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Construction of a gabbro body in the Trinity Complex, northern California

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Construction of a gabbro body in the Trinity Complex, northern California

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Copyright 2001
CONSTRUCTION OF A GABBRO BODY
IN THE TRINITY COMPLEX,
NORTHERN CALIFORNIA
by
Gunilla Kerstin Andreasson
A Thesis Presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
MASTER OF SCIENCE
(GEOLOGICAL SCIENCES)
August 2001
Gunilla Kerstin Andreasson
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UM I Number: 1409575
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UNIVERSITY OF SOUTHERN CALIFORNIA
The Graduate School
U niversity Park
LOS ANGELES, CALIFORNIA 90089-1695
This thesis, w ritten b y
______ G u n illa A ndreasson_____________________
Under th e direction o f h &C .. Thesis
Com m ittee, and approved b y a ll its members,
has been presen ted to and accepted b y The
Graduate School, in pa rtia l fulfillm ent o f
requirem ents fo r th e degree o f
Master of Science
Dean o f Graduate Studies
D ate August 7. 2001
THESI S COMMITTEE
Chairperson
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ACKNOWLEDGEMENTS
I gratefully acknowledge the following people: my advisor, Scott Paterson,
for his guidance, sharing of knowledge and eternal patience with me; my other
committee members, Greg Davis and Jean Morrison, for their comments and
support; Rodney Metcalf, for interesting discussions about the Trinity Complex; the
strain lab group with Aaron. Keegan, Geoff, Michael, Paul, Markus, Melissa, David
and Helge for spending all their spare time in the office and for their big help; and
other departmental folks, Sue, Shoshana, Eric, Shari, Ross, Brian and Brooks.
Thanks for everything. I would also like to thank Dorte and Christian for always
being there, Adam for cooking those falafel in the field and John for saving me from
my field assistant. To all my Swedish friends, Kattis, Patrik, Niklas, Lena, lill-Bjom,
stor-Bjom, Magnus, Johannes, Lars, Ulrika, Mats, Bettan, Ingrid and all the others, I
would like to thank you for the fun and stable friendship you give. To all my horse-
polo and swing-dancing friends, thanks for all the exercise and fun that kept me
going. Thanks Arie, you helped me through my first year in LA.
Finally, I would like to thank my wonderful family, Ulla, Pelle, Ingela,
Lennart, Claes, Liv, Stefan, Karin, Sofia, Christine and Malin, that always stood
behind me and kept telling me to come home. Thanks, “mormor och morfar”, for
being my best friends and thanks “mamma” for all your love and support.
This research was sponsored by the Graduate Student Research Fund, the
Geological Society of America stipend and the Swedish-American stipend.
ii
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TABLE OF CONTENTS
ACKNOWLEDGEMENTS.............................................................................. ii
LIST OF FIGURES............................................................................................ vi
LIST OF TABLES................................................................................................ viii
LIST OF PLATES.............................................................................................. viii
ABSTRACT........................................................................................................ ix
CHAPTER1. INTRODUCTION........................................................................1
Nature of Problem................................................................................... 1
Characteristics of an Ophiolite................................................. 1
Construction o f Magma Chambers in Oceanic Settings 4
Proposed Models..................................................................................... 8
Introduction to the Trinity Complex........................................ 8
Quick's (1981) m odel................................................................11
Boudier et al's (1989) m odel....................................................12
Cannat and Lecuyer's (1991) m odel........................................14
Wallin and Metcalfs (1998) model..........................................15
Limitations with the Models.................................................................. 17
Overview of Present Research..............................................................20
CHAPTER 2. FIELD OBSERVATIONS IN THE TRINITY COMPLEX.. 22
Regional Geology and the Tectonic Setting o f the Trinity Complex 22
Petrology...................................................................................................24
Ultramafic Rocks (Op)............................................................. 24
Harzburgite.................................................................... 25
Lherzolite.......................................................................25
Dunite............................................................................. 26
Pyroxenite (Opx)........................................................... 26
Fracture Zones and Shear Zones................................. 26
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Gabbro (Dg)..................................................................................27
Coarse-grained Phase..................................................... 28
Medium-grained Phase...................................................29
Cumulate Gabbro............................................................30
Pegmatitic and Trondhjemitic phases.......................... 30
Gabbro Shear Zones....................................................... 32
Gabbro Margins (D b).................................................................. 32
Sheeted M argins..............................................................32
Stoped Blocks..................................................................34
Diabase Dikes............................................................................... 36
Structure..................................................................................................... 38
Ultramafic Rocks (Op)............................................................. 38
Harzburgite and Lherzolite............................................ 38
Dunite............................................................................... 43
Pyroxenite........................................................................ 44
Shear Zones and Fracture Zones...................................44
Gabbro Sequence (Dg)................................................................49
Layering...........................................................................49
Magmatic Foliation and Lineation............................... 51
Coarse-grained Phase..................................................... 56
Medium-grained Phase...................................................57
Cum ulates........................................................................59
Pegmatitic and Tronhjemitic Phases............................ 59
Gabbro Shear Zone.........................................................61
Gabbro Margins (Db).................................................................. 62
Sheeted M argins..............................................................62
Stoping o f Ultramafic Rocks....................................... 66
Stoping o f Pyroxenite Rocks......................................... 69
Other Xenoliths............................................................... 71
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Foliations, Serpentinization and Chilled margins 72
Diabase Dikes...............................................................................74
CHAPTER 4. DISCUSSION...................................................................... 76
Relationship between the Seven Lakes Gabbro and the Mantle 76
Gabbro Chamber Construction..............................................................79
Intrusion Sequence...................................................................... 79
Interpretation o f Magmatic Fabrics...........................................80
Displacement o f Host Rock during Gabbro Em placem ent................82
Evaluation of Previous Models for the Trinity Complex....................86
Formation o f the Peridotite.........................................................86
Formation o f the Gabbro............................................................ 87
CHAPTER 5. CONCLUSIONS..........................................................................88
Suggestions for Further W ork............................................................................. 90
REFERENCES...................................................................................................... 92
APPENDIX............................................................................................................ 97
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LIST OF FIGURES
Figure 1. Magma chambers models based on geophysical data...................................5
Figure 2. Magma chamber models based on geological evidence........................... 6
Figure 3. A magma chamber model o f the Oman ophiolite..........................................7
Figure 4. Location o f the Trinity Complex in the Eastern Klamath B elt................... 8
Figure 5. Map of the Trinity Com plex............................................................................ 10
Figure 6. Reconstruction of the Trinity Complex (Boudier et al„ 1989)....................13
Figure 7. Reconstruction of the Trinity Complex (Cannat and Lecuyer, 1991).........15
Figure 8. Reconstruction of the Trinity Complex (Wallin and Metcalf, 1998).......... 16
Figure 9. Distribution of different gabbro phases within the Seven Lakes
complex................................................................................................................................ 29
Figure 10. Microphotograph of a contact zone between gabbro and host rock..........33
Figure 11. Microphotograph of a contact zone between stoped peridotite block and
gabbro magma.....................................................................................................................37
Figure 12. Microphotograph of the foliation in peridotite............................................ 39
Figure 13. Equal area plot of poles to the regional foliation....................................... 40
Figure 14. Map displaying tectonite foliations within the Seven Lakes area............41
Figure 15. Map displaying tectonite lineations within the Seven Lakes area........... 42
Figure 16. Profile across the Gumboat Lake shear z o n e..............................................45
Figure 17. Photograph from the Gumboat Lake shear z o n e ....................................... 47
Figure 18. Shear structures in the Gumboat Lake shear z o n e..................................... 48
Figure 19. Photograph o f pegmatitic layering in the coarse-grained gabbro............ 49
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Figure 20. Equal Area plot of poles to layering in the Seven Lakes gabbro 50
Figure 21. Map displaying magmatic foliation within the Seven Lakes gabbro 52
Figure 22. Equal area plot of poles to magmatic foliation within the Seven Lakes
gabbro.................................................................................................................................. 53
Figure 23. Map displaying magmatic lineations within the Seven Lakes gabbro... 54
Figure 24. Equal area plot o f poles to magmatic lineation within the Seven Lakes
gabbro.................................................................................................................................. 55
Figure 25. Microphotograph of coarse grained gabbro foliation................................57
Figure 26. Microphotograph of medium-grained gabbro foliation............................. 58
Figure 27. Photograph of magma mingling in pegmatitic rocks.................................60
Figure 28. Microphotograph of a gabbro shear zone.................................................... 62
Figure 29. Photograph o f sheeted gabbro dikes intruding peridotite........................63
Figure 30. Photograph of sheeted gabbro dikes intruding gabbro cumulates..........64
Figure 31. Equal area plot o f poles to sheeted dikes.................................................. 65
Figure 32. Profile o f the Gumboat Lake stoping zone................................................66
Figure 33. Photograph o f rounded stoped blocks o f peridotite in a gabbro dike.... 67
Figure 34. Photograph o f peridotite intruded by a network o f gabbro veins...........68
Figure 35. Photograph o f rounded stoped blocks within a pyroxenite matrix.........69
Figure 36. Photograph of stoped pyroxenite rocks within a leucocratic gabbro..... 70
Figure 37. Photograph of small xenoliths o f unknown original composition
completely altered to talc and tremolite......................................................................... 71
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Figure 38. Photograph of a large xenolith, (1.5 xl .5 m), completely altered to talc
and tremolite..................................................................................................................... 72
Figure 39. Pegmatitic gabbro dike obliquely crosscutting the peridotite foliation.. 73
Figure 40. Equal area plot of poles to diabase dikes in the Seven Lakes area 75
Figure 41. A schematic profile across the Seven Lakes gabbro..................................77
Figure 42. A simplified sketch of the fabric and dike pattern within the Seven Lakes
gabbro...................................................................................................................................81
Figure 43. A simplified profile showing displacement of host rock associated with
the gabbro intrusion......................................................................................................... 85
LIST OF TABLES
Table 1. A collection of age data from the Trinity Complex..................................... 19
LIST OF PLATES
Plate 1: 1:20,000 scale geologic map o f the Seven Lakes gabbro.................map
pocket.
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ABSTRACT
Previous analyses of geochemical data suggest that the Trinity Complex is an
ophiolite. The results of the present study, combined with age-data, indicate that the
gabbros in this complex are intrusive into an older peridotite and therefore not part of
an ophiolite. The Seven Lakes gabbro consists of multiple gabbro phases, ranging
from early, pyroxene-rich to late, hornblende-rich, and from medium-grained to
pegmatitic. Internal processes such as stoping and mingling, as well as multiple
intrusive relationships are observed in the gabbro, and help to explain the
construction of the magma chamber. Fabrics in the gabbro are interpreted to
represent tectonic strain. However, the early fabric could also be emplacement-
related. This study further suggests that host rock processes, such as stoping, diking
and ballooning play a more significant role than fault emplacement does in the
intrusion o f the gabbros.
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CHAPTER 1. INTRODUCTION
Nature of Problem
Characteristics of an Ophiolite
In many mountain belts, large slabs o f oceanic lithosphere have been
incorporated onto the continents. These slabs, called ophiolites, often preserve
spectacular sections o f ancient oceanic crust, including cross sections through ancient
mid oceanic ridges and supra-subduction zone magma chambers. Using data gathered
from deep-diving submarines, dredging, deep-sea drilling and seismic exploration,
geologists now explain these ophiolites as fragments o f oceanic lithosphere (crust and
mantle) that were formed along spreading ridges, were transported by spreading and
later emplaced onto continents in an episode of plate collision (Press and Siever,
1994). The Greek terms "ophio" (snake) and "lithos" (rock) were proposed by
Brongniart (1813) to describe a group o f serpentinized rocks that occurred in
Tertiary melanges of the northern Apennines, Italy. In 1972, a Geological Society of
America Penrose Conference was held to evaluate ideas about ophiolites and to create
a general description o f ophiolites as follows:
* Ultramafic com plex, consisting o f variable proportions o f harzburgite, Iherzolite and
dunite, usually with a m etam orphic tectonic fabric (more o r less serpentinized).
* Gabbroic com plex, ordinarily with cum ulus textures com m only containing cum ulus
peridotites and pyroxenites and usually less deformed that the ultramafic complex.
* Mafic sheeted dike com plex
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* M afic volcanic com plex, commonly pillowed.
* Associated rock types include (I ) an overlying sedim entary section typically including
ribbon cherts, thin shales interbeds, and m inor lim estones; (2) podiform bodies o f
chrom ite generally associated with dunite; (3) sodic felsic intrusive and extrusive rocks.
* Faulted contacts between mappable units are com m on. Whole sections may be
m issing. An ophiolite may be incomplete, dism em bered, or metamorphosed. Although
ophiolite generally is interpreted to be oceanic crust and upper mantle, the use o f the
term should be independent o f its supposed origin, (Conference Participants, 1972).
The genesis o f oceanic crust is related to the separation of oceanic plates,
most commonly in mid-oceanic settings but also in supra subduction-related back-
arc, intra-arc and for-arc settings. As the plates separate, hot asthenospheric mantle
rises and begins adiabatic melting. Basaltic magma fills a shallow magma chamber
where the magma crystallizes along the floor and the walls to form gabbro as heat is
extracted. As the plates continue to separate the continued growth o f gabbro
chambers form a subhorizontal panel. The asthenospheric residue consists mostly of
olivine and pyroxene, which form the underlying peridotite section. Magma from the
chamber repeatedly intrudes the rift between the spreading plates and solidifies as
vertical basaltic dikes. Basalt spilling out on the seafloor freezes as pillow lava. As
the sea-floor spreads, the zones of lavas, dikes, gabbros and peridotites are
transported away from the spreading ridge. In a mid-oceanic ridge this process is
relatively simple, while in arc-related supra-subduction zones, spreading and
magmatism can be more complicated. In a supra-subduction zone, slow spreading
2
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can cause overlapping magmatism over millions o f years contemporaneous with
mechanical deformation that overprint the different magmatic intrusions (Karson,
1998). Arc-related spreading might also be immature and not produce fully
developed ophiolites. Slow seafloor spreading forms varying thickness of crust with
short-lived magma chambers that freeze in intervals and interact with major tectonic
deformation, while fast spreading ridges have a more constant crustal thickness and
magma supply and less mechanical deformation (Karson, 1998; Macdonald, 1998).
Therefore, ocean crust and ophiolites display very different characteristics depending
on whether they were formed in a mid-oceanic ridge or supra-subduction zone
environment and whether they were formed in a slow or fast spreading setting.
