GRANITIC THERMAL HISTORIES RECORDED IN TITANITE OF THE TUOLUMNE INTRUSIVE SUITE

The majority of magmas produced at continental arcs do not erupt at the surface. These plutonic roots are windows into the processes that occur before, during and after arc eruptions, and can constrain the chemical and thermal structure of sub-volcanic magmatic and hydrothermal systems. Chemically zoned minerals in granitic rocks are one potential mechanism by which magmatic thermal histories can be assessed. With both an experimentally-calibrated trace-element thermobarometer and several experimentally-determined trace-element diffusivities, titanite (CaTiSiO₅) can provide valuable insights into the thermal histories of magmatic systems. Here, trace-element diffusion profiles retained within titanite from the Tuolumne Intrusive Suite (TIS), in conjunction with Zr-in-sphene thermobarometry, are utilized to reconstruct the range of possible thermal histories experienced by these minerals during and after assembly of the TIS.

The TIS is a compositionally-zoned plutonic system that crystallized over a span of ~8-10 million years. Titanite crystals were investigated from several units across the TIS including the equigranular and porphyritic units of the Half Dome Granodiorite and the Cathedral Peak Granodiorite. Titanite crystals within units have distinctive morphologies and mineral associations that vary between units. These variations also correlate with inter-unit differences in titanite Zr content. Sub-micron resolution trace-element line scans across sector-zoned and growth (oscillatory)-zoned titanite crystals were collected using wavelength-dispersive spectroscopy. The diffusion profiles observed in titanite crystals across the TIS combined with Zr-in-titanite thermobarometry indicate crystallization at or slightly above the nominal granodiorite wet solidus for most analyses. Diffusion profiles reveal that, after crystallization, these systems remained relatively cool, with no evidence of significant time spent above ~750 °C. This supports other lines of evidence from continental arc volcanic and plutonic systems that sub-arc magmas spend the majority of their lives as relatively cool, crystal-rich systems.