Snapshots of magmatic evolution revealed by zircon depth profiling

Upper-crustal granitoids are a late-stage product of crustal differentiation. This last stage in the evolution has been proposed to be the key interval that governs volcanic eruptions, ore formation, and fluid migration. Though numerous techniques have been employed to understand the evolution of late-stage felsic magmas, there remains little agreement regarding their crystallization histories. Here we use laser ablation depth profiling of zircon trace elements to probe the thermochemical evolution of well-characterized granitoid rocks from the Tuolumne Intrusive Suite (CA, USA). In combination with titanite trace-element data, we can determine the T-X trajectory of the magmatic system during zircon growth at the latest stage of crystallization. Our data also allows for a robust empirical calibration of the widely used Ti-in-zircon thermometer arising from the onset of titanite crystallization captured in zircon rims. These data show that Tuolumne zircon growth occurred at an aTiO₂ of 0.4–0.5, which is lower than many previous estimates. We further model zircon growth up- and down-temperature from titanite saturation, and document extended thermochemical growth records preserved in zircons from these upper-crustal melts. The calculated temperatures are consistent with zircon growth from 775 to 625 °C, indicating that zircon growth occurred over a wide temperature range down to, or potentially below, the water-saturated granite solidus. These findings provide empirically-calibrated values for use in other Ti-in-zircon and Zr-in-titanite temperature calculations, particularly to understand late-stage, low-temperature melts in granitoid systems.