There are many uncertainties regarding the recognition o f ophiolites when
they are incomplete, dismembered, metamorphosed or when the ophiolite units
formed during different times in different settings. For example, the Trinity Complex
has for a long time been called an ophiolite, although it has an older peridotite unit
and a younger mafic unit (Boudier et al., 1989). Because o f the uncertainty in the
origin o f the complex, I chose to examine the relationship between the peridotite and
the gabbro and how these gabbro bodies were constructed. I will also evaluate
previous models of the Trinity Complex and whether the Trinity Complex is an
ophiolite.
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Construction o f Magma Chambers in Oceanic Settings
Geophysical evidence preludes the existence o f large, mainly molten magma
chambers beneath active spreading centers like the East Pacific Rise, and instead
indicate that a thin and narrow axial melt lens overlies a zone o f crystal mush, which
is in turn is surrounded by a transition zone o f mostly solidified crust with isolated
pockets of magma (Fig. 1, Yildrim et al., 1998. In a slow spreading setting (Fig. 1),
such as the Mid-Atlantic Ridge, magmatism is more concentrated in discrete zones,
continuous axial magma chambers are missing, and crustal thickness changes more
along axis than in a fast spreading ridge. Geophysical investigations have primarily
been focussed on major rift zones, and supra subduction zone settings are less well
studied.
Geological descriptions of ophiolitic magma chambers in the Oman, Bay of
Islands, and Troodos ophiolites are presented as models in Figure 2 (Quick and
Denlinger 1993). The models of Pallister and Hopson (1981), based on the Oman
ophiolite, show that layers are deposited on the floor and walls o f the axial magma
chamber and then transported away from the spreading axis without significant
deformation. The magma chamber proposed by Smewing (1981) differs primarily by
having upward steepening walls to account for increasing dip with height in the
section. A different model was proposed by Nicolas et al. (1988b) from the Oman
ophiolite, which attempts to reconcile upward steepening layering, evidence for
synmagmatic deformation of the layered gabbros, and concentric structures in the
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Transform taut/
Magmatic Sagmart Cantor
I I
Mapivtie S tg m n t Ctnlsr
Slow-Spreading Ridge
SagmantCanlar
CyrsmMuah
Volcanica/Basalt
I 7T T 7 1 D*aa
[ • > fl Gabbro
unramafic rocfca/Peridorrta
Fast-Spreading Ridge
m M atlant
rm ^ i Crystal mush zone
— ' (paitMy motan)
i zona
(laigetysoSd)
Figure 1. Interpretive model o f magma chambers along a slow spreading ridge (the
Mid-Atlantic Ridge) and a fast spreading ridge (the East Pacific Rise) based on
geophysical and petrological constraints (Dilek et al., 1998; modified from Cannat et
al., 1995b and Sinton and Detrick, 1992).
5
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-15 -10
■ »
MAGMA
MONO
PA LU STER & HOPSON (1981)
MAGMA
SMEWING (1981)
NICOLAS ETAL. (1988b)
DEW EY & KIDD (1977)
Figure 2. Magma chamber models based on geologic evidence in ophiolites.
Horizontal and vertical scales are equal. Symbols: layering in gabbros, continuous
lines: non-layered, non-foliated “isotropic gabbro, stipple: sheeted dikes, closely
spaced vertical lines: volcanics, stippled pattern. Regions labeled “magma” consist of
melts with or without suspended crystals. Region labeled “crystal much” consists of
crystal cumulates with variable amounts of interstitial melt. Diagram from Quick and
Denlinger(1993).
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mantle related to diapiric rise. The subsiding-floor magma chamber proposed by
Dewey and Kidd (1977) from the Bay o f Islands ophiolite predicted that the layers
develop as cumulates and subside beneath an axial magma chamber during crustal
extension. A more recent magma chamber model constructed by Boudier et al. (1996)
of the Oman ophiolite has the shape o f an overturned boat and is constructed by sills
in the lower layered gabbros and by diking in the upper layer. The melt in the upper
layer migrates into a perched magma chamber where crystallization along walls and
the floor subsequent subsidence occur (Fig. 3). Recent workers (Korenaga and
Keleman, 1998 and Yoshinobu, 1999) also proposed that the lower gabbros in
ophiolites formed from multiple sills intruding subhorizontal to the Moho.
Figure 3. A magma chamber model of the Oman ophiolite after Boudier et al. (1996).
7
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Proposed Models of the Trinity Complex
Introduction to the Trinity Complex
The Trinity Complex is located in the Klamath Mountains in northern
California (approximately between the latitudes 41°00 and 41°30, and the longitudes
122° 15 and 122°45) northwest o f the Sierra Nevada Mountains Range (Fig. 4).
Figure 4. Location o f the Trinity Complex in the Eastern Klamath Belt, northwest o f
the Sierra Nevada Mountain Range.
Eastern
Klamath
Belt
Klamath
Mountains
i Oregon
^California Nevada
S ierra
Nevada
8
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This Complex has an area o f 16,000 km2 and is preserved in an easterly
dipping thrust slice sandwiched between the Central Metamorphic Belt to the west
and the Eastern Klamath Belt to the east. These terranes are also easterly dipping as
they were juxtaposed along thrusts above an easterly dipping subduction zone during
the Devonian (Davis, 1968; Wallin and Metcalf, 1998). The Central Metamorphic
Belt preserves an inverse metamorphic gradient and structures that indicate supra-
subduction zone thrusting o f the Trinity Complex onto an accretionary wedge during
subduction and accretion o f the terranes (Cannat and Boudier 1985). However,
recently this inverted gradient has been interpreted to represent a gradient due to the
hot Trinity peridotite overriding the cold CMT (personal communication, Metcalf,
2001). Geophysical data shows that the Trinity complex consists of a peridotite
sheet with a thickness o f less than 4 km with gabbro bodies (with different sizes and
shapes) randomly distributed within the peridotite sheet (Fig. 5).
Previous workers (Boudier et al., 1989; Cannat 1996) believed that the
Trinity Complex was an ophiolite because it contains a complete pseudo-
stratigraphic section o f peridotite, gabbro, sheeted dikes and pillow lava. Subsequent
workers stated that the units are not entirely correlated to each other raising
controversies about how the Trinity Complex was formed. Most workers agree that
the Complex was formed in an oceanic immature slow spreading environment, a
setting that should develop more tectonic extensional deformation than in fast or
intermediate spreading settings (Boudier et al., 1989; Mutter and Karson, 1992).
9
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122 45
12215 '
QV
Areas of study
— 41 00 '
Geology in the Trinity Area
Quaternary alluvium
Quaternary voicanics
Devonian Copley Greenstone
Paleozoic Eastern Klamath belt
Paleozoic Yreka Terrane
Paleozoic Central Metamorphic belt
Q I Quaternary glacial deposits
s ' x 'l Mesozoic plutonic rocks
Devonian-Silurian gabbro
Ordovician-Devonian peridotite
Figure 5. Map o f the Trinity Complex and the surrounding terranes. Diagram
drafted from the Weed Quadrangle map, California, 1:250,000,1987.
10
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However, extensional deformation such as normal faulting and detachment faulting
has not yet been recorded in this area. Several models have been proposed in order to
explain the formation of the complex. Alternative models include the following: a)
formation above a mantle diapir (Quick, 1986); b) formation by sea floor spreading at
an mid-oceanic ridge (Boudier et al., 1989); c) generation by spreading in a back-arc
basin (Brouxel and Lapierre, 1988), d); generation by spreading in a fore-arc setting
(Saleeby, 1990) and e) generation by spreading in a supra-subduction zone fore-arc
lithosphere (Wallin and Metcalf 1998). To date, the peridotite section is thought to
have formed in a mid-oceanic spreading environment, whereas the mafic section
shows signatures o f arc-related (fore-arc) magmatism (Wallin and Metcalf, 1998).
Quick's (1986) model
Quick (1986) proposes that the Trinity peridotite ascended as a mantle diapir
through the upper mantle from a depth of 30 km, based on textural and
compositional evidence for replacement of spinel by plagioclase in lherzolite. Citing
mineralogic and field constrains, he suggested that the peridotite partially melted at a
high pressure (>10 kbar), producing a basaltic melt that fractionated in the direction
of tholeiitic basalts. Multiple pulses of these transient melts formed clinopyroxene-
rich dikes, dunite bodies, gabbro bodies and dikes that intruded the "restite"
plagioclase lherzolite. In conclusion, Quick (1986) interprets the ultramafic and the
mafic units to have formed in the same tectonic setting and relatively
contemporaneously. Volcanic, volcaniclastic and intrusive rocks with with
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calcalkaline affinities are found throughout the Klamath Mountains and Quick (1986)
suggests that such an environment could account for the volcanic and sedimentary
cover overlying an ophiolite formed in proximity to a continental margin (island arc
or back-arc basin).
Boudier et al’s (1989) model
Based on the age of the peridotite (472 ± 30 Ma) proposed by Jacobsen et al.
(1981), Boudier et al., (1989) suggest that the Trinity Complex represents a piece of
Ordovician oceanic lithosphere that became trapped in a marginal sea during the
Devonian. Using field studies and structural data from the Trinity Complex Boudier
et al. (1989) constructed a model of how the Trinity Complex, in its entirety, was
formed at an oceanic spreading ridge on the flank of a mantle plume (Fig. 6). Their
model is primarily based on the identification of steep foliation and horizontal
lineation in the peridotite, which they interpret to be indicative o f flow along a
spreading ridge. The foliation in their model is parallel to the sheeted dikes, which in
turn are suggested to be contemporaneous with the peridotite and indicate the
orientation of spreading during formation o f the Trinity Complex. Boudier et al.
(1989) propose that the gabbro unit formed as a part of a subhorizontal ophiolitic
section from flow in a magma chamber and was later disrupted and faulted into the
steeper contacts that are observed in the Trinity Complex today. This slow flow in
the gabbro was coupled to the underlying faster flow in the peridotite, and the
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coupling is said to have caused shearing along the contacts. However, Boudier et al.
(1989) do not present any evidence from sheared contacts in the Trinity Complex.
m
CRUST
\ \ ■ 01 . \ \ \
\ \ \X \
v A v > \ \ V \
A S T H .! lithosphere
M A N T LE
Figure 6. Reconstruction of the Trinity Complex in its inferred oceanic spreading
setting, based on structural data. S 1 =foliation, representing the flow plane;
Ll=mineral lineation, representing the flow direction. Diagram from Boudier et al.
(1989).
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4
Cannat and Lecuyer’s (1991) model
Cannat and Lecuyer (1991) present a model that differs considerably from the
model of Boudier et al. (1989). In their model, ephemeral magma chambers
developed afrer the end o f the plastic deformation in the surrounding mantle (when it
cooled down to lithospheric temperatures). Figure 7 shows the different stages of
cooling of the asthenosphere, the downward migration o f the boundary between
lithosphere and asthenosphere, and finally the intrusion o f the gabbro magma into the
transition zone (Cannat and Lecuyer, 1991). The model is based on fieldwork in
Castle. Toad, and Tamarack Lakes, where they interpret the
contacts between gabbro and peridotite as being walls and roofs of ephemeral magma
chambers. This model is also based on the pattern o f discontinuous, km-sized
pockets of gabbro within the Trinity peridotite and from field evidence such as
magmatic breccias along the gabbro contacts. Their evidence for a gabbro intruding a
cold lithosphere is concluded from data from Jacobsen et al. (1984) that show
contrasting Nd and Sr isotopic compositions for the Trinity peridotite (MORB
affinity) and two gabbroic dikes (island arc affinity) crosscutting it. Therefore, the
peridotite is thought to have formed in a separate environment from the mafic
intrusives.
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STAGE I STAGE I" STAGE I
Cooling o < IV m iff ntnmOlpAtfl
•n i to»ncrt aigrgiim of if**
lilHOHMft / oW NfiM M fi MnaMry
m iiw m m
r O IK E S
= TRANSITION
IONE
^ lithosphere
a sth en o sph er e
Figure 7. Interpretative sketch showing the two-stage evolution for the Trinity
Complex. Diagram from Cannat and Lecuyer (1991).
Wallin and Metcalfs (1998) model
Wallin and Metcalf (1998) were the first to propose that the Trinity Complex
began to form in a transform plate margin in the paleo-Pacific ocean during the
Ordovician. This was a different approach from the previous spreading ridge models
(Quick, 1986; Boudier et al., 1989; Cannat and Lecuyer, 1991). In the model by
Wallin and Metcalf (1998), intra-oceanic subduction was initiated along this
transform fault due to a density instability. The denser slab sank, causing upwelling
o f the asthenosphere and extension of the hangingwall lithosphere (Fig. 8). Wallin
and Metcalf (1998) suggest that adiabatic melting o f the asthenosphere, coupled with
a minor flux o f slab-derived fluids, resulted in the emplacement o f primitive,
ultradepleted mafic intrusive complexes overlain by a thin cogenetic volcanic
carapace. Continued trench rollback and hanging wall extension produced mafic
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I w—1 twm
Figure 8. Schematic diagram o f the mid-Paleozoic evolution o f the Eastern Klamath
Mountains: A) Intraoceanic subduction initiated due to a density instability along a
transform fault. B), C) and D) Trench rollback, associated hangingwall extension and
adiabatic melting of astenosphere produced mafic igneous complexes and a cogenetic
volcanic carapace over 30 Ma period. E) Present configuration o f mid-Paleozoic
rocks o f the eastern Klamath Mountains. Diagram from Wallin and Metcalf (1998).
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igneous complexes and subduction-related volcanism over a 30 Ma period before
obduction during the Devonian. Wallin and Metcalf (1998) further propose that the
Central Metamorphic Belt represents a remnant of the downgoing oceanic slab; the
Yreka Terrane, situated north o f the Trinity Complex, represents the accretionary
prism and the eastern Klamath Belt is interpreted to be the preserved volcanic
carapace downfaulted by the La Grange normal fault (Fig. 8). Wallin and Metcalf
(1998) are the first workers to put the Trinity Complex into a regional tectonic
perspective and to present that the different gabbro bodies range in age from 404 ± 3
Ma to 431 ± 3Ma.
Limitations with the Models
The previous Trinity tectonic models are based on geochemical, structural and
geochronological data. Early research undertaken on the Trinity Complex was
mostly based on geochemistry and petrology and lacked detailed structural field data
(Brouxel and Lapierre, 1988; Jacobsen et al., 1984; Grau et al., 1995; Quick, 1986).
