Open-system Behavior during Pluton-Wall-rock Interaction as Constrained from a Study of Endoskarns in the Sierra Nevada Batholith, California

Crustal xenoliths (pyroxenites and plagioclase + quartz + pyroxene lithologies) from the Quaternary Big Pine volcanic field on the eastern flank of the Sierra Nevada Batholith in California (USA) represent the products of metasomatic reaction between the margins of a Cretaceous granodioritic pluton and Paleozoic marbles, possibly at mid-crustal depths based on the equilibration temperatures recorded by Ti-in-quartz geothermometry. This interpretation is based on the presence of plagioclase showing relict plutonic textures, pyroxenite characterized by nearly pure diopside clinopyroxene, recrystallized plagioclase with anomalously high anorthite content, textures indicating replacement of plagioclase by clinopyroxene (and vice versa), 'ghost' plagioclase rare earth element signatures in some clinopyroxenes, and the presence of phlogopite endmember micas at the contact between clinopyroxene-rich and plagioclase-rich zones. These observations suggest that the xenoliths represent fragments of an 'endoskarn', the outer sheath of a pluton that chemically reacted with carbonate country-rock. Mass transfer between the carbonate country-rock and the pluton involved transfer of Ca and Mg from the carbonate into the pluton and transfer of Na, K, Al and Si from the pluton to the carbonate, the latter generating extensive endoskarns. The Ca metasomatism of the pluton converted alkali feldspar components into anorthite-rich plagioclase, releasing Na and K, which left the plutonic system. K, in particular, migrated towards the carbonate and precipitated phlogopite upon entering clinopyroxene-rich lithologies. Mass-balance calculations, based on theory and residual enrichments in immobile elements such as Ti, suggest that the pluton experienced net mass loss (> 15%) in the form of Si, Al, Na and K to the surrounding country-rock, but a net gain in Ca and Mg.