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42
The GMBP barometer is based on the breakdown of biotite and plagioclase
under increasing pressure to form garnet and muscovite. It is defined by two reactions,
accounting for both Fe and Mg mineral end-members:
Fe: annite + 3 anorthite = grossular + almandine + muscovite
Mg: phlogopite + 3 anorthite = pyrope + grossular + muscovite
During prograde metamorphism, an increase in pressure would produce a
decrease in the anorthite content of plagioclase and an increase in the grossular content
of garnet. In sample FHa270, for example, this can be seen as a modest increase in
grossular content towards the margins of the garnet (Figure 2.8c) and a decrease in
anorthite content at the margins of the plagioclase (Figure 2.8d). A retrogressive path
would produce the opposite effect, with a drop in grossular content, as can be seen, for
example, in the outer 50 μm of the garnet in FDC62 (Figure 2.7c).
Although kyanite is present in the McCoy Creek Group, its absence from our
seven chosen samples means that we could not apply the garnet-aluminosilicate-quartz-plagioclase
(GASP) barometer of Newton and Haselton [1981].
Mineral composition data were obtained using the JEOL JXA-8200 electron
microprobe at UCLA operated at 15 kV with a ~20 nA beam current. Raw data were
reduced using the ZAF matrix correction [Reed, 1995]. Table 2.2 contains representative
mineral analyses for each sample and Table 2.3 lists the Fe/Mg ratios of garnet and
biotite and the concentrations of all end-member components.
Garnets were analyzed with line traverses and spot analyses close to their
margins. However, due to retrogressive overprints highlighted by X-ray maps and the
traverses, analyses were confined to within 50μm of the rims. Biotites were analyzed
Object Description
| Title | Structural and thermobarometric constraints on the exhumation of the northern Snake Range metamorphic core complex, Nevada |
| Author | Cooper, Frances Jacqueline |
| Author email | fcooper@usc.edu; fcooper@usc.edu |
| Degree | Doctor of Philosophy |
| Document type | Dissertation |
| Degree program | Geological Sciences |
| School | College of Letters, Arts and Sciences |
| Date defended/completed | 2008-08-27 |
| Date submitted | 2008 |
| Restricted until | Unrestricted |
| Date published | 2008-10-22 |
| Advisor (committee chair) | Platt, John P. |
| Advisor (committee member) |
Davis, Gregory A. Morrison, Jean Platzman, Ellen Thompson, Mark E. |
| Abstract | Observations from areas of large-scale continental extension, including the Basin and Range Province in western North America, have revealed the presence of regionally subhorizontal normal faults that appear to have exhumed rocks from mid- to lower-crustal levels. These detachment faults separate upper plate rocks extended on arrays of high-angle brittle normal faults from lower plate rocks exhibiting ductile mylonitic stretching and medium- to high-grade metamorphism. The origin and evolution of these detachments has been a matter of debate for decades, and yet a number of issues remain unresolved: (1) the dip of the faults when they were initiated and were active; (2) their penetration depth into the crust; (3) their role in exhuming high-grade metamorphic rocks; and (4) the origin and significance of the mylonitic deformation in their footwalls.; I explored these issues in the footwall to a classic detachment fault -- the northern Snake Range décollement (NSRD) in eastern Nevada -- using a combination of structural geology, geothermobarometry, paleomagnetism, isotope geochronology, and electron backscatter diffraction (EBSD) analysis. Garnet-biotite-muscovite-plagioclase thermobarometry suggests that the footwall to the NSRD experienced late Cretaceous peak metamorphic conditions of 6–8 kbar and 500–650°C, equivalent to a burial depth of ≤ 30 km. Calcite-dolomite thermometry indicates that Tertiary mylonitic deformation occurred under lower temperature conditions of 350–430°C, equivalent to mid-crustal levels. Structural, paleomagnetic, and EBSD data demonstrate that mylonites experienced two phases of shear (top-east and top-west), inconsistent with movement along a single throughgoing normal fault.; I conclude that exhumation of the northern Snake Range footwall was a two-step process. Initial ductile stretching and thinning of the crust exhumed footwall rocks to the middle crust beneath a discontinuity, referred to as the localized-distributed transition (LDT), that separated extension along brittle normal faults above from localized ductile shear zones below. Mylonites formed along the LDT were subsequently captured by a moderately-dipping NSRD that soled into the middle crust. The NSRD, therefore, appears to be a late-stage brittle normal fault that was responsible for only about half the total exhumation of the footwall, and is not directly related to the mylonitic deformation. |
| Keyword | continental extension; extensional tectonics; Basin and Range province; Cordillera; metamorphism; mylonite zone |
| Geographic subject | tectonic features: Snake Range décollement |
| Geographic subject (state) | Nevada |
| Geographic subject (country) | USA |
| Language | English |
| Part of collection | University of Southern California dissertations and theses |
| Publisher (of the original version) | University of Southern California |
| Place of publication (of the original version) | Los Angeles, California |
| Publisher (of the digital version) | University of Southern California. Libraries |
| Provenance | Electronically uploaded by the author |
| Type | texts |
| Legacy record ID | usctheses-m1695 |
| Contributing entity | University of Southern California |
| Rights | Cooper, Frances Jacqueline |
| Repository name | Libraries, University of Southern California |
| Repository address | Los Angeles, California |
| Repository email | cisadmin@lib.usc.edu |
| Filename | etd-Cooper-2458 |
| Archival file | uscthesesreloadpub_Volume40/etd-Cooper-2458.pdf |
Description
| Title | Page 57 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | 42 The GMBP barometer is based on the breakdown of biotite and plagioclase under increasing pressure to form garnet and muscovite. It is defined by two reactions, accounting for both Fe and Mg mineral end-members: Fe: annite + 3 anorthite = grossular + almandine + muscovite Mg: phlogopite + 3 anorthite = pyrope + grossular + muscovite During prograde metamorphism, an increase in pressure would produce a decrease in the anorthite content of plagioclase and an increase in the grossular content of garnet. In sample FHa270, for example, this can be seen as a modest increase in grossular content towards the margins of the garnet (Figure 2.8c) and a decrease in anorthite content at the margins of the plagioclase (Figure 2.8d). A retrogressive path would produce the opposite effect, with a drop in grossular content, as can be seen, for example, in the outer 50 μm of the garnet in FDC62 (Figure 2.7c). Although kyanite is present in the McCoy Creek Group, its absence from our seven chosen samples means that we could not apply the garnet-aluminosilicate-quartz-plagioclase (GASP) barometer of Newton and Haselton [1981]. Mineral composition data were obtained using the JEOL JXA-8200 electron microprobe at UCLA operated at 15 kV with a ~20 nA beam current. Raw data were reduced using the ZAF matrix correction [Reed, 1995]. Table 2.2 contains representative mineral analyses for each sample and Table 2.3 lists the Fe/Mg ratios of garnet and biotite and the concentrations of all end-member components. Garnets were analyzed with line traverses and spot analyses close to their margins. However, due to retrogressive overprints highlighted by X-ray maps and the traverses, analyses were confined to within 50μm of the rims. Biotites were analyzed |
