study, carbonate-bearing rocks with caldte
contents greater than 95% are dassified as marble, while rocks with less than
95% caldte are distinguished as calc-silicate rocks. The nearly ubiquitous
mineralogy of the carbonate zones is caldte, dolomite and graphite. Whole
rock powder X-ray diffraction analyses of 9 marble samples demonstrates that
the dolomite content is highly variable and ranges from undetectable to as
much as 60%, although most samples contain < 5% dolomite. Primary silicate
minerals present in the marbles indude, in decreasing abundance, diopside,
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54
plagiodase, quartz, forsterite, scapolite, phlogopite, sphene, apatite, potassium
feldspar, orthopyroxene, spinel and zircon. Specific sample mineral
assemblages are given in Table 1. Silicate minerals enclosed within carbonate
are generally discrete or contained in 0.5 to 200 mm thick layers that are
usually concordant with foliation although compositional layers are rarely
involved in intra- and inter-layer folding. Minor secondary or retrograde
silicate minerals include tremolite, phlogopite, serpentine and chlorite.
Carbonate mineralogy is consistent throughout the terrane with the
exception that forsteritic olivine is found only in the San Sevaine Canyon
area and pink spinel is found only in the Deer Canyon carbonates.
In the Cucamonga terrane, foliation is well developed and generally sub
parallel to east-west trending, steeply north dipping lithologic layering (May
& Walker, 1989; Barth & May, 1992). Compositional layering varies in
thickness from = 1 mm calc-silicate layers within marble, up to 3-5 m thick
marble layers. Mapping by May & Walker (1989), Barth & May (1992), and our
own work has shown that layering of meta-sedimentary lithologies in the
Cucamonga terrane has been transposed and disrupted by both mylonitic and
brittle shear deformation so that lithologic layering typically can only be
traced along strike from a few cm for thin layers and up to 100 m for 2-5 m
thick layers. The discontinuous layers appear to be "pinched off" and
resemble large-scale boudin trains. Trains of distinctive lithologies such as
marble can be traced for up to several hundred meters along strike.
Terrane-scale carbonates
Cucamonga terrane carbonate 81 3 C and S1 8 0 results are listed in Table 1
and are shown in Figure 3. The overall pattern observed in solid black and
gray dots in Figure 3 has a weak positive slope suggesting coupled isotopic
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55
depletion. In contrast, the open circles from the Deer Canyon core drilled
location, form a nearly vertical trend with a small variation in oxygen
isotopic composition combined with a large spread in carbon isotopic values
I S 1
that suggest depletion in 5 O and little or no change in 5 C values.
Coupled carbon and oxygen isotopic depletion in metamorphic rocks may
be caused by magma tic and/or meteoric fluid infiltration (Valley, 1986;
Nabelek, 1991) and less so by decarbonation and dehydration reactions
(Valley, 1986; Broekmans et al., 1994). Terrane-scale temperature gradients
during metamorphism can produce coupled isotopic trajectories that
resemble coupled isotopic depletion due to temperature dependent
fractionation effects between co-existing minerals (Valley, 1986).
Most of the calc-silicate (> 5% silicate mineralogy) samples lie in the range
of most depleted 5I3C and 5lsO values (Figure 4). There are several reasons
that the calc-silicate carbonates could be isotopically lighter compared to the
marbles: (1) mass balance isotopic exchange of lsO enriched caldte with
isotopically lighter detrital silicate minerals or igneous minerals (Hoefs, 1997;
Barth et al., 1992) will reduce 5lsO of the resultant caldte, (2) during
decarbonation and devolatilization reactions, (3) isotopic exchange of caldte
with mantle-derived magma tic fluids will lower both 51 3 C and 5lsO (Valley,
1986; Hoefs, 1997). To assess the contribution that each of these three processes
of coupled isotopic depletion may have made, we have evaluated the
magnitude of potential isotopic mass balance equilibration, the extent isotopic
depletion caused by decarbonation and devolatilization reactions and the
potential for mantle-derived magmatic fluid infiltration.
Figures 3 and 4 demonstrate a wide range in carbon and oxygen isotopic
compositions. Cucamonga carbonate 51 3 C values are heterogeneous and likely
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56
reflect original sedimentary values minimally affected by decarbonation
reactions and equilibration isotopically light organic carbon (Hovanitz &
Morrison, in submission). Terrane-wide carbonate 5lsO values range from a
low of 14.5%o to a high of 24.7%o, with an average of 19.3 ± 2.8% o (n=42). By
comparison, the unmetamorphosed carbonates of Veizer & Hoefs (1976) vary
from 11.7 to 40.6%o (average = 23.1%o), the metamorphosed carbonates from
the Esplanade Range vary from 7.0 to 21.8%o (average = 16.1 %o; Ghent &
O'Neil, 1985), the Adirondack carbonates range from 12.3 to 27.2%o (average =
20.0 % o ; Valley & O'Neil, 1984) and the granulite facies Sri Lanka marbles
(Hoffbauer & Spiering, 1994). A comparison of the SlsO characteristics of these
data is shown as histograms in Figure 5.
The results of Veizer & Hoefs (1976) demonstrate that there is considerable
heterogeneity in 5lsO carbonate isotope values in relatively unaltered
carbonate rocks of Mesozoic to Precambrian age from around the world.
Ghent & O'Neil (1985) conclude that heterogeneous 51 3 C and 5lsO values
from the variably metamorphosed carbonates from the Esplanade Range were
produced during largely fluid-absent metamorphism. However, the
Esplanade Range carbonates are depleted by c. 7% o with respect to
unmetamorphosed carbonates and depleted by 3-4 % o compared to the other
metamorphosed terranes. Thus, their oxygen isotopic data suggests the
influence of some oxygen isotopic depleting mechanism. Likewise, Valley &
O'Neil (1984) conclude on the basis of heterogeneous 81 3 C and 5lsO values
that the high grade carbonates from the Adirondacks probably represent
metamorphism in nearly fluid-absent conditions and likely represent original
sedimentary values.
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57
Metamorphic processes such as decarbonation, devolatilization,
infiltration of fluids, and isotopic exchange with other mineral species (e.g.
