TITANITE U-PB AGE AND TRACE ELEMENT CHEMISTRY: TRACING COOLING AND ALTERATION IN SIERRA NEVADA BATHOLITH

Magmatic arcs are important records of crustal growth, with the plutonic framework from which they are built providing important records of magma generation, transport, and emplacement. In this regional survey we assess how magmatic to sub-solidus hydrothermal conditions in the Sierran arc are recorded in isotopic (U-Pb) and trace element (REE, Zr, Nb, Ta) composition in titanite (CaTiSiO₅), a nearly ubiquitous accessory phase in the batholith. Chen and Moore (1982) first analyzed Sierran titanite for U-Pb age, noting that titanite U-Pb ages were often younger than zircon in the same rock. They hypothesized that diffusional Pb loss in titanite led to younger apparent ages.

Building on the modern geochronologic framework in the Sierran arc, including abundant evidence of incremental assembly of plutons, we have assessed this original hypothesis, through LA-ICP-MS analysis of titanite from the central Sierra Nevada, using over a dozen of Chen and Moore's original samples and many others. We used a two-tiered sampling scheme. First, regionally distributed samples from across the batholith were evaluated to see if differences of titanite and zircon U-Pb age reflect slower cooling in older, western batholith domains that may have experienced multiple heating events as younger plutons intruded. A second, localized scheme focused on transects of older plutons (Brush Creek Granite, quartz diorite of Yucca Point) engulfed by younger, larger plutonic systems that would potentially perturb the chemistry of titanite in the older, smaller plutons.

Overall, we note that Sierra Crest plutons have closer agreement of U-Pb age of zircon and titanite, confirming that younger plutons saw a more rapid cooling, thus greater match of zircon and titanite U-Pb age. Western plutons, which may have seen titanite grow in response to changes of redox state in the sub-solidus, don’t present a clear signal between diffusional Pb loss. In the focused studies of small plutons (e.g., Yucca Point) majority of the titanite within the samples analyzed are igneous (magmatic) and have trace element patterns consistent with magmatic growth. Secondary titanite has lower total REEs and positive Eu anomalies suggesting growth in equilibrium with deuteric fluids. We infer that growth of substantially younger (up to 10 million years) titanite was initiated as younger magmas caused re-equilibration of Fe-Ti oxide minerals with the younger deuteric fluids.