Metamorphism and fluid flow in the contact aureole of the Eureka Valley-Joshua Flat-Beer Creek pluton, California

摘要

We examined the petrology and stable isotope compositions of metamorphic rocks in the thermal aureole of the Eureka Valley-Joshua Flat-Beer Creek (EJB) composite pluton, White Mountains, eastern California, to determine the physical conditions of contact metamorphism and associated fluid flow. The Jurassic EJB pluton is one of several bodies intruded during protracted Meso-zoic magmatism in the White-Inyo Mountains. Emplacement of the pluton rotated and stretched the intruded greenschist-facies Cambrian sedimentary rocks to vertical orientations that are subparallel to the pluton contact. Traverses across the contact aureole included marbles, calc-silicates, schists, and quartzites. Each lithology behaved differently during contact metamorphism. Most dolomite and calcite marbles did not equilibrate with an external fluid as revealed by original, unshifted δ~(18)O and δ~(13)C values of 20%-25‰ (relative to standard mean ocean water [SMOW]) and -1‰ to +2‰ (relative to PeeDee belemnite [PDB]), respectively. Largest isotopic shifts occurred in calc-silicates where δ~(18)O decreased to ~15‰. The moderate oxygen-isotope ratio shift suggests that the calc-silicates equilibrated with an aqueous fluid, magmatic and/or metamorphic, whose oxygen-isotope ratio was buffered by the surrounding silicate metamorphic rocks. Infiltration of the calc-silicates by an aqueous fluid in the high-grade part of the aureole is also demonstrated by the presence of grossu-lar garnet and vesuvianite.Schists in the aureole vary in mineralogy and compositions. Harkless and Saline Valley formations have higher Al/K ratios that resulted in more abundant muscovite and andalusite, than the Campito Formation, which contains ubiquitous K-feldspar. On the western side of the pluton, partial melt-ing of the Harkless Formation was driven by flow of aqueous fluids that was focused between the pluton and marbles more distant from the contact. Metamorphic pressure-temperature (P-T) pseudosections for the schists suggest that partial melting has occurred between 2.5 and 3 kbar and >660 °C. The pseudosections also predict a narrow subsolidus cordierite field. However, cordierite, feldspars, and other silicate minerals were altered by pervasive fluid flow through the schists below 500 °C. This fluid flow likely was driven by slow cooling of the large EJB pluton and/or continued magmatism in the White-Inyo Mountains, particularly during later emplacement of the Birch Creek pluton to the west of the EJB pluton.