silicate minerals) can alter original depositional isotopic values.
Outcrop-scale carbonates and silicates
Three outcrops in Deer Canyon (open circle in Figure 2), separated by a
total distance of c. 10 m were selected for core drill sampling because of their
lithologic diversity. The purpose of the core sampling was to examine the
carbon and oxygen isotopic compositions at constant intervals in order to
examine isotopic gradients across different lithologies that would be expected
to have isotopically distinct protolith compositions (Rumble & Spear, 1983;
Valley, 1986; Hoefs, 1997). The lithologies sampled include marble, calc-
silicate, mafic gneiss, quartzo-feldspathic granulite, aluminous gneiss, and a
quartz granulite. Figure 5 shows the carbon and oxygen isotopic distribution
of the three traverses at the same isotopic scale. As discussed by Hovanitz &
Morrison (in submission) the carbon isotopic compositions are
heterogeneous at the terrane, outcrop and thin-section scales, with no
evidence of a pervasive carbon-bearing fluid being present during granulite
metamorphism. Such a fluid, if present, would be expected to have
homogenized the carbon isotopes.
The core drilled oxygen isotopic results, in contrast to the carbon isotopic
data, demonstrate a high degree of isotopic homogenization across all
lithologies. Coupled caldte and clinopyroxene (diopside in marble and calc-
silicates; augite in gneisses and granulites) oxygen isotopic data for the three
core-sample traverses are listed in Table lb and combined with lithologies in
Figure 6. Twenty-two samples that contain caldte have homogeneous SlsO
values that range from 15.4%o to 16.9%o with an average 81 8 0 of 16.1 ± 0.4%o.
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58
Cucamonga terrane carbonates that exclude the three Deer Canyon traverses
have 8lsO values that range from 14.5 to 24.7%o with an average value of
18.1 % o (n = 74). The relationship of the average Deer Canyon carbonate 51 8 0
values with other parts of the Cucamonga terrane is shown as histograms in
Figure 7. The average Deer Canyon core sample carbonate 81 8 0 values are
depleted by c. 2.0 % o from the average Cucamonga carbonate values of 18.1 %»,
but are similar to the isotopic values of Cucamonga Canyon.
The Deer Canyon core sample carbonate 81 8 0 values are from rocks that
range in composition from marbles with > 99.9 % carbonate mineralogy
which would be expected to have relatively enriched 51 8 0 whole rock values
compared to lithologies with < 5% carbonate which would be expected to
have relatively low 8 1 8 0 whole rock values due to exchange with lower 8 1 8 0
silicate minerals.
Thirty-five of the Deer Canyon core samples contained sufficient Cpx
(diopside in marble and calc-silicate lithologies or augite in gneisses and
granulites) for oxygen isotopic composition analysis. 81 8 0 values range from
12.9 to 14.3 % o and average 13.6 ± 0.4% o . Nineteen of the core samples contain
coexisting caldte and diopside with A1 8Occ-cpx values that average 2.50 ±
0.64 % o . There is no statistical difference between diopside oxygen isotopic
values co-existing with carbonate that have 81 8 0 = 13.6 % o and Cpx values
from the gneissic rocks that have oxygen isotopic values that average 13.5 % o .
Coexisting caldte and diopside allows the application of the caldte-diopside
1A
oxygen isotopic thermometer. Using 1000 In a = A OCai-Cpx for A values below
10% o , a temperature of 720 ± 112° C is determined using the temperature
calibration of Chiba et al. (1989). An average temperature estimate of 705° C is
estimated using the refined increment calibration method of Hoffbauer et al
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59
(1994). The relationship between caldte and diopside 5lsO values is shown in
Figure 8.
The coupled caldte-diopside data demonstrate a considerable range in
temperatures from 468 to 867°C. Hand specimen and microscopic
examination of the samples revels that some of the very low temperature and
very high temperature results are from samples that contain veins in the
carbonates or contain unusual doudy caldte (468°C, 536°C & 867°C). Thus, it is
possible that some of the scatter in the data is due to the presence of
undetected veins. Also, some of the apparent scatter in the temperatures
results may be due to the sensitive nature of the calculated temperature to
minor 51 8 Ocai and 81 8 Ocpx analytic uncertainty.
Eight garnet oxygen isotopic analyses were made from traverses #1 and #3.
The range of 8lsO values is from 13.5 to 14.7%o with an average 5lsO is 14.0 ±
0.4% o . Two garnet oxygen isotopic values that co-exist with carbonate have
5lsO = 14.5 % o , while six garnet samples from non-carbonate lithologies have
an oxygen isotopic composition of 13.9 % o . The small sample size of co
existing caldte and garnet limits the statistical significance of the data,
however, the most enriched garnet 5lsO values are assodated with carbonate
which would be expected to have the most pre-metamorphic enriched 5lsO.
Since the bulk rock 81 8 0 carbonate would be expected to be more enriched in
lsO than silicate lithologies this suggests that the garnet isotopic values may
retain some isotopic signature that predates fluid flow.
Seven oxygen isotopic analyses were made of biotite and phlogopite.
Biotite is common in a few schistose rocks commonly in assodation with
clinopyroxene, plagiodase and quartz. Three biotite samples from traverse #1
have an average 8lsO of 13.7 ± 0.3%o and range from 13.4 to 13.9% o .
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60
Phlogopite is present as large isolated grains in many marble and calc-silicate
layers. Four phlogopite samples co-existing with caldte have an oxygen
isotopic composition that range from 12.8 to 13.7%o and averages 13.2 ± 0.4%o.
Using the caltite-phlogopite fractionation of Hoffbauer et al. (1994) a
temperature estimate of 736 ± 54° C is calculated.
Hoffbauer et al. (1994) determined that the oxygen isotopic fractionation
factors for biotite and phlogopite differ by only 0.01-0.05 % o . Therefore, oxygen
isotopic compositions of biotite and phlogopite may be directly compared, so
that within the geographic range of the three traverses, the isotopic
composition of the phyllosilicates is homogeneous irrespective of lithology.
The difference between the average isotopic composition of phlogopite and
biotite is 0.5 % o with biotite being slightly heavier in comparison to
