Properties of fluids attending variable recrystallization of quartzite during contact metamorphism in the White‐Inyo Range, California

Abstract

AbstractFluid inclusions in the metamorphic aureole of the Eureka Valley‐Joshua Flat‐Beer Creek (EJB) pluton in the White‐Inyo Range, California, reveal the compositions and origin of fluids that were present during variable recrystallization of quartzite with sedimentary grain shapes to metaquartzite with granoblastic texture. Metamorphosed sedimentary formations, including quartzites, marbles, calcsilicates and schists, became ductile and strongly attenuated in the aureole during growth of the magma chamber. The microstructures of quartzites have an unusual distribution in that within ~250 m from the pluton, where temperatures exceeded 650 °C, they exhibit relict sedimentary grain shapes, only small amount of grain boundary migration (GBM), and crystallographic preferred orientations (CPOs) dominated by <a> slip. At distances >250 m, quartzites were completely recrystallized by GBM and CPOs are indicative of prism [c] slip, characteristics that are typically associated with H2O‐assisted, high‐T recrystallization. The lack of extensive GBM in the inner aureole can be attributed to rapid replacement of H2O by CO2 produced by reaction of quartz grains with calcite cement that also produced interstitial wollastonite. Fluid inclusions in the inner aureole generally occur in margins of quartz grains and are either wholly aqueous (Type 1) or also contain H2S, CO2 and CH4 (Type 2). Type 2 inclusions occur only in some stratigraphic layers. In both inclusion types, NaCl and CaCl2, in variable proportions, dominate the solutes in the aqueous phase, whereas FeCl2 and KCl are less abundant solutes. The solutes indicate attainment of a degree of equilibrium with carbonates and schists that are interbedded with the quartzites. Some Types 1 and 2 inclusions in the inner aureole show evidence of decrepitation due to high amounts of strain and/or heating suffered by the host rocks, which suggests that they represent pore fluids that existed in the rocks prior to contact metamorphism. In addition to Type 1 inclusions, outer aureole quartzites also contain inclusions that contain CO2 vapour bubbles in addition to aqueous phase (Type 3). These inclusions only occur in interiors of granoblastic quartz that was produced by large amounts of GBM. The aqueous phase has identical ranges of first melting and final ice melting temperatures as Type 1 inclusions, suggesting that they have the same solute compositions. These inclusions are thought to represent the interstitial pore H2O that promoted recrystallization of quartz and reacted with graphite to produce CO2. Absence of significant amounts of CH4 in Type 3 inclusions is attributed to elevated fO2 that was buffered by mineral assemblages in interbedded schists. As opposed to the large amount of CO2 that was produced by the wollastonite‐forming reaction in the inner aureole to inhibit GBM, the amount of CO2 produced in the outer aureole by reaction between H2O and graphite was apparently insufficient to inhibit recrystallization of quartz.