The geochemical data indicated that the different mafic units are related, but were
inconclusive as to whether or not the peridotite is related to the mafic units.
Jacobsen (1984) and Grau et al. (1995) suggested that the mantle source of the
peridotite differ from the source that generated the gabbros, whereas Brouxel and
Lapierre (1988) propose that the units form a cogenetic suite. Both the studies by
Jacobsen (1984) and Brouxel and Lapierre (1988) were based on work including
LREE and Nd-isotopic ratios.
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More recently, Boudier et. al (1998) and Cannat and Lecuyer (1991)
presented structural work based on field studies of a few selected areas. The models
o f Boudier et. al (1989) and Cannat and Lecuyer (1996) differ considerably in their
interpretations of how the gabbro bodies formed. While the model by Boudier et. al
(1989) concludes that the Trinity Complex is an ophiolite, the model by Cannat
(1991) presents the gabbro bodies as isolated magma chambers intruded into a
relatively cold peridotite.
Geochronology of the different units in the Trinity Complex, are important
for evaluating these models. For example, field observations o f sheeted dikes
oriented parallel to the peridotite foliation lead Boudier et al. (1989) to propose that
the peridotite and mafic rocks were contemporaneous. However, the peridotite is
dated as 472 ± Ma. (Jacobsen et al., 1994), whereas the gabbro bodies are dated as
404 to 439 Ma (Table 1, Jacobsen et al., 1984; Lanphere et al., 1968; Wallin and
Metcalf, 1998; Wallin, 1995). These age-data weaken the model o f Boudier et al.
(1989). Given the 32 Ma error margin for the peridotite, field work remains an
important tool to establish the relationship between gabbro and peridotite.
The present study has mainly been focussed on one large gabbro body and its
margins in order to study the relationship between gabbro and peridotite in the field,
and to evaluate the earlier models presented. For example, Boudier et al. (1989)
proposed that local faults have caused rotation of the horizontal gabbro peridotite
contacts into steeper positions, whereas Cannat and Lecuyer (1991) proposed that
the inclined contacts are walls o f gabbro magma chambers. However, the model by
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Age Error Method Rocktype Location Reference
480
?
U-Pb auartz-diorite
?
Mattison and Hopson 1972
480
?
Zircon olagiogranite
?
Mattison and Hopson 1972
469-475
? ?
gabbro-tonalite Yreka-Trinity boundary Mattison and Hopson 1972
472 32 Sm-Nd lherzolite China Mountain Jacobsen et al. 1984
450
?
Zircon olagiogranite Boulder Mattison and Hopson 1972
439 18 K-Ar gabbro Scott Mountain Lanphere and et al. 1968
431 3 U-Pb gabbro Bonanza King Wallin and Metcalf 1998
435 21 Sm-Nd microgabbro Float, Mount Eddy Jacobsen et al. 1984
418 17 K-Ar gabbro Scott Mountain Lanphere and et al. 1968
415 3 U-Pb gabbro China Mountain W allin et al. 1995
414
?
Sm-Nd same gabbro China Mountain W allin et al. 1995
404 3 U-Pb gabbro Porcupine Lake Wallin and Metcalf 1998
Table 1. A collection of age data from the Trinity Complex (and the Neoproterozoic
complex north-west of Trinity). Lherzolite is dated at 472 Ma and gabbro ages vary
between 404 to 439 Ma.
Boudier et al. (1989) does not include any information about local faults that could
have caused the rotation and none o f the models have any observations of magmatic
or subsolidus fabrics at the contacts. A subsolidus fabric close to the gabbro contact
would support the model by Boudier et al. (1989), where coupling between fast
mantle flow and slower mafic flow causes shearing along the contact. A magmatic
fabric would support the intrusive model by Cannat (1991). Another big
disagreement between their models is whether the breccias along the margins are
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intrusive-related (Cannat and Lecuyer, 1991) or fault-related (Boudier et al. 1989).
The data presented by Boudier et al. (1989) give no evidence for local faults or
ductile fabrics at the contact and the intrusive model o f Cannat and Lecuyer (1991)
lack convincing evidence (magmatic fabric and reaction rims) for intrusive breccias
and stoping of the host rock. Therefore, in this study I have performed detailed
mapping of the margins o f the gabbro and evaluated the construction of a gabbro
body in the Trinity Complex.
Overview of Present Research
My research focuses on the following issues: 1) to distinguish if the gabbro is
intrusive into the peridotite or a part o f an ophiolitic pseudo-stratigraphic section; 2)
to characterize the internal construction of the gabbro body; 3) to identify the
external processes associated with the gabbro formation; 4) to evaluate previous
models o f the Trinity Complex and 4) to determine whether or not the Trinity
Complex is an ophiolite.
The Seven Lakes gabbro in the eastern part o f the Trinity Complex (Fig. 5)
was selected for detailed field studies. Fieldwork was conducted during the summers
o f 1999 and 2000, and thin section studies were performed from fall 2000 to spring
2001.
My field and laboratory studies show that the Seven Lakes gabbro is
intrusive into the mantle peridotite and not a part of an ophiolitic pseudo-
stratigraphic section. The main evidence for this is stoping o f the peridotite wall
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rocks and sheeted margins. Other observations, such as stoping and crosscutting
relationships between different gabbro phases have helped determine the
crystallization sequence of phases within the gabbro chamber. External processes
displacing the host rock are displayed as stoping, ballooning, diking and roof uplift.
My studies combined with earlier geochemical and geochronological data have shown
that the model o f Cannat and Lecuyer (1991) is a good, but simplistic model, whereas
the model by Boudier et al. (1989) lack supporting data implying that the Trinity
Complex is not an ophiolite.
The body o f this report is divided into three chapters each relating to the
formation of the Trinity Complex and the relation between gabbro and peridotite
rocks. Chapter 2 presents field and microscopic observations o f all the units in the
investigated area. Chapter 3 includes a discussion about field observations and
previous work and chapter 4 summarizes the results from the previous chapters and
gives implications for what problems still needs to be solved.
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CHAPTER 2. FIELD OBSERVATIONS IN THE TRINITY COMPLEX
Regional Geology and the Tectonic Setting of the Trinity Complex
The Klamath Mountains consist o f easterly dipping tectonic terranes that
record a long history o f subduction-related underplating, metamorphism, magmatism
and sedimentation. The Trinity Complex, which was described in chapter 1, is
surrounded by the Eastern Klamath Belt (PEKB) to the east, the Yreka Terrane
(PYT) in the north and the Central Metamorphic Belt (PCMB) to the west (Fig. 5).
The Eastern Klamath Belt is interpreted to be an intra-oceanic volcanic arc
o f Silurian-Devonian age (Wallin and Metcalf, 1998). The volcanic succession
includes flows and breccias o f the basal Copley Greenstone and pyroclastic rocks of
the overlying Balaklala Rhyolite, both intruded by the Mule Mountain trondhjemite
stock. The Copley basalt is composed primarily o f low K-tholeiite and bonitite with
an LREE-depleted pattern. The composition resembles the magmatic rocks in the
Trinity Complex, which is though to be an immature fore-arc basin to the Eastern
Klamath belt intra-oceanic volcanic arc (Wallin and Metcalf, 1998).
The Yreka Terrane is located north o f the Trinity complex and consists of a
series o f thrust sheets that structurally overlie the northwest margin o f the Trinity
complex and is interpreted to represent an accretionary prism (Wallin and Metcalf,
1998). Between the complex and the main structural sequence lies a disrupted zone
that contains Early Cambrian plutonic rocks, pre-Middle Ordovician keratophyric
volcanics and Ordovician mafic blueschist (Saleeby, 1990). The Early Cambrian
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plutonic rocks o f the disrupted zone have been considered ophiolitic in character due
to their spatial association with the Trinity Complex (Saleeby, 1990). Lapierre et al.
(1987) interpreted the Early Cambrian and Ordovician Yreka units to be calc-
alkaline, island-arc volcanism preceding the formation o f the Trinity Complex.
However, dating o f the sedimentary strata in the Yreka terrane indicated deposition
o f mudstones and turbidites in a fore-arc setting next to a Devonian intra-oceanic
convergent margin (Wallin and Metcalf, 1998).
The Central Metamorphic Belt, west o f the Trinity Complex, is primarily
composed o f metabasalt with minor calc-schist and pelites and has been interpreted
as the footwall o f an easterly dipping Devonian subduction zone (Davis, 1968;
Wallin and Metcalf, 1998). The evidence for this is: 1) metasomatism o f the Trinity
Complex by subduction-related de-watering o f the CMB; 2) an inverted
metamorphic gradient that formed when the Trinity Complex was thrust over the
CMB and 3) Devonian isotopic ages of metamorphism in the CMB (Wallin and
Metcalf, 1998).
The Trinity Complex is sandwiched in between the PCMB and the PEKB and
dip slightly to the east. The Trinity Complex consist o f a large peridotite sheet, km-
sized, irregular gabbro bodies randomly spaced within the peridotite, diabase dikes
intruding gabbro and peridotite and pillow lava and sediments thrust onto the
peridotite. For my study, the Seven Lake gabbro (Fig. 5) was selected because of
good exposures o f peridotite, gabbro, dikes and most importantly, the margins o f the
gabbro. Also included in the field area are a few exposures of pillow lava (the
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Copley Basalt) and sediment (the Bragdon Formation). However, the Copley Basalt
and the Bragdon Formation are not important localities in this study about the
relationship between gabbro and peridotite and will therefore not be further
discussed.
Below, I will describe petrology and structures within the ultramafic rocks
(harzburgite, lherzolite, dunite and pyroxenite), the gabbro phases (medium-grained,
coarse-grained and pegmatitic) and the diabase dikes. All descriptions presented are
based on my field data and microscopic observations from the Seven Lakes gabbro
unless cited differently.
Petrology
This study is based on my petrological observations in the field and
microscopic analysis o f samples collected in the Seven Lakes gabbro area. Rock
composition and alteration products were determined from mineral observations in
large (3x2 inch) thin sections.
Ultramafic Rocks (Op)
Previous studies show that the bulk o f the Trinity peridotite is composed o f
plagioclase lherzolite, lherzolite, harzburgite and dunite (Quick, 1981), and is
probably one of the largest bodies of plagioclase lherzolite exposed anywhere in the
world (Boudier et al. 1989). Lherzolite composition dominates the peridotite while
dunite, harzburgite, websterite and wehrlite occur subordinately. My study shows
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that harzburgite dominates around the Seven Lakes gabbro with minor amounts of
lherzolite, dunite and pyroxenite.
Harzburgite
Harzburgite rocks make up almost 80% o f the ultramafic rock in the area of
investigation. These rocks have a red-brown weathered surface and a dark brown
fresh surface. The harzburgite rocks are composed o f about 50-70% olivine, 20-30%
orthopyroxene, 10-20% serpentine, 3-4% spinel and 0-5% talc, chlorite, amphibole,
clinopyroxene and sausserite. Microscopic observations show that talc, chlorite and
amphibole are alteration products occurring in minor amounts along cleavage planes
and around orthopyroxene rims. Plagioclase grains in the plagioclase-harzburgite
samples are rare, anhedral and altered to sausserite (mostly zoisite and clinozoisite).
These sausseritized grains often display a relatively thin (0.5mm) corona o f chlorite
and serpentine. Microscopic studies from several localities, show that the amount of
talc, amphibole and serpentine increases towards the contact o f the gabbro intrusion.
Peridotite xenoliths are usually extremely serpentinized and altered as observed
during my study.
Lherzolite
Plagioclase lherzolite forms blocky, yellow-brown to red-brown weathered
outcrops with extremely rough weathering surfaces in the studied field area. These
rocks are composed o f about 70-80% olivine, 15-20% ortho- and clinopyroxene, 1-
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2% black spinel, 2-10% plagioclase and sometimes amphibole (Quick, 1981).
According to Quick (1981) plagioclase lherzolite grades into plagioclase harzburgite
with a subtle decrease in abundance o f clinopyroxene. Lherzolite and harzburgite
seem to alternate over short distances and it is often difficult to distinguish these
rocks in the field because of their high degree o f serpentinization and weathering.
Dunite
Dunite bodies are green in fresh surface and brown in weathered surface.
According to Quick (1981), dunite in the China Mountain area consists of 98-99%
olivine, 1-2% black spinel and <1% pyroxene and forms outcrops that are much
smoother than the harzburgite. The dunite investigated in this project consists of
83% olivine and 15% serpentine and 2% spinel, e.g more serpentinized.
Pyroxenite (Opx)
Pyroxenite intrusions consist o f 10-20% olivine, 15-25% orthopyroxene, 45-
55% clinopyroxene, <5% pargasite hornblende, 1-5% spinel and 10-15% plagioclase
(my study). However, the plagioclase composition varies from plagioclase-rich to
plagioclase-poor.
Fracture Zones and Shear Zones
The area of investigation is characterized by the occurrence o f highly
serpentinized zones o f peridotite surrounded by less serpentinized peridotite. These
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zones are referred to in this study as fracture zones and contain undeformed black
clusters of magnetite symplectite surrounded by serpentine. These zones best
described as fracture zones, because they lack evidence for shearing and the high
degree of alteration indicate that they may have been a migration path for
hydrothermal fluids.
A large shear zone in the Gumboat Lake area is also highly serpentinized, but
is defined as a shear zone in this study because of the deformed (sheared) mafic
dikes found here. Microscopic analysis shows that this shear zone consists o f 80%
serpentine, 20% magnetite and <1% relict cryptocrystalline grains. The deformed
dikes consist of 60% chlorite, 20% garnet and 15% actinolite and hornblende, and
5% apatite, sericite, sphene and sausserite. Some deformed dikes in the Gumboat
shear zone are completely altered to dark green chlorite with only a narrow pink
band o f rodingite left. Non-deformed gabbro dikes intruding into this shear zone
have a 5-10 cm reaction rim on either side o f the dike consisting o f garnet, zoisite
and chlorite. Similar gabbro dikes observed outside the shear zone lack this reaction
rim assemblage.
Gabbro (Dg)
The fieldwork o f this study concludes that there are four major gabbro phases
within the Seven Lakes gabbro. These phases include coarse-grained phase,
medium-grained phase, pyroxene-rich cumulates and pegmatitic / trondhjemitic
phases. The coarse-grained gabbro is estimated to make up about 60% of the
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exposures, while the uppermost medium-grained gabbro make up approximately
20%, the foliated cumulates about 20% and pegmatitic / trondhjemitic phases about
10% (Fig. 9).