phlogopite. This is contrary to the expectation that phlogopite should be
isotopically heavy due to equilibration with carbonate.
DISCUSSION
Stable isotope systematics
Evaluation of the potential amount of isotopic heterogeneity that could
have been produced by metamorphic processes is necessary because the
Cucamonga carbonates could have had homogenous isotope protolith
compositions that were depleted by varying amounts during metamorphism.
Processes such as devolatilization, fluid infiltration and isotopic equilibration
between caldte and isotopically lighter silicate minerals could have
contributed to isotopic heterogeneity from a potential isotopically
homogeneous protolith.
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61
To evaluate devolatilization, the Rayleigh devolatilization model is
chosen because it provides the greatest isotopic fractionation. A fractionation
factor of 1.0120 and variable F-values dependent upon the mode % calc-
silicate minerals in the marble or calc-silicate rock are used. Caldte containing
diopside would provide the largest potential isotopic fractionation according
to the equation:
dolomite + quartz = diopside + CO2
CaMg(CQ3)2 + 2Si02 = CaMgSi20 6 + 2CO2 (1)
Using the above values, the maximum oxygen isotopic change for a rock
that is pure diopside is c. 6.1 % o at the "calc-silicate limit" (F=0.60; Valley,
1986). In general the calc-silicate mineral content varies from 1-20% which
means the average oxygen isotopic depletion is less than c. 1.3 % o . Therefore
the terrane-scale heterogeneity in oxygen isotopic composition cannot have
been produced solely by devolatilization reactions. This result is similar to the
conclusions of Valley & O'Neil (1984) and Hoemes et al. (1994).
Oxygen isotopic equilibration between adjacent lithologies would be
1 f t 1 f t
expected to lower 8 O values of high 8 O lithologies such as marble and raise
the whole rock 8lsO of a lithology like mafic gneiss, assuming that the mafic
gneiss protolith was a mantle-derived basalt (Barth, et al., 1992) with a
1 f t
premetamorphic 5 O value c .6 % 0 . It is not possible to model the data too
closely since there is no protolith for comparison. However, using reasonable
isotopic values and mass balance oxygen isotope equilibration between equal
amounts of marble ( 0 = 4 7 . 9 6 % ) and augite ( 0 = 4 4 . 3 3 % ) and starting isotopic
values of 8 180 C ai = 2 5 % o and 81 8 Ocpx= 6% o would leave a closed system
8 180 Wr = 1 3 .5 % o . At the terrane scale, then, carbonate S 180 values would have
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62
to be lower than any measured within the terrane. This assumes a carbonate-
mafic gneiss ratio of 1:1 when volumetrically the carbonate is less than c, 5%
of the Cucamonga terrane. Even in the Deer Canyon area, where the average
S1 8 Ocai = 16.5%o and the average S1 8 Ocp x = 13.6 % o , the calculated outcrop-scale
51 8 Owr is 3% o from a reasonable calculated value.
The last metamorphic process that could have produced heterogeneous
S1 8 0 values from an initially homogeneous protolith is channelized
infiltration of an external fluid. The Cucamonga terrane contains veins that
record the passage of channelized fluid, however, they are not common, and
appear to have altered the SlsO values of carbonate by at most c. 2%o, not
enough to account for the >10 % o range in oxygen isotopic values throughout
the terrane.
Caldte oxygen isotopic compositions from the Cucamonga terrane in its
entirety demonstrate a heterogeneity in marked contrast to the three core
drilled traverses that indicate isotopic homogeneity throughout lithologies
that would be expected to have distinct bulk rock 81 8 0 values (Hoefs, 1997).
Terrane-scale heterogeneity in caldte carbon (Hovanitz & Morrison, in
submission) and oxygen isotopic compositions coupled with a similar isotopic
average value and range as seen in unmetamorphosed carbonates suggest
that the Cucamonga terrane was not pervasively affected by either a KfeO or
CO* fluid, despite local vein evidence for channelized fluid flow. Isotopic
homogeneity over a 10 m interval across numerous profound lithologic
boundaries such as marble-mafic gneiss could have been accomplished by (1)
fluid-absent oxygen diffusion, (2) isotopic exchange promoted by
devolatilization reactions (Rumble et al., 1982), or (3) achieved in the presence
of either a static or dynamic fluid phase (H2 O, CO2, or a combined fluid) that
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63
facilitated oxygen isotopic exchange (Rumble & Spear, 1983; Rumble, 1986). It
is difficult to estimate dry oxygen diffusion through 10 m of rock due to the
large number of different intervening lithologies present and the lack of an
accurate estimate of metamorphic temporal duration. Although high
temperature promotes diffusion, fluid-absent (solid-state) oxygen diffusion
over a distance of 10 m is unlikely on several lines of reasoning.
The first line of evidence that suggests dry diffusion did not occur is that at
temperatures of c. 800°C, diopside (and probably augite) has a very low oxygen
diffusion rate and a high closure temperature (Sharp & Jenkin, 1994). A
second line of evidence that suggests a lack of dry diffusion is by comparison
« Q
with the presence of c. 8% o 5 O heterogeneities at the cm to m scale from the
c. 1000° C granulite Rauer Group, Antarctica (Buick, et al., 1994) and steep
isotopic gradients heterogeneities from Sri Lanka (Hoemes et al., 1994) that
were retained during largely fluid-absent conditions. Lastly, Buick et al. (1994)
conclude that slow oxygen diffusion rates will cause isotopic resetting over a
scale of only a few cm. Devolatilization reactions also increase oxygen
diffusion in carbonates (Rumble, et al., 1982), however, most of the
intervening lithology volume between one end of the core drilled traverse
and the other end are silicate lithologies that would not have participated in
devolatilization reactions. It should be noted that even small amounts of
hydrous fluid can dramatically increase diopside, silicate or whole-rock
oxygen diffusion by several orders of magnitude (Cole & Ohmoto, 1986;
Baker, 1990; Sharp & Jenkin, 1994; Edwards & Valley, 1995),
H2O, CO2 or combined H2OCO 2 fluids would have sufficient oxygen
content to have facilitated the homogenization of the caldte and silicates in
the Deer Canyon core drilled region. The fluid could have been present either
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64
