PLUTON-VOLCANIC CONNECTIONS IN THE SIERRA NEVADA EVALUATED FROM ZIRCON TRACE ELEMENT DATA

The Sierra Nevada batholith formed as a result of multiple large Mesozoic granitic intrusions. The Ritter Range and Saddlebag Lake pendants in the east-central part of the batholith preserve volcanic rocks approximately the same age as intrusions within the batholith and allow us to explore pluton-volcanic connections through time. The purpose of this research is to determine if melts which formed these volcanic and plutonic rocks were chemically similar, using zircon crystals from granites and coeval tuffs. The rocks within the Ritter Range pendant vary between an upper volcanic section and a lower, more deformed and metamorphosed section. Our samples from the upper section yielded ages of about 98 Ma, close to the time of the last magmatic epoch of the batholith. The stratigraphically lowest volcanic rocks within the Saddlebag Lake pendant include three tuffs which have yielded ages of 232 to 219 Ma, much older than the Ritter Range samples and close to the time of initiation of the batholith. Chemical zoning was recognized within the volcanic zircons - Hf and REE increased and Th and U decreased as the crystal grew. REE patterns also vary – Yb/Gd increased and the Eu anomaly deepened with crystallization. The REE data for these zircons was normalized to chondrite and the coexisting melt composition was calculated by using zircon/melt partition coefficients. The calculated melt compositions derived from the plutonic and volcanic zircons are very similar, all showing a strong Eu anomaly. The similarities in the calculated melts of these two rock types suggest a parallel origin.