Coarse-grained Phase
The coarse-grained gabbro phase consists o f 65% sausserite (zoisite,
clinozoisite and albite), 30% pyroxene and amphibole (in some samples totally
altered to chlorite), 5% spinel and rare grains o f garnet. Large grains o f plagioclase
(<10 mm) have almost totally altered to fine-grained sausserite. Prior to alteration,
the phase was probably equigranular and idiomorphic to hypidiomorphic with u n
sized plagioclase and pyroxene. Subsequent to alteration, chlorite and sausserite
display fine-grained textures separated by relict plagioclase boundaries o f well-
preserved albite. Microscopic studies also show that euhedral to subhedral spinel is
clustered in the chlorite regions. A triangular network o f spinel grains is forming a
texture similar to a spinifex texture (bladed olivine thought to provide evidence for
an ultramafic magma which may form in ultramafic plutonic rocks, Hyndman,
1985). The samples collected from the sheet-like intrusion southwest o f the main
gabbro have experienced more alteration compared to the samples within the main
gabbro.
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P
N
t
2 k m
Q uatenw y glacial deposits
Mesozoic granite and diorite
[ m Q Mississippiun Bragdon Form aion
Devonian Copley Greenstone
H | Devonian breccia
Devonian medium grained gabbro
□ Devonian coarse grained gabbro
m Devonian pegmatitic gabbro
Ordovician to Devonian pyroxaiile
O rdoviaan peridotite
Figure 9. Distribution o f gabbro phases within the Seven Lakes gabbro.
N >
vO
Medium-grained Phase
These rocks consist of 60% sausserite and 20% amphibole 20% pyroxene.
The sausserite has 45% zoisite and clinozoisite, 45% chlorite and 10% albite. Garnet
also exists subordinately. There is slightly more amphibole within this phase
compared to other gabbro phases. Otherwise the composition is similar. The
analysis also show that some samples displays coronas of amphibole around 70% of
the pyroxenes, while the other samples lack corona texture and only display
amphibole alteration within the pyroxene. The pyroxenes surrounded by amphibole
coronas have the central part replaced by chlorite. Plagioclase is altered to
sausserite.
Cumulate Gabbro
Plagioclase-rich pyroxenite samples reveal a primary composition of 50-70%
pyroxene, 20-40% o f plagioclase and 10% amphibole. Pyroxene is altered to talc
and tremolite, while plagioclase is altered to sausserite. A few unaltered pyroxenes
show augite composition, amphibole has hornblende composition and plagioclase
consists o f zoisite, clinozoisite, chlorite and sericite.
Pegmatitic and Trondhjemitic Phases
In proximity to the margin of the intrusive gabbro complex, the pegmatitic
gabbros occur as hornblende gabbro dikes within the mantle peridotite and as
homblende-pyroxene gabbro dikes truncating other gabbro phases within the
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intrusive gabbro complex. The pegmatitic hornblende gabbro consists o f large
hornblende crystals (20-30%) in a matrix o f fine-grained sausserite (60-70%) and
spinel (3%). The pegmatitic pyroxene-homblende gabbro consists o f sausserite
(55%), pyroxene (30%), amphibole (15%) and spinel (2%). In both phases, the
amphiboles have a pargasite hornblende composition and the sausserite consist of
zoisite, clinozoisite and albite. The pyroxene in the pyroxene-rich samples consist
dominantly o f clinopyroxene (augite), but some orthopyroxene is also present. The
pegmatitic gabbro phase is water-rich and contains dendritic hornblende crystals,
which are up to 40 cm long. The high water-content may explain the high
serpentinization and hydro-garnet crystallization along the margins o f pegmatitic
gabbro intruding the peridotite observed in the Gumboat Lake area. Within the
pegmatitic homblende-pyroxene gabbro phase, pyroxene is rimmed by dark green
hornblende both seen in the field and in the microscope. The amphibole is in general
not altered. Though in the contact zone where gabbro intrude the Gumboat Shear
Zone, hornblende grains are partially altered to chlorite, uralite, sausserite and hydro-
garnet along cleavage planes, edges and other planes in the amphibole. Some
amphibole grains contain small inclusions o f anhedral magnetite along cleavages and
kink-bands.
Trondhjemite phases are relatively common in the peridotite and interpreted
to be locally derived since they are unusually thin and irregular, lack association to
larger intrusive bodies and appear to be restricted to the plagioclase lherzolite
(Quick, 1981). Trondhjemite veins also crosscut the isotropic gabbro and the upper
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cumulate sequence (Lecuyer and Fourcade 1991). Most trondhjemite melts in the
field area are seen close to pyroxenite, where pyroxenite blocks are fractured and
stoped by trondhjemite melts, which also pre-date and are crosscut by diabase dikes.
Gabbro Shear Zones
Ductile shear zones in the Seven Lakes field area consist o f 45% zoisite and
clinozoisite, 35% amphibole, 10% chlorite, 5% albite and 5% sericite. Compared to
the medium-grained gabbro phase with 60% sausserite and 20% amphibole 20%
pyroxene, this section has more amphibole and less pyroxene, indicating that
pyroxene probably was replaced by amphibole and chlorite. Chlorite and albite are
alteration products that form thin bands that wrap around the larger pyroxene and
amphibole grains, whereas sausserite (zoisite and clinozoisite) is the alteration
product of plagioclase.
Gabbro Margins (Db)
Sheeted Margins
Along the Seven Lakes gabbro margin there are 10-50% gabbro dikes (of the
total area in the peridotite) crosscutting the fabric in the peridotite. These dikes are
mostly pegmatitic and coarse-grained, but medium-grained dikes are also present.
The composition in these dikes is similar to that within the various main gabbro
phases. Reaction rims are observed along the contacts o f the dikes and probably
formed when the hotter gabbro dikes intruded into the colder peridotite. These
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reaction rims were subsequently metamorphosed and now display a mineral
assemblage o f tremolite, talc, serpentine and chlorite (Fig. 10). Microscopic studies
show that the reaction rims are composed o f amphibole (60-85%), chlorite and talc
(10-35%), magnetite (3-4%), chromite (1%), epidote and sphene (0.5%). The
amphibole composition is usually hornblende or tremolite.
Figure 10. Contact zone between gabbro dike within a peridotite host rock. From
the left to the right: serpentinized dunite (01), a small zone o f serpentine (S),
amphibole (A), serpentine and chlorite (C) and sausserized gabbro (PI). Plain light.
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These reaction rims vary in width from millimeters to 20 cm between different
localities. Microscopic studies display that the contact zones have a distinct
compositional transition from serpentine (closest to the peridotite), amphibole
(middle) to chlorite and serpentine (furthest from the gabbro dike) (Fig. 10). The
serpentine band are generally thin (0.5-3 mm) and display a sharp contact to the
amphibole (homblende-tremolite) band, where the lath shaped amphibole grains are
oriented perpendicularly or obliquely to the contact. The amphibole grains grade
into more fine-grained amphibole before they gradually transform to a chlorite and
serpentine band. Closest to the gabbro margin fibrous serpentine (bastite) occur
perpendicular to the gabbro contact.
Stoped Blocks
Important features along the gabbro margin are distinctive zones o f stoped
blocks. Stoped blocks within the gabbro have previously been referred to as
xenoliths and inclusions (Quick, 1981; Boudier et al., 1989; Cannat, 1996; Lecuyer,
1990). The inclusions are appropriately identified as stoped blocks because they are
derived from magma intruding, fracturing, displacing and reacting with the cold
peridotite host-rock. The gabbro carries magmatic foliation and lacks a subsolidus
foliation precluding a tectonic origin for the blocks. Even though most stoped blocks
still preserve a fabric and a harzburgite composition, there are blocks which are
completely metamorphosed and better classify as xenoliths instead o f stoped blocks,
since their original composition, fabric and location are impossible to determine.
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Ultramafic rocks in the investigated field area occur as stoped blocks within
medium-grained gabbro, pegmatitic gabbro and feldspathic pyroxenite adjacent to
the gabbro margins. This study also shows that stoped blocks o f peridotite are found
around the margin o f the gabbro body and within narrow zones within the gabbro
body. Composition and structures in the peridotite xenoliths resemble the host-rock
peridotite samples. Hence, there is a pronounced difference in degree of
serpentinization o f olivine and alteration o f pyroxene within the blocks. Microscopic
studies show that pyroxene grains (20-30%) are completely altered to talc and only a
small amount of olivine (3-40%) is remaining (from originally 30-60%). Instead
serpentine makes up 30-70% o f the peridotite together with smaller amounts of
amphibole and chlorite. Amphibole (tremolite-homblende) is commonly seen as
interstitials and as corona textures around orthopyroxene. The magnetite-symplectite
observed along the cleavage planes in pyroxene and amphibole probably formed
during hydrothermal alteration. Microscopic studies also show that reaction rims
between peridotite and gabbro consist of: a) a 3 mm wide band o f talc (closest to
peridotite); b) a 1-4 mm wide band o f bladed to lath shaped amphibole crystals
oblique or perpendicular to the boundary and c) a 0.5-3 mm wide band o f serpentine
and chlorite (closest to the gabbro-pyroxenite) (Fig. 11). This reaction rim
assemblage is similar to the contact rim assemblage between gabbro dikes and
mantle peridotite.
Within the stoping zones in the field area, most of the stoped blocks are
recognized as peridotite or pyroxenite but there are blocks in the cm- to m- range that
35
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are completely altered and the original composition is difficult to derive. Field
observations show that these xenoliths display purple and red colors from talc and
green colors from tremolite and chlorite. Large xenoliths have a dm- wide, red,
reaction rim of talc surrounding a green interior o f tremolite. The composition
changes with different xenoliths from more talc-rich to more tremolite-rich. My
microscopic studies show that the less altered samples consist of 65-75% amphibole,
20-30% chlorite and ~4% spinel, while the more altered samples consist o f around
50% amphibole, 25% chlorite, 25% talc and 2% spinel. Amphibole composition is
predominantly tremolite. Within the less altered xenoliths, porphyritic large bladed
amphibole crystals are found. These crystals might have replaced pyroxene in an
altered peridotite or pyroxenite. Microscopic spinel grains have crystallized along
the cleavage planes within these grains. The matrix is totally replaced by tremolite
and chlorite. Chlorite increases towards the margin o f the xenolith, where it
completely dominates.
Diabase Dikes
Diabase dikes are the most common dikes within the Seven Lakes complex.
My microscopic study shows that the diabase dikes consist o f tremolite (90%),
sericite and sausserite (7%) and chlorite (3%). Plagioclase has altered to sericite and
sausserite and chlorite is partially replacing tremolite. Tremolite is replacing
clinopyroxene according to studies done by Quick (1981).
36
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Figure 11. b) Reaction rims around a stoped block. From left to right: gabbro
(sausserized plagioclase) (PI), a zone o f chlorite (C), a zone o f amphibole (A) and
furthest to the right, talc (T) and olivine (01) within the peridotite. Plain light.
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Structure
In this section, my field and microscopic observations of structures from
various units o f the Seven Lakes gabbro are described. These observations include
orientations and textures o f foliations and lineations within the gabbro and peridotite
units, as well as textures within shear zones, sheeted dikes and stoped blocks. These
observations help distinguish the construction of the Seven Lakes gabbro and
associated emplacement processes affecting the mantle host rock.
Ultramafic Rocks (Op)
Harzburgite and Lherzolite
The harzburgites display a well-defined foliation characterized by alignment
o f black spinel and elongated brown pyroxene. This fabric is easily recognized in
Seven Lakes field area due to weathering o f olivine and piagioclase. In thin section
(samples collected from the Seven Lakes field area), this foliation is expressed as
aligned pyroxene phenocrysts (axial ratio 1:2), olivine grains (axial ratio 1:1 to 1:3)
and spinel grains clustering in thin veins parallel to the alignment o f the pyroxene
grains (Fig. 12). Peridotite foliation and lineation mapped by Boudier et al. (1989)
show a consistent pattern at the scale of the complex. Their data indicate that the
foliation dips steeply and strikes NW-SE, while a subhorizontal lineation well
defined by pyroxene and spinel trends NW-SE (Fig. 13). These data are consistent
with foliation and lineation mapped in the northwestern and western part o f my field
area (Fig. 14-15). However, this study suggests that foliation and lineation close to
38
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the Seven Lake gabbro is deflected from the regional peridotite pattern and wrap
around the gabbro intrusive with a margin parallel orientation (Fig. 14-15). This
margin parallel structure is interpreted as the result of strain caused by the intrusion
o f the gabbro. The proposed structural aureole as measured from the intrusive
contact to the outer aureole where margin parallel foliations switch to NW-SE
striking foliations is 1-2 km wide.
Figure 12. Microphotograph of foliation in peridotite defined by elongate ortho
pyroxene (Opx), olivine (01) and spinel (black) (polarized light).
39
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Microscopic studies show that the harzburgite texture within the aureole is
porphyritic with subhedral, elongated orthopyroxene phenocrysts (2-7 nun) aligned
in a matrix of olivine (1-4 mm) with microcrystalline and hypidiomorphic character.
The olivine crystals contain a network o f serpentine veins, which has also been
observed in the Oman peridotite (Boudier et al., 1996). In all the peridotite samples,
olivine crystals are recrystallized (polygonized) into smaller euhedral to subhedral
grains separated by a network o f serpentine veins and almost free o f dislocation
structures. This texture is also described by Boudier et al. (1989) and by Quick
(1986) for samples from the Trinity peridotite. In contrast, orthopyroxene crystals
preserve deformation features as 80% o f the grains display deformation lamellae,
undulose extinction, bending, kinking and fracturing.
Figure 13. Poles to regional foliation (N=547) and lineation (N=374) mapped in the
Trinity peridotite (from Boudier et al., 1989): a) the foliation is steeply dipping and
strikes NW-SE, while the lineation (b) is subhorizontal and trends NW-SE.
40
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4
Soapstone
Pond _
Lakes
Castle
Lake
L Tectonile foliation
| (dip 60*90 degrees)
f
Quaternary glacial deposits
E 3 Mcaozoic granite and diorite
Ivvl Mtssisaippian Bradgon Formation
m H Devonian Copley Greenstone
m Sloping zones
Devonian gabbro
Ordovician to Devonian pyroxcnite
■ Ordovician peridotite
O 1 2 K m
Figure 14. Peridotite foliation around the Seven Lakes gabbro (this study).