as a static, in ter granular fluid with self-diffusion or as a dynamic system with
fluid-flow (Rumble & Spear, 1983). It is difficult to distinguish between either
a static or dynamic fluid with the available isotopic and petrologic data, except
to note that fluid was not present during peak metamorphism, and must
therefore have been post-peak metamorphism, which requires infiltration
and consequent flow of fluid into the Deer Canyon core drilled region.
Hovanitz & Morrison (in submission) have demonstrated that the
Cucamonga terrane has a highly heterogeneous carbon isotopic composition
at the terrane, outcrop and thin-section scale. On this basis, Hovanitz &
Morrison (in submission) concluded that the Cucamonga terrane was not
subjected to pervasive infiltration of a C0 2 ~rich fluid. If the fluid involved in
homogenization was not C0 2 -rich, then it must be a H2 0 -rich fluid, but must
have a carbon-bearing component (Ohmoto & Kerrick, 1977; Ferry &
Baumgartner, 1987; and Lamb & Valley, 1984). It is important to note that
while the pervasive fluid infiltration that affected the Deer Canyon core
drilled area must have been H2 0-rich with a minor carbon-bearing
component, there is evidence throughout the terrane of channelized flow of a
carbon-rich fluid from the presence of graphite veins.
Fluid characteristics
Fluid composition
Isotopic mass balance calculations and thermodynamic principles provide
limits on the fluid composition. Ohmoto & Kerrick (1977), Ferry &
Baumgartner (1987) and Lamb & Valley (1984) have discussed oxygen fugadty
and fluid composition of a water-bearing fluid in equilibrium with graphite
and carbonate. At granulite temperatures of c. 850° C, Ohmoto & Kerrick
(1977) suggest a fluid in contact with graphite would have a maximum PH2O
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65
of about 0.8. The remaining fluid would contain variable quantities of CO2,
CO, and CH4 depending on the oxygen fugadty (Ohmoto & Kerrick, 1977).
Lamb & Valley (1984) calculated the effect graphite (aC = 1) would have at
granulite conditions of 800° C and 8 kb on the fluid composition. These
results are shown in Figure 10 (Lamb & Valley, 1984). The results of their
calculations limit maximum XH 2O to about 0.9 at LOG/0 2 °f c. -15.5. In the
absence of a phase outside of the system C-O-H, X C O 2 and XCH4 would be in
nearly in equal concentration and make up the remainder of the fluid. This
means that in the presence of graphite, at least 10% of the fluid phase must be
carbon-bearing at LOG J0 2 of c. -15.5. If LOG J0 2 varies from -15.5 in the
presence of graphite then XH 2O would be less. For instance at LOG/O2 = -18,
XH 2O is approximately 0.15 and at LOG/0 2 = -14.8, XH 2O and X C O 2 are about
equally abundant (Lamb & Valley, 1984).
If graphite is not present, like most of the Cucamonga terrane, and aC =
0.1, then XH 2O may approach 1.0 at LOGfOi = -16.0 to -14.0. However, almost
all the Deer Canyon core traverse samples contain graphite which implies
that the fluid that infiltrated this area of the Cucamonga terrane might have
dissolved carbon if it weren't already saturated with a carbon phase.
Fluid Source
We have determined that a pervasive hydrous fluid was present near the
peak of granulite grade metamorphism in a limited area of the Cucamonga
terrane. An estimate of the oxygen isotopic composition of the fluid may be
made using one of the temperature dependent caldte-water fractionation
calibrations (Sharp & Kirschner, 1994; O'Neil et al., 1969; Hoffbauer et al., 1994;
Chacko et al., 1991). Assuming, for simplicity that the fluid is pure H2O and
does not contain dissolved salts or other chemical species that may potentially
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66
affect the isotopic fractionation of an t^O -bearing fluid and caldte (Driesner &
Seward, 1994), then an estimate of the oxygen isotopic composition of the
hydrous fluid may be made. This estimate is dependent on the temperature
attained during isotopic equilibration. Sharp & Kirschner (1994) noted that
there is good agreement among researchers on the A1 8 Ocai-H 2 o fractionation
calibration, so we use the calibration of O'Neil et al., (1969); A18Ocai-H2 o = 2.78 *
(106 * T"2) - 3.39) which is near the middle of the range of calibrations. Using
1 A
an average measured 6 0 = 16.1 % o at the peak metamorphic temperature of
c. 850°C (Hovanitz & Morrison, in submission) yields a hydrous fluid with an
isotopic composition of c. 15.7%o. If the average A18Ocai-Di =720° C then 8
18O water would have been about 14.1 % o . Fluid infiltration at a temperature less
than c. 720°C is unlikely because of the quantity of data that suggests higher
temperatures. Thus, a fluid composition of c. 14% o is probably the isotopic
minimum composition.
A fluid with an isotopic composition of 81 8 Owater c. 14 % o may be generated
by several processes. Fluid 5lsO of this value implies extensive interaction
with 1 8 0 enriched metamorphic rocks, or fluids derived from devolatilization
reactions occurring during prograde metamorphism. The isotopic
composition of a mantle derived igneous fluid of 51 8 Owater c. 6% o is only
possible at a temperature of c. 450° C, or a fluid of 51 8 Owater = 11% > at a
temperature of 572° C. This range of temperatures is unreasonable in that at
temperatures between 450 and 575° C, one would expect complete retrograde
hydration of the granulite mineralogy to hydrous greenschist or lower
amphibolite grade mineralogy. However the Cucamonga rocks preserve good
granulite mineralogy and precludes the fluids having been a mantle derived
magmatic fluid.
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67
If the pervasive fluid is not of igneous origin, it must therefore be of
metamorphic origin. Two possible sources for metamorphic fluids include
fluids generated internally within the Cucamonga terrane, possibly derived
from devolatilization reactions or metamorphic fluids derived from an
external source. Except in the case where a high metamorphic temperature
gradient exists, it is difficult to generate metamorphic fluids from the
Cucamonga terrane that had reached temperatures of c. 850° C, because any
hydrous minerals such as biotite would have succumbed to biotite
dehydration melting reactions.
A likely source for significant quantities of hydrous fluids in a subduction
zone/volcanic arc setting, such as the Cucamonga terrane, could be the
