GEOCHEMISTRY AND DISCRIMINANT ANALYSIS OF ZIRCONS FROM TRIASSIC PLUTONIC AND VOLCANIC ROCKS IN THE SIERRA NEVADA: TRACING THE ORIGINS OF GRANODIORITE AND IGNIMBRITES FOLLOWING ARC INITIATION

The connection between plutonic and silicic volcanic rocks following arc initiation is a key to understanding the evolution of arc middle crust. Triassic batholithic and volcanic rock suites in the Sierra Nevada preserve the record of granodioritic to granitic plutonism and broadly contemporaneous ignimbrite eruptions in the initial pulse of Sierra arc magmatism. Zircons were extracted from the Mount Olsen and Wheeler Crest granodiorites, the Rush Creek and Tioga Pass granites of Lee Vining Canyon, the Pine Creek granite, and the tuff of Saddlebag Lake. SIMS analysis of 24 trace elements in 150 zircons is used in combination with discriminant function analysis to highlight geochemical differences between groups of zircons based on rock type. There is no difference between zircons from granites and granodiorites in terms of Ti and Hf, and only minor enrichment in some REE in granitic zircons. Volcanic zircons show a narrow Ti range clustered within the low end of the granitic zircon range and similar Hf and REE. Sc abundance is a major discriminator between plutonic rocks, with zircons from granodiorites possessing significantly lower Sc than granitic zircons. We argue this represents the effect of early-stage amphibole fractionation on granodioritic melts, depleting Sc relative to granitic melts. Sc abundances in volcanic zircons are no different from granitic zircons and significantly higher than granodioritic zircons. This relation suggests a link between large scale volcanic eruptions and granitic melt sources. Additionally, analysis of zircon cores vs. rims suggests occasional intrusion of granitic melt into granodioritic systems, demonstrating a complex and dynamically interconnected melt system. U/Sc vs. Hf shows how zircons from the Rush Creek granite of Lee Vining Canyon possess cores comparable to granitic zircons and rims identical to granodioritic zircons. Zonation disconformities between cores and rims of Rush Creek zircons further support their transfer from one melt system to another. This study presents a linkage between granitic melt sources and large-scale ignimbrite eruptions in the first major pulse of a long-lived arc system and highlights the usefulness of high-dimensional zircon trace element data in elucidating complex relations between volcanic and plutonic systems.