Scholarly
    Communications Project


Document Type:Master's Thesis
Name:Nancy A. Brauer
Email address:nbrauer@vt.edu
URN:1998/00230
Title:Fluid inclusions as a monitor of progressive grain-scale deformation during cooling of the Papoose Flat pluton, eastern California
Degree:Master of Science
Department:Geological Sciences
Committee Chair: R. J. Bodnar and R. D. Law
Chair's email:bubbles@vt.edu
Committee Members:R. J. Tracy
Keywords:fluid inclusions, deformation, brittle-ductile transition, Papoose Flat pluton
Date of defense:December 18, 1997
Availability:Release the entire work for Virginia Tech access only.
After one year release worldwide only with written permission of the student and the advisory committee chair.

Abstract:

Analyses of fluid inclusions and microstructures within the Papoose Flat pluton were used to investigate the chemistry and temperatures of fluids circulating with the pluton during cooling. Based on previous microstructural analyses, the interior of this late Cretaceous granitic to granodioritic pluton has been divided into three domains: i) a central core characterized by magmatic microstructures, ii) a middle domain of high temperature (>500C) solid-state deformation, and iii) an outermost domain characterized by relatively low temperature (<500C) solid-state deformation. According to previously published anisotropy of magnetic susceptibility analyses and pluton cooling models, plastic flow occurred in both the outer part of the pluton and within its aureole rocks while the core of the pluton was still molten. Solid-state deformation is proposed to have stopped when the pluton interior cooled through its solidus less than 100,000 years after magma emplacement.

Microstructural analysis of samples from all three domains confirmed the transition from magmatic flow in the core of the pluton to solid-state deformation at the pluton margin. However, weakly developed solid-state microstructures overprint the dominant magmatic microstructures in samples from the core domain. The existence of solid-state microstructures in all three domains indicates that deformation continued during and after crystallization of the interior of the pluton.

Two phase, low salinity (< 26 wt% NaCl equivalent), liquid-rich aqueous fluid inclusions predominate within both quartz and feldspar grains in all samples. Throughout the pluton, the majority of fluid inclusions are hosted by deformed grains. Feldspar-hosted primary inclusions are associated with sericitic alteration. Inclusions were also observed in feldspar as secondary or pseudosecondary inclusions along fractures. Inclusions in quartz are frequently found near lobate grain boundaries or near triple junctions; linear pseudosecondary inclusion assemblages are commonly truncated against lobate boundaries between adjacent quartz grains, indicating that discrete microcracking events occurred during plastic deformation.

Homogenization temperatures overlap for all three microstructural domains. Coexisting andalusite and cordierite in the contact aureole, and the intersection of the Mus + Qtz dehydration reaction with the granite solidus, indicate trapping pressures between 3.8 and 4.2 kb. Ninety-eight percent of the calculated fluid inclusion trapping temperatures at 3.8 - 4.2 kb are below the granite solidus of 650C. Seventy-six percent of the trapping temperature data fall within the more restricted range of 350-500C; i.e. at temperatures which are lower than the commonly cited brittle-ductile transition temperatures for feldspar at natural strain rates, but above those for quartz. No correlation could be established between trapping temperatures and either host mineral or microstructural domain within the pluton.

The similar, relatively low trapping temperatures indicate that the majority of inclusions preserved in all three domains were trapped during the late low strain magnitude stages of solid-state deformation. The most common fluid inclusion trapping temperatures (400-500C) in all three microstructural domains are similar to the deformation temperatures indicated by microstructures and crystal fabrics in the outer part of the pluton; these trapping temperatures are obviously lower than temperatures associated with contemporaneous solid state and magmatic flow in the pluton interior. The similar trapping temperatures within the pluton core and margin must indicate that the inclusion-trapping event migrated from the margin to the core of the pluton as it cooled, because fluid inclusions would rapidly equilibrate to a density appropriate for the PT conditions of their host minerals.


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