underlying, dewatering subducting slab particularly when the subduction
angle is unusually low and the subducting plate is vertically close (Cross &
Pilger, 1978; Dickinson & Snyder, 1978). The remnant of such a slab is the
Pelona Schist terrane portion of the Pelona-Orocopia-Rand schist (Ehlig, 1981;
Jacobson, 1983b). The Pelona Schist terrane is a thick sequence of
predominately continentally derived sediments with minor intercalated
greenschist of oceanic basalt affinity, minor limestone and quartzite with a
chert protolith (Ehlig, 1981). Metamorphic conditions are generally thought to
be of high pressure greenschist facies, with some rocks of the equivalent Rand
Schist being retrograded from blueschist facies (Dillon, 1976; Graham, 1975;
Jacobson et al., 1988; Ernst, 1984). Thus during underthrusting of the Pelona
Schist terrane, ample opportunity existed for extensive dewatering of the
Pelona Schist metasediments and consequent interaction w ith overlying
upper plate rocks like the Cucamonga terrane. However, the P-T-t
relationship between the Cucamonga and Pelona Schist terranes puts the
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68
Cucamonga terrane at a temperature of < 300° C (Barth & May, 1992; Miller &
Morton, 1980) at the time of inferred juxtaposition of the two terranes.
Therefore the metamorphic fluids could not have originated from
dewatering of the underthrusting Pelona Schist.
Another possible source for the fluids is the minor mafic and felsic
intrusions within the Cucamonga terrane (May & Walker, 1989; Barth & May,
1992). If these intrusions are anatectic products from melting of nearby
portions of the Cucamonga terrane (Barth et al. 1992) then a sufficiently
enriched oxygen isotopic fluid composition could have been expelled during
18
crystallization of the anatectic magma to have caused high 5 O isotopic
homogenization in the Deer Canyon core drilled outcrop.
Fluid Flow Timing
The localized episode of pervasive fluid flow in the Deer Canyon
portion of the Cucamonga terrane can be bracketed in time. The presence of a
hydrous fluid during peak metamorphism (c. 850° C) would have produced
extensive melting of most lithologies (e.g., Huang & Wyllie, 1974; Thompson
& Algor, 1977; Thompson, 1996). If fluid flow occurred during prograde
metamorphism, then isotopic re-equilibration at peak metamorphism would
be expected. Therefore we conclude that hydrous fluid flow occurred during
retrograde cooling while the Cucamonga terrane was still at a temperature of
c. 720-800°C based on the range of caldte-dinopyroxene isotopic temperatures.
High Temperature Carbonate Permeability
Marble is generally thought to be impermeable at high metamorphic
conditions due to its ductile behavior (Bickle et al., 1995; Peters & Wickham,
1995; Wickham & Peters, 1992) though evidence of a layer parallel
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69
permeability has been noted (Nabelek et al. 1984). A wide range in lithologies
in the Deer Canyon region of the Cucamonga terrane have been isotopically
homogenized and there is no recognized oxygen isotopic gradient across the
10 m and 3 outcrops that were core sampled. One marble/ calc-silicate layer in
traverse 3 (Figure 5) is c. 60 cm in thickness and is both isotopically
homogenous with respect to oxygen isotopes and in isotopic equilibrium with
the surrounding silicate gneisses. This implies that the hydrous fluids
involved in isotopic homogenization pervasively permeated this carbonate
layer.
Some form of permeability enhancement must have occurred to have
accomplished oxygen isotopic homogeneity. Possible granulite metamorphic
grade mechanisms that could increase marble permeability and porosity
include reaction enhanced permeability due to the volume of metamorphic
products being greater than the reactants (Rumble, 1994) and reaction-
enhanced microcracking during melt production (Connolly et al., 1997). Since
the enhanced permeability presumably took place following peak
metamorphic temperatures, it is unlikely that either of these contributed to
the increased permeability.
Calcite-Graphite thermometer stability
Metamorphic temperatures > 850° C are recorded by caldte-graphite carbon
isotopic thermometry in the Deer Canyon core drilled outcrops (Hovanitz &
Morrison, in submission). This temperature estimate is significantly greater
than temperatures recorded by Cal-Cpx isotopic thermometry and cation
exchange thermometry (Barth & May, 1992) which likely record temperatures
prevailing at the time of high temperature hydrous fluid infiltration that
followed peak metamorphism.
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70
H ydrous fluid flow of sufficient quantity to homogenize 5 O over c. 10 m
occurred follow ing peak m etam orphism and on therm odynam ic grounds
m ust have contained a small portion of a carbon-bearing specie like CH4
(Lamb & Valley, 1984) because of the near ubiquitous presence of graphite in
nearly all of the Deer Canyon core drilled outcrops. The A1 3 Coa_cr systematics
wore not significantly affected by the presence of the carbon-bearing fluid
because very low A^CcaKGr values are retained. It is possible that the range of
in A^Ccatcr values (13-32.%o; excluding samples w ith cloudy caldte or
graphite vein) m ay be due to the interaction of the carbon-bearing portion of
the fluid w ith carbonate a n d /o r graphite, the A^Ccai-Gr system largely retained
the higher tem peratures.
CONCLUSIONS
Carbon and oxygen compositions from throughout the m arble and calc-
silicate portions of the Cucamonga terrane are heterogeneous. The degree of
heterogeneity of both 81 3 Ccd and 51 3 Cgt data from throughout the Cucamonga
terrane strongly im plies that there was not a pervasive (fluid/rock = 0.1-03)
carbon bearing fluid present during granulite fades m etam orphism
(Hovanitz & M orrison, in submission). The lack of a pervasive CO2 bearing
fluid suggests that granulite fades metamorphism was not accomplished by a
reduction of w ater activity by dilution. Overall, the 51 3 C of the Cucamonga
m arbles and calc-silicate rocks is little different from that of
unm etam orphosed carbonates (Hovanitz & M orrison, in subm ission).
Terrane-scale SlsO compositions also have a range sim ilar to that of
unm etam orphosed carbonates. However, the m ean value of the Cucamonga
rocks is depleted by c. 3.8%o relative to the S1 8 0 mean value of
unm etam orphosed carbonates. This is likely due to a com bination of factors