Reproduced with permission o f th e copyright owner. Further reproduction prohibited without permission.
8 $
US#
8
B W rw rar
p s K g m
N N \ \ M L r A
✓ / /
Tectonile lineation
(d ip 0*30 degrees)
f
E 3 Quaternary glacial deposits
□ Mesozoic granite and diorite
□ Missiisippian Bradgon Formation
H | Devonian Copley Greenstone
| Sloped zones
| S J Devonian gabbro
Ordovician to Devonian pyroxcnite
| ^ | Ordovician peridotite
O 1 2km
Figure IS. Tectonite lineation around the Seven Lakes gabbro (this study).
Dunite
Dunite can be subdivided into three different groups based on their geometry:
1) large and tabular bodies with a thickness up to 100 m; 2) small (cm- to m- wide)
and irregular-shaped patches; and 3) small (cm-wide) tabular bodies that flank some
clinopyroxene-rich dikes (Quick, 1981). A gradation exists between plagioclase
lherzolite and dunite in the range o f 15 cm to several meters (Quick, 1981). In the
area o f investigation, dunite is seen as tabular bodies within the peridotite and as
decimeter-wide bands at the intrusive margin zone. Internal fabrics can not be
clearly recognized in the field, since the more resistant orthopyroxene is missing in
the dunite and the weathering creates a smoother outcrop than for harzburgites.
Olivine grains are polygonized and recrystallized at high temperature
according to studies from dunite in the China Mountains (Quick, 1986). This texture
is also observed in samples from the Seven Lakes area during this study, which also
shows that the olivine texture is hypidiomorphic and equigranular, where olivine
crystals are rounded to slightly elongated in different orientations. The olivine
contains fractures filled with serpentine and spinel grains, which were possibly
formed by hydrothermal activity during intrusion o f the mafics. This concentration
o f aligned magnetite along serpentinized channels is described by Peacock (1987) as
a strong foliation that can be measured in the field. Gabbro veins intruding the
dunite are cm-thick parallel to the serpentinized, magnetite-filled channels, which
thin out to mm-width when crosscutting the channels. This structure indicates that
43
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the gabbro veins either pre-date or were formed contemporaneously with the
serpentinized channels.
Pyroxenite
Pyroxenite occurs within the Seven Lakes area as large (5-50m wide and 10-
100’s-m long), green and tabular bodies both within the peridotite and the gabbro.
Pyroxenes are parallel and elongated forming a cumulate texture and foliation
together with elongated plagioclase grains (Quick, 1981). His microscopic
observations indicate that the texture is hypidiomorphic granular and anhedral
plagioclase fills the interstices between subhedral ortho- and clinopyroxene (Quick,
1981).
Shear Zones and Fracture Zones
Fracture zones are best recognized along the topographic ridges in my field
area where exposures are excellent and where peridotite gradually transfers into light
green and serpentinized zones. These zones are generally 20-50 m wide, but they
can also be significantly wider or narrower. For example, in the Picayune Lake area,
there are serpentinized zones over 150 m wide. These zones may have been highly
migrated by hydrothermal fluids and therefore display high degree of
serpentinization. Field and microscopic studies indicate that the fabric is relatively
weak in these zones and defined by magnetite symplectite. Magnetite symplectite is
44
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formed when magnetite pseudomorphically replaces pyroxene starting to crystallize
along cleavage planes and eventually cover almost entire pyroxene grains.
The Gumboat Lake shear zone is 200 m wide and can be mapped for least a
few hundred meters within the peridotite before it is covered by talus. This shear
zone is located 100 m away and parallel to the contact o f the gabbro. Field and
microscopic studies indicate that the shear zone contains both undeformed and
deformed gabbro dikes that are both assumed to be related to the adjacent main
gabbro, since these dikes are included in a swarm of dikes forming a sheeted margin.
From north to south along a profile across the shear zone (Fig. 16), harzburgite
grades into serpentinized peridotite displaying a network of small shear zones and a
spheriodal weathering pattern.
Elevation
(m eters)
Gabbro
Peridotite
Shear Zone
2060
2040
2000
Figure 16. The Gumboat Lake shear zone including serpentinized peridotite with a
spherical weathering pattern (circles), chloritized peridotite (gray) and gabbro dikes
(white). The solid lines represent the peridotite foliation. (Profile A-A’ in Figure 9).
45
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Several meter-wide, light blue-green zones o f highly serpentinized peridotite occur
within this zone. Furthermore, there are dark green zones o f serpentinized and
chloritized peridotite often associated with pink melt residues (deformed gabbro)
randomly distributed within the shear zone (Fig. 17). Towards the margin o f the
gabbro body, the serpentinized zone progressively grades into a 100 m wide zone o f
harzburgite and a zone o f stoped peridotite blocks within the gabbro. A microscopic
view o f the shear zones, reveal that magnetite forms 1mm thin anastomosing bands
that wrap around mm- wide clusters of serpentine. The anastomosing bands of
magnetite in the shear zones are similar to magnetite bands parallel to foliation
planes in harzburgites. However, these bands in the shear zone are more developed
(e.g. more magnetite crystals connecting into more continues bands) than in the
harzburgite. One likely explanation is that magnetite crystallized from fluids
migrating along grain boundaries through the peridotite. No kinematic indicators
can be found at the outcrop or microscopic scale in the highly serpentinized zones.
However, microscopic studies o f the deformed gabbro dikes within the shear zone
display pressure shadows around garnet and amphibole crystals, indicating a south-
side-up (pluton side up) and a reverse sense o f shear (Figure 18).
46
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Figure 17. Dark green zones of serpentinized and chloritized peridotite associated
with pink melts o f rodingite (altered and deformed gabbro dikes) and surrounded by
serpentinized fault planes (solid white lines) in the Gumboat Lake shear zone
(looking south).
47
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Figure 18. Ductile shear structures in the Gumboat Lake shear zone indicating
pluton side up kinematics. S= serpentine, G= garnet, A= amphibole, C=chlorite.
Plain light.
48
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Gabbro Sequence (Dg)
Layering
Field observations show that layering within the gabbro unit is rarely seen as
grain size (pegmatitic) layering (Fig. 19), modal (compositional) graded layering and
schlieren (wispy) layering, the latter characterized by thin and vaguely defined
modal layers that unobtrusively appear and fade out.
Figure 19. Pegmatitic layering in the coarse-grained gabbro phase at one location
along the Pacific Crest Trail. The lens cap is 5 cm wide.
49
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Pegmatitic layering occurs occasionally within coarse-grained gabbro as
discontinuous, 10 cm-wide layers and 10 cm-wide nodules o f coarse grained to
pegmatitic gabbro in a medium to coarse-grained gabbro phase. These layers are
rich in feldspar and pyroxene and poor in mafic minerals such as hornblende and
biotite. Within the field area, the medium-, coarse- to pegmatitic layers can be found
for several meters before they grade into a more isotropic texture. The orientation o f
the layering varies depending on locality. For example, data from the Dunsmuir
area, show that the cumulate gabbros have a foliation o f N80E/ 20S (Brouxel and
Lapierre, 1988). The rare layers found in the Seven Lakes gabbro vary in orientation.
The central parts o f the field area (C) is shallowly (10-30°) dipping layering, whereas
the other parts closer to the contacts show steeper (50-85°) layering (Fig 20).
Figure 20. Equal Area plot of poles to layering in the Seven Lakes gabbro domains:
A) Gumboat Lake, B) Soapstone Pond, C) PCT 2, D) Castle Lake, E) Cliff Lake and
F) PCT 1.
N
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Magmatic Foliations and Lineations
Magmatic foliations and lineations are defined by elongated pyroxene and
hornblende crystals aligned parallel to feldspar crystals in the medium-grained and
coarse-grained gabbro phases. Fabric intensities are based on the scale M0-M5,
where MO is isotropic, M l is very weak, M2 is weak to moderate, M3 is moderate,
M4 is relatively strong and M5 is seen as distinct layering (Paterson, 1998). Field
observations show that the planar fabrics are moderately strong in the cumulate
phase (M3) and weak in the medium- and coarse-grained phases (M l-M2). The
pegmatitic phase often lacks a fabric (MO). Measurements and foliation analyses
from this field study are summarized in maps and local stereonet analyses for
different domains o f the Seven Lakes gabbro (Fig. 21-22). The analysis in these
figures show that there are probably multiple fabrics (or one fabric with multiple
orientations) involved in the formation of this gabbro body. There is a steeply
dipping magmatic foliation striking NE-SW preserved in the Cliff Lake, Castle Lake
and Boulder Peak area. This foliation is parallel to the orientation of sheets in the
southwestern part of the Seven Lakes gabbro. Another foliation changes orientation
from NW-SE in the Soapstone Pond area and the Gumboat Lake area to striking E-
W in the middle parts o f the mapped area. This foliation changes to more shallowly
dipping in the central part o f the field area. Magmatic lineations also show a
relatively scattered pattern (Fig. 23-24). However, most lineations in the middle
parts are shallowly trending NE-SE, except for the slightly steeper lineations within
the domains called Boulder Peak and Soapstone Pond.
51
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*
Soapstone
Basin
Lakes
Helen
Castle
Lake
Magmatic foliation
(60*90 degrees)
L M agmatic foliation
f (30*60 degrees)
foliation
l degrees)
f
h
Magmatic
(0*30 degn
[Qgd | Quaternary glacial deposits
E l Mesozoic granite and diorite
E 3 Mississippian Bradgon Formation
H i l l Devonian Copley Greenstone
^ ^ g Sloping zones
[jjS lJ Devonian gabbro
^ ^ g Orovician to Devonian pyroxenite
Ordovician peridotite
O 1 2 k m
Figure 21. The magmatic foliation pattern in the Seven Lakes gabbro (this study).
U 1
N >
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Gumboat Lake
N
PCT2 Cliff Lake
Castle Lake
N - J 7 \
1
r )
Y S J I J
w *
PCT1
N
N -
Helen Lake
N
Boulder Peak
N
Figure 22. Equal area plots
o f poles to magmatic foliation
in the Seven Lakes gabbro
<
I (•)
\ ^ c
^ y
N ■
$*0
£
N » 3 0
V
2 k m
E 3 Quaternary Glacial dcpoaita
Meaozoic gran tie and dkttite
I M b! Misaiasippian Bragdon Formation
|P c g | Devonian Copfcy Gircnatooe
E D Devonian breccia
Devonian gabbro
E l
Ordovician to Dcvooian pyroxcnite
m | Ordovician peridotite
— StructuralDomaina
Soapstone Pond
< _ n
O J
io n o f th e copyright owner. Further reproduction prohibited without permission.
H M M M M
Castle
f
1
Magmatic lineation
(dip 0*30 degrees)
;
Magmatic lineation
(dip 3 0 4 0 degrees)
|Q fJ Quaternary glacial deposits
Mesozoic granite and diorite
^ Mississippian Bradgon Formation
^ Devonian Copley Greenstone
m Sloped zones
^ Devonian gabbro
Ordovician to Devonian pyroxenite
Ordovician peridotite
O 1 2 k m
Figure 23. The magmatic lineation pattern in the Seven Lakes gabbro (this study).
Reproduced with permission o f th e copyright owner. Further reproduction prohibited without permission.
Gumboat Lake
N
PCT2
N
/ *J
w ) > J
’ (v I
i* y
-----------
N • 4
Helen Lake
N
Boulder
N
N • 1§
Figure 24. Equal area plots
o f ploes to magmatic lineation
in the Seven Lakes gabbro.
< _ n
Ui
Castle Lake
N
r
N • 14 N » l J
2 k m
E 3 Quaternary glacial depoaita
Fm « 1 Mesozoic granite and diorite
|M b j MUeiuippien Bngdoa Focnutioo
|D « g | Devonian Copley Greautane
E D Devonian breccia
Devonian gabbro
Ordovician to Devonian pyroxcnite
Ordovician peridotite
StructuralDomains
Coarse-grained Phase
More than 60% o f the gabbro exposures consist o f coarse- to medium-grained
gabbro and this phase is seen in the main field area and in the western part o f the
Castle Lake area. Field observations indicate that this phase consists mostly o f
coarse-grained gabbro but grain-size changes locally within meter-scale into
medium-grained, fine-grained and pegmatitic gabbro. The coarse-grained gabbro
■'hase has a magmatic but relatively weak fabric (M l-M2). Foliation is determined
by the alignment o f green pyroxene (and more rarely hornblende) and plagioclase
(Fig. 25). Microscopic studies show that the coarse-grained gabbro phase displays
deformation features in plagioclase such as bending, undulose extinction, subgrains
with cuspate boundaries and recrystallization to small grains along the boundaries.
However, this deformation is minor and the foliation is primarily magmatic.
A clear crosscutting relationship between the foliated pyroxene-rich cumulate
gabbro phase and the less pyroxene-rich coarse-grained gabbro phase (this unit) has
not been found. Instead, field observations show that there is a gradational transition
from the cumulate phase to medium-grained gabbro and finally coarse-grained
gabbro in the Castle Lake area. The coarse-grained gabbro is overlying the cumulate
gabbro phases and the foliation is steepening as cumulates turn into the coarse
grained gabbro toward the center o f the gabbro body.
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Figure 25. Microphotograph o f coarse grained gabbro foliation in the Cliff Lake
area. Plain light. P=Pyoxene, Pl=Plagioclase, white line=foliation.
Medium-grained Phase
Field observations show that the medium- to fine-grained gabbro section
predominantly occurs on ridges, in the southwestern part o f the gabbro, and as
scattered entities (Fig. 9). The medium-grained gabbro is interpreted, based on my
mapping, to represent an upper gabbro section overlying the coarse grained gabbro
phase observed in the central part o f the gabbro. However, there are also other
localities with medium-grained gabbro lower in the coarse-grained section.
Microscopic analyses show that the texture is hypidiomorphic with relatively large
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(2-5 mm) amphibole and orthopyroxene grains forming a magmatic foliation in a
more fine-grained sausserite matrix (0.25 mm) (Fig. 26). Microstructures indicate
that the fabric in the medium-grained phase is magmatic with a weak subsolidus
overprint o f fractured and boudinaged pyroxene linked by fibrous chlorite.
Microscopic analysis also show that chlorite is neo-crystallized in pressure shadows
around larger grains o f pyroxene and amphibole, which are bent and display
deformation twins. Plagioclase is commonly completely altered to serpentine.
