ZIRCON GEOCHEMISTRY RECORDS THERMAL AND COMPOSITIONAL EVOLUTION OF PLUTONS AND VOLCANIC ROCKS IN THE EASTERN SIERRA NEVADA BATHOLITH

The Mesozoic Sierra Nevada batholith preserves a long and widespread record of continental arc magmatism that can be used as a model for similar processes around the world. This region is important in that the batholith and corresponding ash-flow tuffs, which originated from the same magmatic system, have been tilted by tectonic forces so that rocks from different levels of the “plumbing” of the system can be studied. The purpose of this study is to determine the petrogenesis of such magmatic systems using geochemical analyses of zircons from Triassic and Jurassic ash-flow tuffs and plutonic rocks of the eastern Sierra Nevada batholith. The zircons display visible zoning, partial dissolution, and regrowth. Analyses were taken on both homogenous spots, usually at the core of the crystal, and over bands of zoning, typically toward the rim of the crystal. Analyses indicate geochemical variability both within grains and among grains in a single sample. Visible zoning is chemically related to total REE, U, and Th contents, which are highly correlated to each other but show no simple relation to Hf content. Cores and rims show no systematic differences in composition. Titanium thermometry was used to calculate zircon crystallization temperature, yielding temperatures as low as 631°C and as high as 885°C. Samples of Jurassic age yield higher crystallization temperatures in general, indicating a higher temperature magma system at that time. Calculated temperatures and compositions show significant overlap for ash-flow tuffs and batholithic rocks of the same age, suggesting that the thermal evolution of ash-flow tuff magmas was similar to intrusive rocks prior to eruption. Geochemical analyses of the zircons show elevated U contents, U/Yb and Yb/Gd in the Triassic zircons, and elevated Ce contents and Th/U in Jurassic zircons. These differences may suggest relatively greater fluid interaction in Triassic magmas and relatively greater crustal involvement in Jurassic magmas.