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
71
including devolatilization reactions and isotopic equilibration between
isotopically enriched m arble and isotopically depleted gneisses. However we
have shown that these effects are m inor and the oxygen isotopic values of
Cucamonga terrane is are close to original sedim entary values.
The hypothesis of Newton et al., (1980) that passage of a C C > 2-rich fluid or
CQ2-rich magma (Frost & Frost, 1987) is difficult to reconcile w ith the isotopic
systematics of the Cucamonga terrane. The carbon and oxygen isotopic
heterogeneity of the Cucamonga carbonates and calc-silicate rocks suggests
that granulite-fades m etam orphism was not accom plished in the presence of
a pervasive C0 2 -rich fluid or magma.
In contrast to the heterogeneous carbon isotopic signature, caldte,
dinopyroxene, garnet and biotite oxygen isotopic compositions are locally
homogeneous throughout the three core-drilled outcrops that span c. 10 m.
Localized isotopic hom ogenization m ust be due to pervasive infiltration of
all lithologies by an H 2 0 -rich fluid. The presence of graphite throughout the
traverse outcrops requires the fluid have a small carbon com ponent that did
not result in significant change in either carbonate or graphite S1 3 C because
A l3Ccai-Gr values are not affeded and high tem perature signatures are retained
(Hovanitz & M orrison, in submission).
Carbonates are thought to be largely im pervious to fluids at m id to lower
crustal pressures due to the ductile behavior of caldte at high confining
pressures sealing fractures, however, at least for relatively thin marble layers
(c. 20 cm), the Cucamonga marbles were perm eable to the degree suffident for
H2 0 -rich fluids to homogenize c. 10 m of diverse lithologies. Enhanced
marble perm eability is probably due to the intense m ylonitic deform ation that
accom panied m etam orphism .
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
72
Peak metamorphic tem perature was in excess of 850° C and likely about
880° C (Hovanitz & M orrison, in submission) using the caldte-graphite
therm om eter of Kitchen & Valley (1995). A t this tem perature dehydration
m elting of many lithologies should have occurred (e.g., H uang & Wyllie,
1974; Thompson & Algor, 1977; Le Breton & Thompson, 1988; Vielzeuf &
Montel, 1994) and furtherm ore the data of Barth et al. (1992) suggests that
there was a melting episode of mafic and alum inous gneisses despite the lack
of migmatitic structures anywhere in the terrane. This im plies that melt
extraction was efficient in that it did not cause disruption of fine lithologic
layering or form migmatites and that the Cucamonga terrane is therefore a
restite.
It is likely that pervasive melt production w ould hom ogenize oxygen
isotopic values in the silicate rocks, however, it is unlikely that significant
quantities of melt could infiltrate the marble and calc-silicate lithologies
w ithout chemically altering the marbles or the fine silicate-m ineral layering
w ithin the carbonates. Therefore we conclude that m elt m ovem ent through
the Deer Canyon core drilled section did not produce oxygen isotopic
homogenization and that a hydrous fluid phase was present to produce
oxygen isotopic homogenization w ithin the localized c. 10 m range of the
three core sample traverses.
Barth et al., (1992) concluded on the basis of isotopic and geochemical
evidence that a partial m elting occurred in the Cucamonga terrane and thus
generated the mafic intrusions found in the terrane. Significantly, there are
no recognized migmatites in the Cucamonga terrane despite the presence of
several semi-concordant mafic and felsic syn-m etam orphic intrusions (Hsii,
1955; May & Walker, 1989). Enigmatically, m igm atites are pervasively present
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73
in the adjacent upper amphibolite grade San Antonio portion of the Placerita-
San Antonio terrane (Hsti, 1955; Ehlig, 1981; May & W alker, 1989) that was
m etam orphosed and extensively intruded at a higher crustal level and lower
grade m etam orphic conditions (Barth et al, 1992).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
74
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8 1
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82
Table la . Sample location, rock type, rock m ineralogy and isotopic
com position. Sample data for the Deer Canyon core drilled traverse are
show n in Table lb.
S a m p l e
R o ck
T y p e Rock Mineralogy
5«OcaI
5 J3C CaiS
L o c a tio n
9 3 C B -1 -2 M C a l, C p x , G r 18.07 -0 .0 6 -3 .6 7 C C
9 3 C B -1 -3 M C a l, C p x , G r 18.41 0 3 9 -3 .1 1 C C
9 3 C B - l a M C a l, C p x , G r 18.00 0 .4 6 -5 .7 3 C C
9 3 C B -lb M C a l, C p x , S e p , G r 17.19 -5 .4 7 - 1 1 3 C C
9 3 C B -2 a M C a l, C p x , G r 1 7 3 5 2 3 4 -3 .1 7 C C
9 3 C B -2 c M C a l, C p x , P h i, G r 16.77 0 .9 9 -3 .4 3 C C
9 6 C B -1 1 C S C a l, C p x , G r 1 4 3 4 1.16 C C
9 6 C B - l lb C S C a l, C p x , G r 1 7 3 1 -2 .1 7 C C
9 6 C B -1 8 b M C a l, C p x , G r 16.60 - 0 3 7 C C
9 6 C B - lg M C a l, D i, G r, S e p 15.74 -4 .4 6 C C
9 6 C B -2 0 M C a l, C p x , P I, G r, (T r) 15.92 -4 .4 3 C C
9 3 C B - l l c - l C S C a l, S ep , C p x , P I, G r 16.60 -0 .6 5 -9 .0 5 D a C
9 3 C B -1 2 c M C a l, Phi, C p x , G r 1 8 3 3 4.11 D C
9 3 C B -1 2 d M C a l, S ep , P h i, C p x , G r 20.09 3 3 5 D C
9 3 C B -1 3 a M C a l, C p x , S e p , G r 1 8 3 3 - 0 3 1 - 6 3 7 D C
9 3 C B -1 3 b M C a l, C p x , G r 17.95 2 3 6 -4 .4 7 D C
9 3 C B -1 4 a C S C p x , C a l, P I, G r 17.18 - 4 3 4 -6 .8 7 D C
9 3 C B -1 4 b C S C a l, C p x , P I, G r 1 6 3 4 - 1 3 8 D C
9 2 C B -2 M C a l, Q tz , P I, O x , C p x , G r 19.07 0 3 2 S S C
9 2 C B -3 a C S C a l, PI, C p x , Q tz , G r 18.91 8 .68 2 3 3 S S C
9 2 C B -4 a M C a l, C p x , P I, G r 18.47 8 .0 7 2 .4 0 S S C
9 2 C B -5 a M C a l, C p x , P h i, G r 18.41 8 .49 -7 .0 7 S S C
9 2 C B -6 a M C a l, C p x , P I, P h i, G r 1 6 3 5 - 1 3 1 -4 .3 5 S S C
9 2 C B -7 a M C a l, C p x , P I, G r 1 8 3 8 2.11 S S C
9 2 C B -8 a M C a l, O I, G r 20.18 0 3 S S C
9 2 C B -9 a M C a l, (C p x , G r) 2 4 .4 4 2 .44 S S C
Rock Types: M, marble; CS, calc-silicate; MG, mafic gneiss; FG, felsic granulite;
AG, alum inous gneiss; BS, biotite schist; GQ, granular quartzite; G, gneiss.
M ineral abbreviations: Ap, apatite; Bio, biotite; Cal, calcite; Cpx,
dinopyroxene; Fo, forsterite; Gr, graphite; Grt, garnet; Ksp, potassium
