ZIRCON AGE SPECTRA, ZIRCON CRYSTALLIZATION TEMPERATURE, AND THE GROWTH OF THE HALF DOME AND CATHEDRAL PEAK GRANODIORITES, SIERRA NEVADA BATHOLITH, CALIFORNIA

In situ SIMS dating of zircon and application of new geothermometers based on incorporation of Ti in zircon and quartz are greatly enhancing our understanding of pluton construction. We have conducted SIMS analysis of zircons (dating, thermometry, trace element analysis) from the Half Dome (Khd) and Cathedral Peak (Kcp) granodiorites in the 1200 km² Cretaceous Tuolumne Batholith, to better understand its growth and evolution. We focused on samples where appreciable concordant U/Pb age dispersion had been recognized from previous TIMS work. In situ ²⁰⁶Pb/²³⁸U ages (SHRIMP-RG) on zircons from each unit span over 1 million years. Zircon crystallization temperatures estimated by Ti-in-zircon thermometry (T_Ti) range from 640-780°C (a_TiO2 = 0.75) but there is no difference in average T_Ti between Khd (n=22) and Kcp (n=20), and no correlation exists between T_Ti and zircon age (as determined by SIMS). Temperatures from crystal centers are within uncertainty of the T_Ti at rims or show slight core to rim decreases, and T_Ti distributions from both samples show peaks around 670-680°C, with about 1/3 from each having distinctly higher T_Ti. Trace elements vary systematically with T_Ti (e.g. decreasing Th/U and increasing Hf with decreasing T_Ti). Both plutons have low average Zr (Khd = 111 ppm; Kcp =126 ppm) and low zircon saturation temperatures (T_zrn = 736°C in Khd; 755°C in Kcp). When compared to experimental melts of likely Sierran granitoid protoliths, the low Zr and low T_zrn indicate that magmas were zircon-undersaturated initially, and low T_Ti indicates late zircon crystallization in the magmas. The age spectra and presence of zircon that crystallized at temperatures higher than the dominant temperature peak implies recycling of zircon, but general lack of resorption features suggests that zircon antecrysts were armored in other crystals; thus recharge and mixing occurred in cool, crystal-rich magma. Interestingly, the average T_zrn is higher than the average T_Ti, and the distribution of T_Ti is generally shifted to lower temperatures in comparison to the T_zrn distribution. This is at odds with predicted temperature spectra for zircon-undersaturated intermediate melts, and requires either extremely low Zr concentrations in initial melts, substantial removal of zircon elsewhere in the magma system, or partitioning of Zr into other minerals during magma crystallization.