GEOCHEMICAL SIGNIFICANCE OF A JURASSIC IGNIMBRITE FROM A SECTION OF THE RITTER RANGE PENDANT, CENTRAL SIERRA NEVADA, CALIFORNIA

A 100- to 250-m-thick layer of phenocryst-rich ignimbrite from a section of the eastern Ritter Range pendant in the central Sierra Nevada, CA, records explosive eruption of at least ~35 and perhaps up to 100 km³of silicic magma in Middle Jurassic time. Fossils were found indicating a marine depositional environment (Sorensen et al., 1998). The overarching intent of this study is to discover information on processes leading up to and during an explosive submarine volcanic eruption. The tuff of Rosalie Lake has a northwest strike and dips steeply and is indicative of a single explosive submarine eruption sequence. The ignimbrite is porphyritic, with ~50% subhedral and broken euhedral crystals composed primarily of feldspar and quartz. Both quartz and feldspar phenocrysts have observable embayments and relict melt inclusions, and quartz phenocrysts record post-emplacement deformation. The ignimbrite has silica content ranging from 69 to 74 wt. % and so is compositionally intermediate between Group 1 and Group 2 ignimbrites of Bachmann and Bergantz (2008), represented by the Bishop Tuff and Fish Canyon Tuff, respectively. Whole rock alkali contents are high (K₂O = 6 - 8.6%), and along with whole rock and quartz d¹⁸0 = 11 to 12 (Hanson et al., 1993) suggest post-emplacement marine hydrothermal alteration. Rare earth element abundances show light rare earth element enrichment and plagioclase fractionation in these hydrothermally altered samples. High crystallinity and intermediate silica content suggest the tuff of Rosalie Lake represents explosive submarine eruption from a voluminous mushy magma chamber in the Jurassic east Sierran magmatic arc.