173 Structural Geology: Advances in Understanding Deformational Processes Ancient and Modern (Posters)

Tuesday, 20 October 2009
9:00 AM - 6:00 PM
Hall A

LOCAL METAMORPHIC FLUID CONTROL ON SLIPS SYSTEMS AND RECRYSTALLIZATION ACROSS THE EJB AUREOLE, EASTERN CALIFORNIA

Paper number: 173-29
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

A series of 18 quartzite samples and 5 marble samples collected across the intensely deformed NE aureole of the Jurassic Eureka Valley-Joshua Flat-Beer Creek (EJB) pluton in eastern California were analyzed for crystallographic preferred orientations (CPO), microstructures, metamorphic phase relationships, fluid inclusions (FI) and oxygen isotopes. Aureole metasedimentary layering is vertical. Aluminum in hornblende barometry from the pluton suggest a 8-10 km depth of emplacement and sillimanite is common in the 1 km wide aureole. Adjacent to the contact, quartzites exhibit y-axis maxima and cross-girdle patterns indicating that basal and prism slip operated. Recrystallization is incomplete and many quartz grains maintain their sedimentary grain shapes. In the quartzites further from the pluton (>280m), the slip systems, interpreted from the CPO’s, indicate that prism [c] slip operated. Patterns exhibit single or double maxima centered about the lineation, or X-strain axis. With more distance from the contact, the CPO’s switch back and forth again between slip and [c] slip patterns.

The data suggest that the primary variable controlling the type of CPO in the quartzites, and therefore slip systems, is the proximity to marble layers. Our interpretation is that aH2O in the quartzites adjacent to the marble layers was low due to production of CO2 during metamorphism of marbles and calc-silicates. The result is that the “effective temperatures” of deformation and recovery were reduced for “dry” rocks and resulted in slip. In contrast, higher aH2O in some quartzite layers, most likely from prograde metamorphic reactions in the surrounding and interbedded pelites, promoted [c] slip and complete recrystallization.

Stable isotope data supports our assumption of “dry” (or dryer) quartzites adjacent to marbles and “wet” quartzites interbedded with pelites. The most unrecrystallized samples have the highest δ18O(qtz) values and slip patterns while the most recrystallized samples have the lowest δ18O(qtz) values and [c] slip patterns. FI data suggests that the unrecrystallized quartzites maintained their pre-intrusive FI’s. Recrystallized samples have similar FI compositions, but underwent some reequilibration with a small amount of pore fluid.