LOW-T ALTERATION OF VOLCANOPLUTONIC ARC ROCKS; PROCESSING FLUIDS IN THE UPPERMOST SUBDUCTION FACTORY

The tuff of Skelton Lake from the Mammoth Crest area of the Sierra Nevada batholith displays Ca gains, Na losses, and scattered abundances of K relative to expected values for silicic volcanic rocks. In contrast, typically immobile element groups resemble those of rhyolite from calc-alkaline arc settings. The intensity of alkali and alkaline earth element alteration does not correspond to proximity to contacts with the Round Valley Peak granodiorite.

CL, PLM, and SEM petrography show: well-preserved igneous matrix and phenocryst textures even within meters of igneous contacts; variable replacement of groundmass grains and plagioclase phenocrysts by Ca-rich plagioclase (locally >An₉₀) ± epidote ± muscovite; and near end-member K feldspar and albite grains, with microstructures and compositions that may record early alkali metasomatism, locally replaced by plagioclase. Locally abundant calc-silicate-mineral veins display selvages with ~An₉₀ plagioclase. Early thulite-bearing veins are cut by epidote + quartz ± grossular ± calcite veins. Still younger sets of quartz veins and joints display Na-rich bleached margins. Some veins parallel folia similar in geometry to features related to batholith emplacement whereas other veins cut them.

Quartz and whole rock δ¹⁸O values for the tuff (9.7-13.2 ‰) and vein quartz (12-14 ‰) are greater than values for the granodiorite (quartz typically <10 ‰). The data are best explained by low-T (<250°C), alkali and alkaline-earth alteration related to pre-batholithic metamorphism. Fluid sources were likely seawater or basin fluids. Some calcic alteration reflects remobilization by contact metamorphic fluids whose oxygen signature was inherited from earlier alteration.

A complex scenario of alteration and metamorphism is emerging for metavolcanic rock-dominated pendants of the Sierra Nevada. A typical history, as exemplified by the Skelton Lake tuff and tuffs from the Ritter Range, Alabama Hills and Inyo Mountains, is characterized by pre-plutonic alkali and alkaline-earth element alteration, followed by contact-metamorphic reworking of the early signature. In volcanoplutonic arcs, early low-T alteration can hydrate syn- to early post-eruptive settings of volcanic rocks; these processes can impart alteration signatures to later contact metamorphic events.