Modeling the Thermo-Rheological Evolution of the Papoose Flat Contact Aureole in the White-Inyo Range, California

Abstract:

The Papoose Flat pluton in the White-Inyo Range is one of the best examples of forcefully emplacement of magma with room created by softening of its contact aureole. The western part of the aureole, consisting of schists, marbles and quartzites, displays a ductile deformation with strong foliation and lineation. The softening of the aureole under lithostatic fluid pressure conditions and variable H₂O activity was modeled using the Fortran program SUTRAMET. The model assumes a geothermal gradient of 30^oC km^-1 and instantaneous magma intrusion with liquidus of 970^oC and solidus of 650^oC. The Papoose Flat pluton was emplaced at the depth of 10-15 km.

Assuming the power-law dependence of rheology, under hydrous conditions consistent with the observed greenschist-facies regional metamorphism, the brittle-ductile transition for quartzite in the crustal section is at the ~13 km depth and for limestone at the ~11 km depth. Under anhydrous condition, the brittle-ductile transition would have been at the ~15 km depth for quartzite and the ~12 km depth for limestone. There is no obvious brittle-ductile transition zone for schist due to its inherent weakness and very small temperature-dependence of strength.

During cooling of magma, the model pluton was completely crystallized by ~233,330 y. Water fugacity plays an important role in the rheology and deformation of the host rocks. A ductile deformation aureole that is produced due to conductive heat flow above the pluton and in the presence of H₂O is 1-2 km wide, which is consistent with observations. The limestone produces a wider ductile aureole than the quartzite. At low H₂O fugacities the model aureoles are narrower.

These calculations suggest that the Papoose Flat and other plutons in the White-Inyo Range were emplaced at the brittle-ductile transition. The relatively hot host rocks at these depths caused slow cooling of the plutons and promoted the development of broad ductile aureoles that permitted growth of plutons. On the other hand, emplacement of magma in the shallower crust within the brittle regime is less likely to produce ductile aureoles due to more rapid cooling. Growth of plutons in this regime may require other means for creating space.