OPEN SYSTEM MAGMA RESERVOIR FRACTIONATION MODEL FROM JURASSIC EXPLOSIVE VOLCANISM IN SIERRA NEVADA, CALIFORNIA

Serial daughter magmas generated by closed-system crystal fractionation will differ in composition, and some may also erupt to form silicic ash-flow tuffs. The purpose of this study is to examine closed-system fractionation in a typical Sierra Nevada granodiorite, and place limits on the role of fractionation in generating coeval ash-flow tuffs. The Jurassic Palisade Crest Intrusive Suite was emplaced in the eastern Sierra Nevada between about 171 and 165 Ma. The Tinemaha pluton of the Palisade Crest Intrusive Suite is granodioritic to granitic, with silica ranging from 57-70% and was modeled as a closed magma system undergoing sidewall crystal fractionation (Sawka et al., 1990). The southern Tinemaha pluton underlies volcanic rocks of the Oak Creek pendant, including the rhyodacitic tuff of Sardine Canyon, with 68-72% Here we model the Tinemaha pluton as an open system that could have generated coeval voluminous ash-flow tuffs. Solidification of the chamber by sidewall crystallization of a 70% silica granodiorite magma that produces plagioclase, hornblende, biotite, and magnetite, will ensue at approximately 740⁰C and 3 kbar pressure (Piwinskii, 1968). Petrography of Sardine Canyon tuffs show large phenocrysts of quartz and plagioclase that range from 1-3mm with smaller phenocrysts of hornblende and biotite ranging from 0.5-1mm. Inclusions of matrix within the phenocrysts are indicative to reheating potentially during eruption. All volcanic rocks have been hydrothermally altered and metamorphosed progressively with time based on metamorphic and hydrous mineral abundance, foliation and quartz deformation. Major element analysis shows compatible relationships with silica for all major elements except potassium oxide, and the tuffs are similar in composition to relatively silica-rich granodiorite and granite. REE analysis shows similar abundances, extent of HREE depletion, and Eu fractionation in intrusive rocks and tuffs. The data suggest a model where eruption to produce ash-flow tuff occurred in the late stages of double diffusive crystal fractionation, before the crystallization of significant quantities of sphene.