feldspar; Ol, olivine; Ox, opaque oxide; Phi, phlogopite, PI, plagiodase; Qtz,
quartz; Rt, rutile; Srp, serpentine; Sep, scapolite; Sil, sillim anite; Tr, tremolite;
Ttn, titanite (sphene); Wo, w ollastonite; Zir; zircon. M inerals in parentheses
are in trace am ounts; italics are texturally secondary. Location abbreviations:
CC, Cucamonga Canyon; DaC, Day Canyon, DC, Deer Canyon, SSC, San
Sevaine Canyon area.
§ Data from H ovanitz & M orrison (in submission).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
83
Table lb. Samples from Deer Canyon Core Drilled Traverses.
Sample Rock
Type
Rock Mineralogy
»
5 180
C a l
5 lsO
C px
5 lsO
Bio
5I80
G rt
T ra v . #1
95 C B -0 8 FG Q tz , G rt, C p x , B io, O x , G r, Z ir 13.46 13.95 14.11
95 C B -0 9 A G B io , P I, C p x , G r t, Q tz , W o , O x , A p , Z ir , G r 13.06 1 3 3 2 1 3 3 4
95C B -10 C S C S C p x , S e p , Ceil, T tn , G r 16.05 13.89
9 5 C B -1 0 G G - 1 3 3 7 1 4 3 9
95C B -11 C S C p x , P I, Q tz , T tn , O x , S e p , O r, G r, Z ir , T r, Chi 13.63
9 5 C B -1 2 M C a l, C p x , F o , G r 15.73 13.82
95 C B -1 3 M G P I, C p x , B io , Q tz , T tn , O x , G r, (W o ) 13.42
9 5 C B -1 4 M C a l, C p x , S e p , T tn , F sp , G r 15.94 14.12
9 5 C B -1 5 M C a l, C p x , G r ( in v e in ) 16.89 13 3 7
9 5 C B -1 6 M G C p x , P I, Q tz , C a l, T n t, G r, (Z ir, Srp) 15.93 13.45
9 5 C B -1 7 FG P I, Q tz , C a l, C p x , B io , T tn , Z ir , G r, (Chi) 13.15
9 5 C B -1 8 FG Q tz , PI, C p x , O x , C a l, A p , T tn , G r, (Chi) 13.64
9 5 C B -1 9 FG Q tz , PI, C p x , T tn , O x , G r, (Chi, Cal) 12.92
9 5 C B -2 0 M C a l, C p x , G r 1 6 3 3 1 3 3 2
95C B -21 FG Q tz , P I, C p x , (C a l, Ser, A p , Id, G r)
9 5 C B -2 2 G Q Q tz , (P I, B io, C p x , Id, G r, O x , O p x , Chi, Tr)
T ra v . # 2
9 5 C B -2 3 M C a l, B io, C p x , G r, (K sp ) 1 6 3 9 14.03 13.71
9 5 C B -2 4 C S C a l, C p x , P I, P h i, T tn , A p , G r 16.13 13.72
9 5 C B -2 5 FG Q tz , P I, B io, C p x , A p , G r 1 3 3 1
9 5 C B -2 6 FG PI, Q tz , K s p , O x , G r
9 5 C B -2 7 M G PI, C p x , B io, Q tz , G rt, O p x , O x , G r, Id 13.95
9 5C B -28 M C a l, C p x , G r 15.85 1 3 3 5
9 5 C B -2 9 M C a l, C p x , G r 15.95 13.18
9 5 C B -3 0 C S S e p , C p x , P I, C a l, T tn , (S ill, G r) 15.91 1 4 3 2
95C B -31 C S C p x , S e p , P I, C a l, T tn , ( C r d , A p , Chi, G r) 1 6 3 0 13.90
T ra v . #3
9 5 C B -3 2 C S C p x , S e p , Q tz , P I, T tn , O x , G r, Hem 13.40
9 5C B -33 C S Q tz , C p x , P I, O x , Chi, G r 14.06
9 5 C B -3 4 FG Q tz , PI, C p x , C a l, G r 1 3 3 5
9 5 C B -3 5 M C a l, G r, C p x , P h i, A p 16.61
9 5 C B -3 6 M C a l, C p x , S e p , O x , G r, A p 15.40 1 3 3 2
9 5 C B -3 7 M C a l, C p x , G r 15.99 13.83
9 5 C B -3 8 C S C p x , C a l, G r 15.97 13.14 1 3 3 3
9 5 C B -3 9 M C a l, G r, G r t, C p x , B io, A p , (Chi, S e p ) 15.89 14.67
9 5 C B -4 0 M C a l, F o , S r p , C h i, C p x , G r, G r t 15.97 12.82
95C B -41 M G C p x , P I, B io , T tn , G r
9 5 C B -4 2 C S C p x , C a l, G r, P h i, A p , S e p 15.99 1 3 3 2 12.85
9 5 C B -4 3 C S C p x , C a l, O x , Chi, P h i, G r 16.71 1 2 3 9
9 5 C B -4 4 M G C p x , B io , P I, T tn O x , A p , G r, Z ir
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
9 5 C B -4 5 C S C p x , C a l, S e p , T tn , G r, Z ir 16.63 13.68
9 5 C B -4 6 C S C p x , C a l, S e p , G rt, P h i, A p , G r 1 6 5 4 1 4 3 2
9 5 C B -4 7 F G p i, C p x , q tz , G r, (Chi, T tn , Cal) 1 3 5 4
9 5 C B -4 8 M G P I, C p x , B io, G rt, A p , O x , G r, (O p x , Chi, Z ir) 1 4 3 3
9 5 C B -4 9 M G P I, C p x , G rt, Q tz , O x , A p , G r 13.69
95 C B -5 0 M G P I, C p x , G rt, O x , G r, Z ir 1 3 3 5
84
14.28
13.48
1 3 5 4
14.03
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
85
FIGURE CAPTIONS
Figure 1. Location of the Cucamonga terrane in southern California. The San
Gabriel, Placerita-San Antonio and Cucamonga terranes were located
w ithin the w estern N orth American Cretaceous m agma tic arc. Pelona
Schist terrane likely represents a portion of the underthrust subducting
plate m etasedim ents. These four terranes have been displaced by
num erous Cenozoic faults probably related to the San A ndreas transform
fault from their Late Cretaceous position c. 240 km to the southeast. Blank
areas are recent sedim ents or Cenozoic sedim entary rock units.
Figure 2. Simplified m ap of the Cucamonga and adjacent San Antonio
terrane (m odified from May & W alker, 1989; Barth et al., 1992). The shaded
areas are variably affected by mylonitic deform ation and retrograde
m etam orphism (May & W alker, 1989). W hite area in the central region of
the Cucamonga terrane is generally free of m ylonitic fabrics and relatively
unaltered except that most of the sam ples from the Cucamonga Canyon
area are m ylonitically deformed in thin section. The contact between the
Cucamonga terrane and San Antonio terrane is the "Black M ylonite" (Alf,
1948). H and sam ple locations are shown by black dots; the location of the
core drilled sam ples is shown by a w hite dot.
Figure 3. Coupled calcite 81 3 C and 51 8 0 plot of Deer Canyon core drill samples,
hand sample outcrops in Deer and Day Canyon (central) area, and other
hand sample outcrops in the Cucamonga terrane. Isotopic data for Deer
Canyon cores sam ples have nearly the same range in 51 3 C as the terrane in
it's entirety, however the 81 8 0 is highly restricted in isotopic range and
significantly depleted in 81 8 0 com pared to the terrane in as a whole.
Isotopic com positions from other outcrops in Deer Canyon have a slightly
larger range in 81 8 0 values.
Figure 4. Coupled calcite 81 3 C and S1 8 0 plot com paring the isotopic
composition of calc-silicate (< 95% calcite) rocks and marbles (> 95%
calcite). Most of the calc-silicate rocks have depleted isotopic compositions
com pared to marbles. This trend w ould be expected from decarbonation
reactions and isotopic equilibration betw een silicate m inerals and marble
(e.g., Valley, 1986), though several rocks have relatively enriched carbon
and oxygen isotopic compositions.
Figure 5. H istogram comparison of S1 8 0 values from different portions of the
Cucamonga terrane. The San Sevaine Canyon area has the largest range in
isotopic com positions and the m ost enriched 81 8 0 values. Both the
Cucamonga Canyon and Deer Canyon areas have m ore restricted isotopic
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86
ranges and lower average 5lsO. The data may be skewed by fewer marble
