DECONVOLUTING MAGMA MIXING AND SOURCE CHARACTERISTICS OF A LONG LIVED (~9 M.Y.) SIERRA NEVADA MAGMATIC COMPLEX: O, HF AND TRACE ELEMENTS OF ZIRCON FROM THE JOHN MUIR SUITE

Compositionally and temporally zoned intrusive suites in the Sierra Nevada batholith epitomize the complex processes and extended timescales of magmatic evolution. In this work, we have used zircon to obtain a single-mineral perspective of the mixtures of magma that contribute to construction of the John Muir Intrusive Suite (JMS) between 96 and 87 Ma. In situ analyses by SIMS (O isotopes, Trace elements), and LA-MC-ICP-MS (Hf isotopes) show considerable ranges of geochemical indicies: δ¹⁸O within the suite is 4.8–6.5‰ (n = 149). εHf(t) = 1.6 to –8.6, Ti- and Si-activity corrected Ti-in-zircon apparent temperatures are 650­–925°C (n=152), define a broad trend of decreasing average temperature with increasing SiO₂ and decreasing age. In some cases, a single hand sample, or single zircon spans nearly the entire range of an isotopic or trace element index confirming the diverse crystal mixtures that make up JMS magmas. Taken together, samples from the units of the JMS define broad trends in O-Hf space. Lower-silica diorite and quartz-diorite units (Lamarck (n = 3) and Inconsolable (n=2)), including samples of mingled domains within the Lamarck Granodiorite, define arrays toward at least two potential mantle “reservoirs”. In contrast, intermediate compositions of the Lake Edison Granodiorite (n = 3) are broadly distributed, suggesting this pluton may be among the most heterogeneously mixed plutons, recording a change in the overall bulk source characteristics of the JMS. The youngest and highest silica plutons (Round Valley Peak (n=1) and Mono Creek (n=2)), which have the lowest eHf and highest δ¹⁸O overall, show bi-to tri-modal distribution of δ¹⁸O showing variable crustal input potentially as variable high-silica melts that produced the different δ¹⁸O modes. Collectively, our findings require the JMS to have simultaneously tapped multiple lower crustal melt sources during construction of different plutonic domains. Mixing of zircons from these domains must be accomplished through interconnected plumbing networks that are broadly zircon-saturated to prevent dissolution of chemically distinct populations, or where zircon is armored by other crystals from dissolution, especially in small volume diorite and gabbro bodies that preserve some of the greatest isotopic variation within single hand samples.