PRESSURE-TEMPERATURE CONDITIONS OF EOCENE SYN-EXTENSIONAL METAMORPHISM AND DEFORMATION IN THE FOOTWALL OF THE PIONEER METAMORPHIC CORE COMPLEX, IDAHO-US

The deepest exposed levels of the Pioneer Core Complex (PMCC) expose metasedimentary rocks and metabasites that were metamorphosed at ~50-48Ma during a syn-extensional, magmatically induced high-temperature event. The P-T conditions that these rocks reached prior to, and during extension are still uncertain. In this study we use metamorphic phase equilibrium diagrams and thermobarometry linked to petrographic analysis of metapelites and metabasites at different levels of the footwall to constrain the P-T evolution. We also use petrographic observations and EBSD analysis of quartz to constrain synextensional deformation temperatures.

At the deepest levels two metabasites record T conditions of upper amphibolite and granulite facies at P= ~4.5 -6 kb. A Grt+-Opx+Pl metabasic granulite preserves equilibrium textures of Grt, Pl and Opx growth at the peak T (>800C). The rock preserves textures of Pl, Opx and Mt symplectite around resorbed garnet. The phase equilibrium diagram from a Grt+Pl metabasite with coexisting Cummingtonite+Anthophyllite suggest equilibrium conditions at ~4.5-5 kb and ~700-750C. These temperatures are consistent with inferred deformation temperatures in quartzite from the same level, where microstructures from quartz indicate the operation of grain boundary migration dynamic recrystallization. EBSD maps and their derivative quartz CPO patterns reveal the presence of Dauphiné twinning and high-temperature slip systems, including prism <c> slip, indicative of temperatures >600C and >650C, respectively.

At higher structural levels of the footwall, thermobarometry and equilibrium diagrams of Grt-metabasite and Gt-Sil-Cd metapelite record T locally >700°C at P~3.5-4.5kb. Coronas of Crd+Pl around resorbed garnet in metapelite suggest minor isothermal decompression.

Our data indicate that the deepest exposed rocks in the PMCC were exhumed from pressures of ~4.5-6 kb and 3.0-4.5 kb, consistent with an Andalusite-bearing contact aureole at slightly higher levels. Our metamorphic analysis permits only minimal isothermal decompression. These P estimates indicate that the PMCC records less exhumation than most of the core complexes to the north, but are similar to those south of the Snake River Plain.