FLUID-MELT-MINERAL INCLUSION STUDY OF THE EVOLUTION OF INTERCUMULUS FLUIDS ASSOCIATED WITH THE DEER MOUNTAIN OLIVINE GABBRO, SIERRA NEVADA BATHOLITH, CALIFORNIA

Deer Mountain, located in the central Kern Plateau, southern Sierra Nevada Mountains, is composed of olivine-plagioclase-pyroxene cumulate gabbro surrounded by Cretaceous granodiorite. Insight into the composition of the mantle-derived parental magmas of the Mesozoic Sierra Nevada arc is difficult because gabbro outcrops are rare. Since the Deer Mountain gabbros are cumulates, their bulk-rock compositions undoubtedly differ from their parental magma composition. Information about magma compositions, however, may be provided by a variety of inclusions observed in cumulate plagioclase and clinopyroxene crystals. Among these are fluid inclusions (FI), as well as crystallized melt inclusions (MI). The inclusions may provide insight into the nature of the parent magma as well as the history of subsolidus fluid-rock interaction.

Crystallized MI are composed of quartz-labradorite-magnetite aggregates ±vapor, with anorthite likely crystallized on the inner walls of the MI. The presence of quartz and a more sodic plagioclase within the MI is consistent with trapping of a differentiated intercumulus magma by the crystallizing gabbroic cumulates. In addition, two types of FI were identified: type-1, single-phase FI; and type-2, two-phase liquid-rich FI containing unknown daughter crystals. Type-1 FI occur in plagioclase and clinopyroxene crystals. Plagioclase-hosted type-1 FI are trapped in growth zones along with clinopyroxene inclusions and crystallized MI. The spatial association of clinopyroxene, melt, and fluid phases within a plagioclase host indicates that crystallization of cumulate phases continued down-temperature into volatile-saturated conditions. The type-2 FI are extremely small (1-5µm) and are typically found near calcite-sulfide mineral assemblages within cumulate plagioclase. Type-2 FI are therefore likely secondary and may have formed within late-stage hydrothermal systems. Reconnaissance heating/freezing experiments on isolated type-2 FI indicate that these inclusions are H₂O-rich fluids with a homogenization temperature of ~ 260°C. The results suggest that fluid and crystallized melt inclusions can be used to track the evolution of the intercumulus liquids through crystallization and subsolidus cooling of gabbroic cumulates.