Hence, the magmatic fabric was affected by a low-temperature deformation.
Figure 26. Microphotograph o f medium-grained gabbro in the Soapstone Pond area.
Plain light. P=Pyroxene, Pl=Plagioclase, solid line=foliation.
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Cumulates
Field mapping shows that the gabbro cumulate units only occur in the Castle
Lake area, where they consist o f feldspathic pyroxenite grading into pyroxene-rich
gabbro in a distance o f 300 m (further away from the ultramafic margin). Structural
data indicate that the foliation in these units dips moderately to the NW. Microscopic
studies show that the texture is hypidiomorphic and the grains vary in size.
Pyroxenes are parallel and elongated (axial ratio 1:2), forming a cumulate texture
and foliation together with elongated plagioclase grains (axial ratio 1:4), where
amphiboles are seen as corona textures and interstitials to pyroxene and plagioclase.
The observations indicate further that the gabbro cumulate phases intruded into the
peridotite. Several localities show blocks o f stoped peridotite entrained within the
cumulate gabbro phases.
Pegmatitic and Trondhjemitic Phases
Field observations show that the pegmatitic phase occurs as steeply to
moderately dipping intrusive dikes mostly along the margins o f the gabbro, but also
within the gabbro. This phase crosscut all other phases except for the leucocratic
phase and the diabase dikes. The leucrocratic phase occurs as m-wide dikes and cm-
wide veins crosscutting all other phases but the diabase dikes (my study). This
leucocratic phase, is located at the gabbro margins but also occurs close to large
pyroxenite bodies within the gabbro (for example in Soapstone Pond area).
Microscopic studies indicate that the pegmatitic gabbro phases are porphyritic with
59
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large euhedral to subhedral phenocrysts o f pyroxene and amphibole in a fine-grained
matrix o f anhedral to subhedral sausserite (zoisite, clinozoisite, chlorite, serpentine
and albite). Amphibole grains vary in size (1 mm-20 cm) depending on whether the
sample was collected close to the intrusive margin where the grains are smaller or in
the middle of a dike intrusion where the grains are larger. Relict plagioclase grains
are equigranular (0.5-1 mm) and euhedral to subhedral. Recrystallized albite grains
(part o f the sausserite) are anhedral and interstitial to pyroxene and amphibole. At
some locations, clinopyroxene grains are rimmed by hornblende (Fig. 27).
Figure 27. Pyroxene crystals rimmed by hornblende in the pegmatitic phase in the
Gumboat Lake area. Lens cap is 5 cm wide.
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My microstructural analysis shows that plagioclase displays undulose extinction and
sometimes exhibits subgrains, recrystallization and corrosion shape with very
irregular and sharp boundaries. The amphibole and the pyroxene grains are kinked,
bent, broken and display subgrains.
Gabbro Shear Zone
Field observations indicate that there are few brittle and ductile shear zones
present within the gabbro. Ductile shear zones are typically 10-30 cm-wide and
display SC-fabrics. In this study, ductile shear zones are an important tool to
compare and contrast the mineralogy and fabrics formed at subsolidus conditions to
the magmatic conditions in the gabbro phases since the rocks are strongly altered and
deformation features are almost never preserved. Microstructural studies indicate
that the shear zone texture is allotriomorphic with large, anhedral amphibole grains
which are mechanically deformed (bend, kinked and sheared) and range in grain size
from 0.25 mm to 4 mm. The matrix o f sausserite, chlorite and albite is equigranular
and the grains are <0.25 mm. The foliation is relatively weak and indicated by thin
bands of fine-grained chlorite, albite and amphibole wrapping around larger
amphibole grains in a subsolidus foliation (Fig. 28). Pressure shadows with chlorite
occurs at the edges o f the amphibole grains. The deformation displayed in the
sample is due to an extensional crenulation cleavage with a right lateral sense of
shear. Albite, chlorite and amphibole are neo-crystallized and show evidence for
minor deformation.
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Figure 28. Microphotograph of gabbro shear zone from the Cliff Lake area, where
S=Shear plane, C=Cleavage plane, P=Pyroxene and Pl=Plagioclase.
Gabbro Margins (Db)
Sheeted Margins
Field observations show that the sheeted marginal gabbro dikes display dips
varying from steep to moderate (Fig. 29) and sometimes even shallow (Fig. 30).
Stereonet analyses of the dike orientations are presented for the Lake Helen area, the
Gumboat Lake area and the Castle Lake area (Fig. 31). The dikes adjacent to the
margins o f the gabbro are intrusive into the peridotite and in to the cumulate rocks
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(crosscut foliation), probably related to the main gabbro body (same composition and
grain size) and have variable dip. Field evidence o f interfingering and branching in
several of the dikes in the Gumboat Lake area, indicates that the dike tips propagated
towards the north. Dike dispersion and branching into a large network of cm-wide
veins, occurs at a relatively short distance o f ~ 10 m. In addition, the dikes display
chilled margins.
Figure 29. Moderately to steeply dipping sheeted gabbro dikes intruding peridotite
in the Lake Helen region. Dike widths in the photo 0.5-1 m. Looking east.
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Figure 30. Shallowly dipping coarse-grained dike intruding into the gabbro
cumulates and peridotites in the Castle Lake area. Upper part show stoped peridotite
blocks in a matrix o f pyroxenite and below the dike, dunite is preserved as a
fragment within the gabbro dike. The book is 15 x 10 cm long.
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Castle Lake
N
Gumboat Lake
N
N * 21
2 k m
m4 Quaternary glacial depoaiti
| m | ] Meeoeoic granite and diorile
Mtaatotppian Bragdoo Formation
Devonian Copley Greenetone
|^D b J Devonian breccia
Devonian gabbro
p x | Ordovician to Devonian pyroxenite
Ordovician to Devonian peridotite
Helen Lake
N
N« 18
Figure 31. Equal area plots of poles to sheeted margins in the Seven Lakes gabbro
O n
U l
Stoping o f Ultramafic Rocks
Field observations of ultramafic rocks across the stoping zones show that: (a)
host rock peridotite is intruded by a network o f thin gabbro veins; (b) small rounded
peridotite blocks are completely isolated within the main gabbro and (c) large tabular
peridotite rafts are situated within the gabbro magma adjacent to the margin (Fig.
32).
A A-
P egm otltie coarse-grained
Harzburgite (Hz) Serpentine Hz Stoping zone gabbro gabbro
• * *
|M > • • • • • • • \ j Y ''
S L iimmim
Elevation
(meters)
M W
2020
2000
Figure 32. Profile of the Gumboat Lake stoping zone (A-A’ in Figure 9).
Further, pegmatitic gabbro dikes intruding the host-rock contain rounded fragments
(10-20 cm) o f peridotite host-rock (Fig. 33). In zone (a) a network o f thin
hornblende-rich gabbro veins (1-10 cm wide) starts to intrude and fracture the
peridotite host-rock into smaller fragments. These fragments range in size from cm
to meters and are mostly square-shaped (Fig. 34). Further towards the gabbro, small
peridotite blocks (b) are completely isolated in a gabbro or feldspathic pyroxenite
melt. These blocks are partially altered to chlorite, tremolite, serpentine and talc
along their contacts and reaction rims due to later metamorphism. The black band of
chlorite surrounded by a pink band o f talc is called “black wall” (Schwindinger and
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Figure 33. Rounded stoped blocks o f peridotite in a gabbro dike intruding peridotite
adjacent to the gabbro margin in the Gumboat Lake stoping zone.
Anderson, 1987) and seen in most o f the stoped peridotite blocks (Fig. 35). The
more isolated the ultramafic blocks are within the gabbro, the more mineralogical
changes they have suffered. Further into the main gabbro the peridotite host-rock
blocks are seen as large (5-15 m) tabular rafts (c) completely surrounded by gabbro
magma, but completely preserve the initial composition, fabrics and orientation.
There are also large tabular bodies that are tilted or slightly rotated by the upwelling
gabbro melt. These rafts and tilted blocks are relatively well preserved and not
crosscut by gabbro veins. There is a large variety in size, shape and reaction rims of
stoped blocks within the stoping zone, which indicates that hot gabbro and
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feldspathic pyroxenite magmas fractured and translated fragments o f peridotite host-
rock within the magmas. The magmatic foliation within the matrix magmas suggests
that there was no major component o f ductile deformation or shearing involved
during this process. The magmatic fabric is mostly discordant to the blocks and only
in one case has fabric been observed to wrap around a block o f dunite.
Figure 34. Peridotite intruded by a network o f gabbro veins in the Gumboat Lake
stoping zone. The veins are in general 1-3 cm wide in the photo and around 10 cm in
the center of the photo.
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Figure 35. Rounded stoped blocks o f peridotite within a pyroxenite matrix in the
Castle Lake stoping zone. The black band of chlorite surrounded by a pink band o f
talc is termed "black wall" (Schwindinger and Anderson, 1987) and seen in most o f
the stoped peridotite blocks.
Stoping o f Pyroxenite Rocks
Field observations indicate that pyroxenite rocks are seen as stoped blocks
within the medium-grained gabbro phase, pegmatitic phase and trondhjemite phase,
which indicates that pyroxenite predated these phases and cooled enough to break
into blocks. These blocks are usually 10-30 cm in diameter, rounded within the first
two phases and rounded to angular within the leucocratic gabbro phase (trondhjemite
or plagiogranite) (Fig. 36). Pyroxenite is also intraded by a network of gabbro melts.
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The angular shape o f some stoped pyroxenite rocks within the trondhjemite phase
might be a result of lower intrusion temperature and faster cooling of the melt, which
is further supported by the absence of reaction rims.
Figure 36. Stoping o f pyroxenite rocks within a leucocratic (trondhjemite) gabbro
melt along the Pacific Crest Trail. The lens cap is 5 cm in diameter. The blocks are
both rounded and angular. No reaction rims or chilled margins are observed and the
matrix lacks a distinct fabric.
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Other Xenoliths
Field observations show that within the medium-grained and coarse-grained
gabbro phases there are 10-30 cm blocks that are totally altered to talc and tremolite
(Fig. 37). These blocks usually occur adjacent to the margins associated with the
other stoped blocks but also kilometers away from the gabbro margin towards the
center of the gabbro body. For example, one large block (1.5 x 1.5 m) and several
small blocks (-10 cm) were found at Upper C liff Lake, about 1km from the gabbro-
peridotite contact, completely reacted to talc and tremolite (Fig. 38).
Figure 37. Small xenoliths o f unknown original composition completely altered to
talc and tremolite in the Cliff Lake area. The matrix consists of medium- to coarse
grained gabbro. The lens cap is S cm in diameter.
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Figure 38. A Large xenolith, (1.5 xl.5 m), completely altered to talc and tremolite in
the Upper Cliff Lake area. The matrix consists o f medium- to coarse-grained gabbro
and the lens cap is 5 cm in diameter.
Foliations, Serpentinization and Chilled margins
The foliation in the peridotite is often crosscut and truncated by pegmatitic
gabbro dikes, indicating that the peridotite foliation formed and cooled prior to the
intrusion. Figure 39 shows a foliation obliquely crosscut by a pegmatitic gabbro
dike, although, on a larger scale the peridotite foliation is deflected to a margin
parallel pattern around the main gabbro intrusion. Other field observations also show
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that serpentinization o f the peridotite seems to increase with proximity to the gabbro
intrusion from (Quick, 1981). However, serpentinization within the peridotite is
highly irregular and hence, no distinctive gradient can be observed. Serpentinization
is definitely higher within the 1-1 Ocm wide reaction zones between gabbro and
peridotite around xenoliths or dikes. Chilled gabbro margins are mostly observed
within gabbro dikes intruding into the peridotite.
Figure 39. Pegmatitic gabbro dike obliquely crosscutting the peridotite foliation in
the Gumboat Lake area (white lines show peridotite foliation). The lens cap is S cm
in diameter.
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Diabase Dikes
The diabase dikes were previously interpreted to represent a 200 m thick
sheeted dike section overlying the gabbro section (Boudier et al., 1989). However, a
complete sheeted dike section is not observed in the Seven Lakes gabbro. Within the
investigated field area, the number o f dikes increases upward in section as seen in the
Lake Helen area. Field observations show that diabase dikes occur in swarms and
covers around 30-50% o f the area. Data collected by Boudier et al. (1989) from all
over the Trinity Complex, indicate that the diabase dikes strike NW-SE in both the
gabbro and the peridotite. However, dike orientations within the Seven Lakes
gabbro change from an NW-SE trending orientation in the south-eastern part o f the
gabbro to an E-W orientation and slightly shallower dip in the more westerly parts of
the gabbro (Fig. 40). The pattern is similar to that seen in the gabbro foliation
analyses from in the areas o f PCT2, Helen Lake, Boulder Peak and Soapstone Pond
(Fig. 21-24). However, the orientations differ slightly between other areas such as
Gumboat Lake, Cliff Lake, PCT and Boulder Peak.
The diabase dikes are in texture aphanitic and altered, and no foliation is
present in the thin sections collected in the Seven Lakes gabbro. These microscopic
studies indicate that the texture is homogeneous and equigranular. Tremolite grains
are lath-shaped, radial or randomly oriented in the sample. Deformation mechanisms
cannot be identified within these aphanitic samples and therefore the cause o f
deformation within the gabbros cannot be determined.
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Gumboat Lake
PCT2
Cliff Lake
N
I •©•■
® J
V
N ■ 20
r
Helen Lake
N
Boulder Peak
N
N - 31
Figure 40. Equal area plots
o f poles to diabase dikes
in the Seven Lakes gabbro.
\
\
N *
O 1 2km
Quaternary glacial deports
l ^ n Mesozoic granite and dkvrte
|M b l MUsissippian Bragdon Formation
|D c g | Devonian Copley Greenstone
E D Devonian breccia
Devonian gabbro
E D Ordovician to Devonian pynncnte
Ordovician peridotite
^ ^ Structural Domains
Soap&tone Pond
CHAPTER 4. DISCUSSION
Whether or not the Trinity Complex is an ophiolite has been controversial for
some time (e.g. Wallin and Metcalf, 1998). A particularly contentious issue is how
the gabbros are related to the mantle peridotite section. Therefore, my research has
been an attempt to 1) establish the relationship between the gabbro and peridotite
units; 2) determine how the Seven Lakes magma chamber grew; and 3) use the
above information to try to reevaluate tectonic models for the processes involved in
the formation o f the complex. In this chapter, I first discuss whether the gabbros are
intrusive or not, then examine the complex internal and external processes associated
with the formation o f the gabbro, and in the third section, I reexamine the previously
proposed tectonic models.
