TRANSITION IN MICROSTRUCTURE AND CRYSTALLOGRAPHIC PREFERRED ORIENTION IN QUARTZITES WITH DISTANCE FROM THE EJB PLUTON, EASTERN CALIFORNIA

The Eureka –Joshua Flat- Beer Creek (EJB) pluton (~175 Ma) of eastern California is surrounded by an intensely deformed metasedimentary contact aureole that is 1.3 km wide. The Cambrian Harkless quartzite can be traced from 450 m from the pluton contact, where it is intensely sheared, to 2300 m out from the pluton where it is undeformed. We collected a suite of Harkless quartzites to examine how the microstructures and crystallographic preferred orientations (CPO) change with distance from the contact. Ripple marks are observed at 2300 m and skolithos tubes may still exist at 950 m from the pluton contact.

The quartzites exhibit minor amounts of bulging (BLG) and subgrain rotation recrystallization (SGR) between 2000 and 1000 m. Deformation appears to be heterogeneous as some relic sedimentary grains remain up to 1000 m from the pluton. Grain boundary migration (GBM) becomes increasingly important and within 800 m from the pluton is the only recrystallization mechanism. Recrystallized grain size also increases towards the contact. Grain boundary pinning by minor elements dominates the microstructure where interstitial micas exist. CPOs gradually transition from very weak, low temperature prism <a> patterns far from the contact to double concentrations of c-axes close to the lineation indicating prism <c> slip. The CPO begins to strengthen abruptly where GBM begins to dominate.