Contact aureole rheology of the White Horse pluton
Marko, Wayne T.
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The 160 Ma White Horse pluton intruded a thick sequence of miogeoclinal Paleozoic carbonate rocks in the northeastern Great Basin Region, Nevada. The dominantly quartz monzonite pluton (-16 km^ of exposure) lacks internal fabric, concentric zoning, and stoped blocks, but hosts several smaller granite and granodiorite bodies as well as numerous microdiorite mafic enclaves. The structural aureole extends 7 km along the eastern side of the elliptical intrusive body. Continuous and discontinuous spaced axial planner foliations and harmonic to disharmonic, tight to isoclinal folds wrap around the western margin of the pluton. Folds verge toward and away from the pluton and a rim anticline is preserved along the pluton margin. In several locations fold axes are cut by the pluton host rock contact. The aureole was shortened approximately 54% during emplacement. However, a maximum of 52 % of the exposed pluton area is unaccounted for after the ductile aureole strain is restored. The pluton contact geometry is highly variable and parallel to subparallel with host rock anisotropy. Along the southern aureole the contact dips 14° to the north (towards the intrusion) and in the southeast the contact turns over to dip 45° to the southeast (away from the intrusion). In the northeastern aureole the contact dips 31° to the northeast while at structurally higher elevations the contact dips 45° to the southwest. Several dikes cut host rock structure and turn to propagate towards one another, preserving the late stage stoping process. Aureole tectonites consist of carbonate mylonites and coarse mylonites interlayered with annealed marble. Intracrystalline slip, grain boundary migration, subgrain rotation and grain size sensitive diffusion creep accommodated ductile deformation. Significant syntectonic grain growth occurred during carbonate grain recrystallization. Minimum stress estimates from piimed calcite tectonites range from 30 to 34 MPa. Minimum strain rate estimates range from 10""s'' at 600° C to 10''^s"' at 400° C. Temperatures correspond with the peak thermal gradient predicted for the aureole. Magma chamber construction was accomplished by contemporaneous brittle and ductile translation of host rocks. Stoping accounts for missing host rock area, truncated bedding, and the consistent subparallel orientation of the pluton contact with host rock structure. Ductile deflection of the host rocks was accomplished by lateral expansion of the magma chamber after magma ascent had ceased. This process produced plutonvergent and divergent folds and a rim anticline. Calculated aureole strain rates preclude the possibility of simple end-member emplacement diking and diapiric models. The large viscosity contrast between the host rocks and the magma as well as the lack of fabric within the intrusion suggests aureole deformation (chamber construction) was driven by buoyancy and/or overpressure and was completed before the magma had achieved a yield strength.