Relationship between the Seven Lakes Gabbro and the Mantle Peridotite
What is the relationship between gabbro and peridotite in the Trinity
Complex? Field observations from the Seven Lakes gabbro strongly indicate that
gabbro intruded into the peridotite and is not equivalent to the layered gabbro in
ophiolite sections. Evidence for an intrusive relationship includes intrusive sharp,
steeply oriented contacts, stoped blocks of peridotite within the gabbro, and dikes of
gabbro (e.g. sheeted margins) intruding into the peridotite. A simplified profile
across the Seven Lakes gabbro shows some o f these stoping and intrusive
relationships (Fig. 41). The stoped blocks are strong evidence for an intrusive
relationship, but have previously been claimed to be related to faulting along the
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Pegmatitic
gabbro
intruding
peridotite
Diabase dikes
crosscutting
all other
phases
Stoping of pyroxenite
within coarse and
pegmatitic gabbro
Fine to medium
grained gabbro
accumulating
at the top section
of the body
Gradual transition from
pyroxenite to felspathic
pyroxenite and pyroxene-
rich gabbro
S o ap sto n e p o n d
G u m b o a t L a k e
C a stle L ak e
toood
K i t
Strain in the
peridotite
because of
expansion
of gabbro
body
Stoping and
rafting of
peridotite blocks
within pegmatitic
gabbro
Intrusion of
pyroxenite
dikes
Medium to
coarse grained
phase of gabbro
intruding into
the medium
grained phase.
Stoping of pyroxenite and
peridotite blocks within Stoping of
pegmatitic gabbro and peridotite
feldpathic pyroxenite > n pyroxenite
Figure 41. A schematic profile across the Seven Lakes gabbro showing critical relationships within the complex
(this study).
gabbro contact (Boudier et al., 1989). The relatively large, squared-shaped stoped
block (l.S xl.5 m) found in the gabbro by Upper C liff Lake (>1 km away from the
intrusive margin) is strong evidence that the stoped blocks could not possibly be
related to local faulting or shearing along the gabbro contact. Other evidence that
peridotite blocks are stoped rather than a result o f faulting, includes blocks that are
commonly rotated and surrounded by a magmatic fabric instead o f a subsolidus
fabric. For several well-known ophiolites, such as the Oman Ophiolite (Boudier et
al., 1996; Korenaga and Keleman, 1998) and the Josephine Ophiolite (Yoshinobu,
1999), sill-like intrusions above the Moho form the lower gabbros. Are the sheeted
margins in the Seven Lakes gabbro equivalent to these sill-like intrusions? In these
ophiolites, the gabbro sills are oriented parallel to the Moho and the subhorizontal
foliation in the peridotite, whereas in the Seven Lakes gabbro the gabbro dikes are
moderately to steeply dipping and crosscut the steeply dipping peridotite foliation.
Therefore, the sheeted margins in the Trinity Complex cannot be associated with the
horizontal to subhorizontal, sill-like gabbro intrusions in the lower section of regular
ophiolites. In conclusion, the nature of the contact between gabbro and peridotite is
clearly intrusive and different from an ophiolite, with features such as stoped
peridotite blocks surrounded by a magmatic fabric and moderately to steeply dipping
gabbro dikes intruding into the peridotite.
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Gabbro Chamber Construction
In the Seven Lakes gabbro, several pulses with different composition have
been identified. These pulses range from pyroxene-rich to hornblende-rich rock types
and range in grain-sizes from medium-grained to pegmatitic. Below, I discuss these
phases with consideration to: a) the intrusion sequence and b) the fabric forming
processes. These features can help to distinguish emplacement processes for the
gabbro or tectonic strain during or after emplacement.
Intrusion Sequence
The Seven Lakes gabbro chamber at present level o f exposure was
constructed by multiple batches o f magma and is very complex (Fig. 40). Field
observations in the Seven Lakes gabbro have shown that pyroxenite and feldspathic
magmas were the first to intrude the peridotite, as these rocks are located at the
margins of the gabbro and carry an abundant amount o f stoped peridotite blocks. The
cumulate, feldspathic pyroxenite grades into pyroxene-rich gabbro upwards and
towards the center o f the gabbro body displaying a segregation of the amount of
pyroxene. However, this gradual transition displays a minor stepwise decrease in
pyroxene from multiple parallel magma batches. The coarse- and medium-grained,
pyroxene-rich, gabbro phases dominate in the Seven Lakes gabbro and are clearly
intruded by pegmatitic hornblende, trondhjemite and diabase dikes. All gabbro
phases carry stoped peridotite and pyroxenite blocks. The intrusion sequence is
relatively clear except for several generations of pyroxenite dikes that not only
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intrude pyroxenite before the gabbro intruded, but also intrude the different gabbro
phases as well.
Interpretation o f Magmatic Fabrics
Do fabrics within magma bodies represent magma flow, magma ascent and
emplacement, or the last increment of strain and tectonic deformation recorded in an
area? According to Balk (1937) a primary flow fabric results from hydrodynamic
alignment o f mineral grains suspended in melt, where planar and linear alignment
define respectively flow and flow lines, and map-scale fabric patterns arise from
internal magma chamber processes. However, recent fabric studies suggest that
alignment o f minerals may instead record strain and tectonic deformation during
flow and crystallization and be poor recorders of chamber processes (Paterson et al.,
1998). This means that ascent and emplacement processes may not be preserved at
all or at most only partly preserved.
Major features o f fabrics are reviewed in the Seven Lakes gabbro. Magmatic
fabrics in the Seven Lakes gabbro may have been formed by two major processes.
First, it is possible that the foliations and lineations represent one fabric with two
different orientations formed within the same strain field. A fabric formed by such a
process should show few crosscutting relationships. A second possibility is that two
different fabrics formed at slightly different times. These fabrics could have
characteristics such as crosscutting relationships, inhomogeneous fabric intensities
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across the gabbro, and one fabric stronger than the other. Fabrics observed in the
Seven Lakes gabbro are presented as lines within Figure 42.
Figure 42. A simplified sketch o f the fabric and dike pattern (s) within the Seven
Lakes gabbro.
For fabrics observed in the Seven Lakes gabbro, the second model is a preferred
model for the following reasons: I) a more distinct bending fabric (F2) crosscuts the
less distinct straight fabric (FI); 2) the general fabric pattern is inhomogeneous with
respect to orientations and intensities in the gabbro; 3) the straight fabric (FI) is
parallel to sheet-like gabbro intrusions in the southwest o f the pluton and may
indicate that the main gabbro was originally emplaced as NE-SW striking sheets,
which now preserve a foliation; 4) diabase dikes are oriented parallel to the younger,
bending foliation and crosscut the older, straight foliation; 5) the bending fabrics
Gabbro
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crosscuts gabbro phase boundaries and are therefore interpreted to represent tectonic
strain. Therefore, a likely explanation for fabrics in this area is that a magmatic
foliation formed in a NE-SW-striking and steeply dipping orientation due to
emplacement of sheet-like gabbro batches or a fabric reset by tectonic strain.
Crystallization probably started at the margin of the gabbro intrusion and proceeded
towards the center. The next batch o f magma was incorporated in the center of the
body and mineral grains started to crystallize in a bending pattern due to the tectonic
strain field at that time.
Work done by (Willse, 1999) shows that diabase dikes in the Bonanza King
gabbro (west of the Seven Lake gabbro) bend in a similar pattern to the Seven Lake
gabbro dikes. This observation supports the hypothesis that the dikes (and fabrics)
are affected by a regional strain field and are not a result o f internal magma chamber
processes.
Displacement of Host Rock during Gabbro Emplacement
Likely processes that transfer host rock to make space for the gabbro
intrusion include: a) stoping o f the host rock, b) ballooning o f chamber and c) roof
uplift and d) elastic diking. I suggest that stoping, ballooning and diking were the
dominant processes, whereas fault emplacement processes were minor during the
gabbro intrusion. This hypothesis is based on the following observations, which are
discussed in more detail below: 1) There are abundant stoped blocks preserved; 2)
The structural aureole is relatively wide compared to the width o f the gabbro body;
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3) There is evidence in the field for only one minor shear zone; and 4) There are
multiple gabbro dikes along the gabbro margins. A simplified sketch (Fig. 43)
describes the expansion and mass transfer processes in the Gumboat Lake area.
These large stoped blocks in the Upper Cliff Lake area may be relicts from
roof or wall peridotite host rock trapped in the gabbro. Some o f the gabbro dikes
closest to the gabbro intrusion in Gumboat Lake contain rounded blocks o f host-rock
that could not possibly have been rounded from the m-wide dike. One possibility is
that these blocks were already rounded when captured in the stoping zone at depth
and brought up to the surface within the dike.
Ballooning has been controversial in arc-settings and appears to never have
been suggested for oceanic settings. The deflection o f host rock markers to a margin
parallel pattern around the Seven Lakes gabbro from the regional Trinity Complex
pattern explains a part o f the space-making problem during pluton intrusion. Some
ballooning of the Seven Lakes gabbro is supported by a fabric intensity gradient that
increases with proximity to the gabbro intrusion. In the field, this gradient is apparent
from host rock foliation that is easily discemable within meter-distance o f the gabbro
contact and diminishes at greater distances. Microstructures within this gradient
include recrystallization o f olivine and deformation structures in orthopyroxene,
represented by strain lamellae, undulose extinction, bending, kinking and fracturing.
The regional fabric displays recrystallized olivine and pyroxene, which has been
interpreted to represent solid-state flow under asthenospheric conditions at 12500 C
during formation o f an ophiolite (Boudier et al. 1989). The aureole deformation
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textures in orthopyroxene are probably due to dislocation glide at lower
temperatures, since orthopyroxene would form ribbons at high temperature mantle
conditions (Passchier and Trouw, 1996). These deformation structures probably
formed during emplacement of the gabbro intrusion.
The Gumboat Lake shear zone could have been related to the expansion o f
the gabbro by uplift o f the peridotite roof (Fig. 43). There is good evidence from
deformed and undeformed gabbro dikes found in the shear zone that the shear zone
was active during the intrusion o f the gabbro. These deformed gabbro dikes show
consistent pluton-side-up reverse sense of shear and together with the orientation of
the shear zone. However, this shear zone is minor (-200 m wide) compared to the
size o f the gabbro body (5 km wide) and the structural aureole (2 km) and this is the
only shear zone that is observed in proximity to the gabbro intrusion. Therefore, this
process was minor in comparison to the others.
Diking, on the other hand, is estimate to represent a major process, since
there are abundant gabbro dikes intruding the peridotite and the marginal gabbros.
Hence, from the fabric model presented in the previous section, the main gabbro
appears to have intruded as large sheets and therefore elastic diking processes were
important. However, stoping o f the host rock suggests that diking did not represent
the entire expansion process and therefore may have been replaced by sheeMike
visco-elastic diapirs. In conclusion, major spacemaking processes for gabbro
intrusion include stoping, ballooning and diking, whereas shear zones played a minor
role.
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Figure 43. A simplified sketch o f a profile across the Seven Lakes gabbro indicating
hot rock processes such as stoping, faulting, diking and ballooning associated with
the gabbro intrusion.
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Evaluation of Previous Models for the Trinity Complex
Formation of the Peridotite
The Trinity peridotite sheet has been proposed to represent a mantle diapir
(Quick, 1986), a piece o f a ridge segment formed on the flank of a diapir (Boudier et
al., 1989; Cannat, 1991) and a piece o f mantle formed in a transform fault (Wallin
and Metcalf, 1998). The regional steep foliation and subhorizontal lineation pattern
mapped by Boudier et al. (1989) is incompatible with the diapir model. If the mantle
part o f the Trinity Complex had formed at the top o f a mantle diapir (as in Oman) the
foliation planes should be mostly horizontal in the center and steepen outward
towards the edges o f the diapir. Lineations should trend radically in all directions
and plunge shallowly at the center and steeply towards the margin. The second
model is already determined as an incorrect model, since the Trinity Complex is not
an ophiolite, where mantle peridotite, gabbro and dikes formed contemporaneously.
Instead the third model, in which the peridotite was formed in an oceanic transform
setting, is a variation of the second model and appears to be appropriate for the
Trinity peridotite. Features consistent with this interpretation are: a) a steep foliation
formed in situ with horizontal lineations; b) a high-temperature plastic flow
represented by the foliation and lineation planes; and c) minimal magmatism, which
means that an overlying mafic section may not have been present at this time. If the
model by Boudier et al. (1989) was correct, it would have been difficult to explain
how the mafic sections were dismembered, because normal or detachment faults
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have not been described for the Trinity Complex. Thus, peridotite formed in oceanic
setting, best fit the transform fault model.
Formation o f the Gabbro
The key observation from this study with regards to the genesis o f the gabbro
is that it is intrusive into peridotite rather than part o f an ophiolitic sequence. This
contradicts the ophiolite model o f Boudier et al. (1989). The model o f Cannat
(1991) is a good primary model, where ephemeral magma chambers form within an
older peridotite sheet. Several observations fit this model. For example,
geochemical analyses of the Trinity Complex characterize the mafic phases as arc,
fore-arc and back-arc related (Brouxel and Lapierre, 1988; Grau et al., 1995; Wallin
and Metcalf, 1998), whereas the peridotite is suggested to be formed in a mid-
oceanic ridge environment (Jacobsen et al., 1984; Grau et al., 1995). Furthermore,
this model fits geochronological data. The ages o f peridotite and gabbro are
significantly different, 404 ± 3 Ma to 431 ± 3 Ma and 472 ± 32 Ma respectively. The
gabbro intrusions appear to show a broad range as well. Thus, in contrast to the mid-
oceanic peridotite, the gabbros formed in an arc environment and whereas the
peridotite was emplaced as a sheet on top of the Central Metamorphic Belt, the
gabbro probably was emplaced as more sheet like diapirs into a fore-arc setting.
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CHAPTER 5. CONCLUSIONS
The Trinity peridotite sheet dips slightly to the east and consists mostly of
harzburgite and lherzolite. The sheet is moderately to highly serpentinized and
intruded by several gabbro bodies. These gabbro bodies, ranging in age from 404±3
Ma to 439±13Ma, and are randomly distributed within the peridotite sheet. The
Devonian Seven Lakes gabbro is located in the eastern part o f the peridotite sheet
and represented the area o f investigation. Field observations and microscopic studies
from this gabbro body have revealed a complicated history o f how the gabbros and
peridotite were constructed and processes related to this construction.