outcrops in the Cucamonga and Deer Canyon areas.
Figure 6. H istogram com parison of 81 8 0 values from the Cucamonga terrane,
relatively unm etam orphosed w orld-w ide carbonates (Veizer and Hoefs,
1976), the Esplanade Range (Ghent & O'Neil, 1985), the Adirondacks
(Valley & O'Neil, 1984) and Sri Lanka (Hoffbauer & Spiering, 1994). The
Cucamonga terrane is depleted by c. 4% o compared to unm etam orphosed
carbonates and has about the sam e oxygen isotopic composition as the
other high-grade m etam orphic terranes. The Esplanade Range carbonates
are significantly depleted in 5lsO com pared to the other terranes despite
m etam orphism being fluid-absent (Ghent & O'Neil, 1985).
Figure 7. Plot of caldte, graphite an d dinopyroxene isotopic trends across the
three core drilled traverses. C aldte and graphite carbon isotopic data is
heterogeneous while the oxygen isotopic data has been homogenized
across lithologies that w ould be expected to have different isotopic
com positions (i.e. m arble and mafic gneiss). Isotopic fractionations of
2.7%o for A1 3 Ccai-Gr and A1 8 Ocai-Cpx are shown for reference. This
dem onstrates the presence of a high O-bearing fluid with a m inor C-
com ponent. C aldte and graphite carbon isotopic data is from Hovanitz &
M orrison (in submission). The left side of each of the traverses is
structurally highest. Each of the cores were drilled c. 10 cm apart parallel to
lithologic layering and dow n dip.
Figure 8. Plot of 19 co-existing caldte-diopside pairs from Deer Canyon.
Isotherm s shown for reference are from Chiba et al. (1989). The average
tem perature estim ate is c. 720° C, low er than the caldte-graphite
tem perature estim ate of > 850°C, but the data suggests retention of
generally high tem perature (c. 850°C) isotopic equilibrium that has been
m odified by lower tem perature equilibration, possibly by channelized
retrograde fluid flow.
Figure 9. Plot of 4 co-existing caldte-phlogopite pairs from the Deer Canyon
core-drilled traverse. A tem perature estim ate of c. 736 ± 54°C is made using
the calibration of Hoffbauer & Spiering (1994). This dem onstrates the
retention of granulite m etam orphic tem peratures.
Figure 10. Relationship of XCO2 and Log^t^ which shows that a hydrous fluid
in contact w ith graphite m ust have a carbon-bearing com ponent like CO2
or CH4. Due to hom ogeneous oxygen isotopic composition and
heterogeneous carbon isotopic signatures, we would like to minimize the
carbon content. The m inim um possible fluid carbon content is between
-15 to -16 Logf&z (Lamb and Valley, 1984).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
87
Afadre
C u cam o n Q ® -
Fault
L o s A n g e le s B a s in
I I - San Gabriel Terrane
M B A - Placerita Terrane
E 3 - San Antonio Terrane
1773 - Cucamonga Terrane
IH I - Pelona Schist Terrane
k i l o m e t e r s
Figure 1.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
88
N
i
mylonite zones
S a ^ A n to n io
C a n y o n
k C u c a m o n g a
C a n y o n
D e e r ' D a y X
C a n y o n C a n y o n
S a n S a v a in e
C a n y o n
Cucamonga Terrane
Figure 2.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
5 13C cal V S. 8 ISO cal
8
4
0
81 3 C
-4
-8
-12
10 14 18 22 26
5 1 8 0
Figure 3.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
r n i i i 1 1
•
>
/
i
1
5 <
0
• ~
Q 0 T ® *
•
—
e g ® • •
A "
§Q ®
—
•
O
— O Deer Cyn (cores)
—
# Deer Cyn (hand samples)
• _
# Cucamonga terrane
1 1 1 I 1
1 1
Marble
513C vs. 5180
90
T "T 1 1 1 1 1
8 — 19 —
4 • •
° H
#
• ”
0
o
* 9 * • • •
J o ° I
—
-4
8
§ep
O
—
O
-8
-- —
o calc-silicate >5% silicate
o
” • marble <5% silicate
12
1 1
1 1 1 1
1
10 14 18 22 26
Figure 4.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Cucamonga Canyon
16.98 ± 1.01% o
Frequency 3
2
0
1 2 . Deer Canyon
1 (J 16.54 ± 0.97% o
8
Frequency
6
4
2
0
San Sevaine Canyon
19.98 ±3.02%o
Frequency 3
2
1
0
10 12 14 16 18 20 22 24 26
8 ! 8 0
Figure 5.
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92
25
20
Frequency 15
10
5
0
5
4
Frequency 3
2
1
0
12
10
8
Frequency
6
4
2
0
6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42
5180
Figure 6.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
W orld-W ide
U n m etam o rp h o se d
C arb o n ates
23.1 ± 3.6 % o
n=177
16.1 ± 3.6 % o
A d iro n d ack s
20.0 ± 3.7 % o
n=87
93
8
4
0
6 i3 C ^
-8
-12
-16
24
20
16
8 I S O 1 2
8
4
0
0 10 20 30 0 10 20 0 10 20 30 40 50
I Aluminous gneiss
1 Quartzfeldspathic gneiss
Lithology: ■ Mafic
0 Calc-silicate
D Marble
1 Quartzite
H gure 7.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
: , ’ l i
i j i
” 0 0 ° °oo
o
u i 5 ,
0 I
rn c a k ftp
2 . 7 % , T
L O g ra p h ite
. V j u I :
■ m w m m m m
■ - H - t f H 1 1 1 1 I I 1 1 1 1
U.4P
T n v c s e C
DilllTDIO
■ l 1 1 1 1 l l 1 1 1
“ » [ j r :
. . . ‘i l l " I I I U ;
Traverse <3
U lQ Q D im illllillllll
1 1 1 H 1 I 1 1 1 1 1 n n 1 11 1 +
94
S lS Q c a l V S . 5 l8 0 q > x
18
17
15
14
16
5l^O clinopyroxene
Figure 8.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
95
20
19
18
17
16
5180Caldte 1 5
14
13
12
11
10
10 11 12 13 14 15 16 17 18 19 20
8 l8 0 p h lo g o p ite
Figure 9
O l^ O ca l V S. O l^ O p h l
average A i8 0 c c - p h i = 2.88 ± 0.31 %o
average temperature = 736 ± 54 °C
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
1 .0
CH,
0.8
HoO
0.6
0.4
0.2
CO.
0.0
-20 -18 -16 -14
LOG fO 2
1.0
0.8
CH,
0.6
0.4
0.2
CO.
0.0
-14 -16 -18 -20
LOG fO 2
Figure 10
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Asset Metadata

Creator Hovanitz, Eric William (author)

Core Title Carbon isotopic heterogeneity and limited oxygen isotopic homogeneity in the cretaceous Cucamonga terrane, southeastern San Gabriel Mountains, California

Degree Doctor of Philosophy

Degree Program Geological Sciences

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag geochemistry,Geology,OAI-PMH Harvest

Language English

Contributor Digitized by ProQuest (provenance)

Advisor Morrison, Jean (committee chair), Davis, Greg (committee member), Ehlig, Perry (committee member), Weber, William P. (committee member)

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c17-383459

Unique identifier UC11353532

Identifier 9919052.pdf (filename),usctheses-c17-383459 (legacy record id)

Legacy Identifier 9919052.pdf

Dmrecord 383459

Document Type Dissertation

Rights Hovanitz, Eric William

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the au...

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus, Los Angeles, California 90089, USA