First, the Devonian Seven Lakes gabbro is clearly intrusive into the
Ordovician mantle peridotite. The intrusive nature of the gabbro is best described
by steep gabbro contacts, stoped blocks of host rock peridotite within the gabbro and
gabbro dikes intruding peridotite. These features occur at several locations along the
gabbro contact. Dike contacts and edges of stoped blocks display mm- to cm-wide
reaction rims from when hot gabbros intruded into colder peridotite. These reaction
rims are now metamorphosed and display a low-temperature mineral assemblage of
chlorite, talc, tremolite and serpentine.
Second, the gabbro chamber formed by multiple phases intruding into the
peridotite. Pyroxenite and feldspathic pyroxenite phases were the first to intrude and
are found along the margin o f the gabbro carrying abundant blocks o f host rock
peridotite. Medium- and coarse-grained gabbro phases dominate in the gabbro and
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were emplaced after the pyroxenite phases. Pegmatitic hornblende gabbro and
trondhjemitic melts intruded all other phases as dikes and were later intruded by the
late diabase dikes. These phases occur frequently at the margins of the gabbro.
Third, complicated fabric patterns with many possible causes are observed in
the Seven Lakes gabbro. These fabrics observed in the gabbro mostly represent
tectonic strain, although information about emplacement o f sheet-like gabbro bodies
may be present in the earlier, NE-SW striking fabric.
Fourth, emplacement studies of the Seven Lakes gabbro indicate that there
are multiple emplacement processes interacting during the gabbro intrusion. Some
o f these processes include major processes such as: stoping, ballooning and elastic
diking of host rock, whereas the shear zones contributes with a minor part. This
result is based on the abundance of stoped blocks in the gabbro, number o f shear
zones associated with the gabbro intrusion, width o f the ballooning aureole and the
amount of gabbro dikes in the peridotite, as well as the hypothesis that part of the
main gabbro may have been emplaced as elastic dikes.
The results from field data and microscopic analysis in this study suggests
that the gabbro intruded at the time when the peridotite had cooled down enough to
result in fractured and stoped blocks. These results are consistent with geochemical
data and age-data, which suggest that the peridotite formed in a different setting from
the gabbro. The evaluation o f the earlier tectonic models concludes that the Trinity
peridotite formed in an transform mid-oceanic setting (472 ± 3 2 Ma), whereas the
intrusive gabbros (404 ± 3 to 439 ± 18 Ma) formed in a supra-subduction zone
89
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setting. Therefore, the peridotite and the gabbro are not genetically related and the
complex is not an ophiolite, even though it has been called an ophiolite for many
years. Since studies o f ocean floor are limited to geophysical data o f oceanic
spreading centers and geological observations o f ophiolites, there are still many
unsolved issues about how magma chambers form and what role magmatism plays.
Therefore, the study o f the unique Trinity Complex and its gabbro bodies intruding
mantle peridotite, is an important tool to understand the complexities within a slow
supra-subduction zone spreading setting, where magmatism occurred during at least
30 Ma.
Suggestions for Further Work
Some critical issues remain regarding the Trinity Complex. These issues
include: 1) the duration of formation of individual complexes, like the Seven Lakes
gabbro and all the various phases involved; 2) an estimate o f the significance of the
different emplacement processes within the gabbro; and 3) differences and
similarities o f magma chamber construction with other gabbro bodies in the Trinity
Complex. These issues are important in order to understand the complexities
resulted from a long period of magmatism and to be able to understand and compare
emplacement processes in oceanic settings with magmatism in, for example, arc
settings. Comparing different magma chambers within the same setting can be
useful in determining if all these chambers form with the same processes or with
completely different processes. Research that would address these problems
90
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includes: 1) age-dating of the different phases, from early pyroxenites to late
pegmatitic hornblende dikes within the same gabbro body; 2) field observations
concentrated on emplacement processes as stoping, ballooning, elastic diking and
fault emplacement, as well as fabrics in gabbros and host rock; and 3) detailed field
mapping phases within other gabbro bodies and surrounding peridotite in the Trinity
Complex.
91
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REFERENCES
Balk, R., 1937, Structural behavior o f igneous rocks. Geological Society o f America
Memoir: no. 5, p. 177.
Boudier, F., Le Sueur, E., and Nicolas, A., 1989, Structure o f an atypical ophiolite:
The Trinity Complex, eastern Klamath Mountains, California: Geological
Society o f America Bulletin, v.101, p. 820-833.
Boudier, F., Nicolas, A., and Ildefonse, B., 1996, Magma chambers in the Oman
ophiolite:fed from the top and the bottom. Earth and Planetary Science
Letters: v. 144, p. 239-250.
Brouxel, M., 1991, Geochemical consequences o f flow differentiation in a multiple
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Brouxel, M., and Lapierre, H., 1988, Geochemical study o f an early Paleozoic
island-arc-back-arc basin system. Part 1: The Trinity Ophiolite (northern
California): Geological Society of America, v. 100, p. 1111-1119.
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?: Journal of Geophysical Research, v. 101, no. B2, p. 2847-2857.
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Klamath Mountains, northern Calfiomia: Implications on accretion geometry:
Tectonics, v. 4, p. 435-452.
Cannat, M., and Lecuyer, C., 1991, Ephemeral magma chambers in the Trinity
peridotite, northern California: Tectonophysics, v. 186, p. 313-328.
Conference Participants, 1972, Report on the Penrose Field Conference on
ophiolites: Geotimes, v. 17, p. 24-25.
Davis, G. A., 1968, Westward thrust faulting in the south-central Klamath
Mountains, California: Geological Society o f America Bulletin, v. 79, no. 7,
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crystallization in granitoid rocks: Lithos, v. 21, n. 4, p. 237-245.
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Grau, F., Bemard-Griffiths, J., Lecuyer, C., Henin, J., Mace, J., and Cannat, M.,
1995, Extreme Nd isotopic variation in the Trinity Ophiolite Complex and
the role o f melt/rock reactions in the oceanic lithosphere: Contributions to
Mineral Petrology, v. 121, p. 337-350.
Jacobsen, S. B., Quick, J. E., and Wasserburg, G. J., 1984, A Nd and Sr isotopic
study of the Trinity peridotite: implications for mantle evolution: Earth and
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Karson, G., 1998, Faulting and Magmatism at Mid-Ocean Ridges: Geophysical
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Korenaga, J., and Keleman, P. B. 1998, Origin o f gabbro sills in the Moho transition
zone o f the Oman ophiolite: Implications for magma transport in the oceanic
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Lanphere, M. A., Irwing, W. P., and Hotz, P. E., 1968, Isotopic age o f the Nevadan
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1027-1052.
Lapierre, H., Brouxel, M., Albarede, F., Coulon, C., Lecuyer, C., Martin, P., Mascle,
G and Rouer, O., 1987, Paleozoic and Lower Mesozoic magmas from the
eastern Klamath Mountains (North California) and the geodynamic evolution
of the northwestern America: Tectonophysics, v. 140, p. 155-177.
Lecuyer, C., and Fourcade, S., 1991, Oxygen isotope evidence for multi-stage
hydrothermal alteration at a fossil slow-spreading center: the Silurian Trinity
ophiolite (California, U.S.A): Chemical Geology, v. 87, p. 231-246.
Lecuyer, C., Brouxel, M., and Albarede, F., 1990, Elemental fluxes during
hydrothemal alteration o f the Trinity ophiolite (California, U.S. A) by
seawater: Chemical Geology, v. 89, p. 87-115.
Macdonald, K. C., 1998, Faulting and Magmatism at Mid-Ocean Ridges:
Geophysical Monograph, v. 106, American Geophysical Union, Washington,
D. C.
Mattison, J. M., and Hopson, C. A., 1972, Paleozoic ages o f rocks from ophiolite
complexes in Washington and northern California, (abstract): Eos Trans.
AGU, v. 53(4), p. 543.
93
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Mutter, J. C., and Karson, J. A., 1992, Structural Processes at Slow-Spreading
Ridges: Science, v.257, p. 627-634.
Nicolas, A., 1989, Structures of Ophiolites and Dynamics o f Oceanic Lithosphere,
Petrology and Structural Geology: University o f Montpellier, France, 367 p.
Nicolas, A., Reuber, I., and Benn, K., 1988b, A new magma chamber model based
on structural studies in the Oman ophiolite: Tectonophysics, v. 151, p. 87-
105.
Pallister, J. S., and Hopson, C. A. 1981, Semail ophiolite plutonic suite: Field
relations, phase variation and layering and a model o f a spreading ridge
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Passchier, C. W and Trouw, R. A. J., 1996, Microtectonics:. Berlin, Heidelberg, 289
P-
Paterson, S. R. and Miller, R. B., 1998, Mid-crustal magmatic sheets in the Cascade
Mountains, Washington: Implications for magma ascent: Journal o f
Structural Geology, v. 20, no. 9-10, p. 1345-1363.
Paterson, S. R., Vernon, R. H., and Tobish, O. T., 1989, A review o f criteria for the
identification o f magmatic and tectonic foliations in granitoids: Journal of
Structural Geology, v. 11, no. 3, p. 349-363.
Paterson, S. P., Fowler, T. K., Schmidt, K. L., Yoshinobu, A. S., Yuan, E. S., and
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53-82.
Peacook, S. M., 1987, Serpentinization and infiltration metasomatism in the Trinity
peridotite, Klamath province, northern California: implications for
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70.
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Quick, J. E. 1981, The Origin and Significance o f Large, Tabular Dunite bodies in
the Trinity Peridotite, Northern California: Contribution to Mineral
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Mantle Diapir in the Eastern Klamath Mountains, Northern California:
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94
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Quick, J. E., and Denlinger, R. P., 1993, Ductile deformation and the Origin of
Layered Gabbro in Ophiolites: Journal o f Geophysical Research, v. 98, no.
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dissertation, University o f Nevada, Las Vegas, 126 p.
95
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Yildrim, D., Eldridge, M. M., Fumes, H., 1998, Faulting and Magmatism at Mid-
Ocean Ridges: Geophysical Monograph, v. 106, American Geophysical
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spreading centers: Unpublished, Ph.D dissertation, University o f Southern
California, Los Angeles, 239 p.
96
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APPENDIX
In the "Hans Ramberg Tectonic Laboratory" in Uppsala, I performed some modeling
work with the aim to see how oceanic spreading environments and faults develop
over time and extension in order to relate to the Trinity Complex. The model
consisted o f a brittle upper layer o f "plastelina" and a more ductile lower layer o f
daucoming silicone (PDMS). The brittle layer had an average density of 1.44 g/cm3
(which correlate to 2.8 g/cm3 for oceanic upper and lower crust) and the more ductile
layer had an average density of 1.99 g/cm3 (3.3-3.4 g / cm3 for ultramafic rocks in
the mantle section). The diapir material (placed in the middle-bottom of the model)
had a density of 1.28 g/cm3 in order to have a density less than both layers, as the
diapir material represented warm astenospheric material rising due to buoyancy and
at the surface cool down and form new crust. The upper layer turned out to be 12cm
* 3.85cm * 7.2cm, while the lower layer was 11.4cm * 2 cm* 7.1cm. The upper
brittle layer had four thin sheets in brown and white colors, while the lower ductile
layer had 20 thin sheets in pink and black colors. The diapiric material was measured
to 1cm * 1cm * 7.2 cm. A fault was pre-cut in the upper brittle layer with a 40-
degree angle from the horizontal. The model was placed four times in a big
centrifuge and the extension continued for two minutes every time. The result and
amount o f extension is shown in the table below.
97
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Table 1. Amount o f extension in the spreading model.
Test Round per
minute
Time Initial
length
Length Extension
1 850 2 11,4 13.2 15%
2 847-848 2 11,4 14.4 26%
3 850 2 11,4 15.3 34%
4 875 2+3 11,4 16.1 41%
As seen in the table, the first 2 minutes gave 15% of extension and the model showed
a mushroom shaped diapir initiated underneath the pre-cut normal fault (Figure 42).
The brittle upper crust is relatively undeformed (except for the normal fault) but the
ductile lower crust shows flow structures around the diapir. Another 2 minutes in the
centrifuge did not affect the model very much. Test 3 gave a total extension of 34%
and the diapir had a more flattened shape with the roof pushing the footwall of the
brittle layer upwards (Figure 43). The last 2+3 minutes in the centrifuge gave 41% o f
extension and a very flat diapir in between completely separated brittle crust (Figure
44). The modeling show the correlation between extension, diapiric upwelling and
flow lines in the asthenosphere beneath a spreading ridge. Unfortunately the diapiric
upwelling prevented us from seeing structures in the lower ductile layer caused by
pure extension in the upper brittle layer. The purpose was to see if the major brittle
fault has continuos, wide ductile fault zone in the lower layer or if a detachment fault
would initiate in between the layers. In order to see this, the next model needs to be
modeled without the diapiric material.
98
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Figure 42. A mushroom shaped diapir initiated after 15% extension
Figure 43. The diapir starts to flatten after 34% extension.
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Figure 44. A flattened diapir after 41% extension.
100
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NOTE TO USERS
Oversize maps and charts are microfilmed in sections in the
following manner:
LEFT TO RIGHT, TOP TO BOTTOM, WITH SMALL
OVERLAPS
This reproduction is the best copy available.
U M I*
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Map sym bols
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^ Magmatic foliation
Magmatic lineation
Bedding
Shear zone
Ia J ______
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Qgd| Quaternary glacial deposits
0 Mezozoic granite
|iyibf| Mississippian Bragdon Form ation:( Marine: shale, graywacke
1 I and minor conglomerate)
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0 Devonian breccia
0 Devonian gabbro
|opx| Ordovician to Devonian pyroxenite
0 Ordovician peridotite
2 Km
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Asset Metadata

Creator Andreasson, Gunilla Kerstin (author)

Core Title Construction of a gabbro body in the Trinity Complex, northern California

School Graduate School

Degree Master of Science

Degree Program Geological Sciences

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

Tag Geology,OAI-PMH Harvest

Language English

Contributor Digitized by ProQuest (provenance)

Advisor Paterson, Scott (committee chair), Davis, Gregory (committee member), Morrison, Jean (committee member)

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c16-40974

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