LOW-TEMPERATURE CRYSTALLIZATION OF QUARTZ IN GRANITIC ROCKS FROM THE TUOLUMNE INTRUSIVE SUITE

Understanding of the temperatures of granite crystallization is critical to interpreting the formation and compositional evolution of the continental crust— from the Hadean to the present day— as well as the thermal and structural profile of the crust and the petrogenetic link between granitic and volcanic rocks. Experimentally-determined solidus curves have contributed to the widespread perception that granitic mineral assemblages do not crystallize below ~650-700 °C. However, evidence from some experiments^1,2suggests that the most abundant minerals in granitic rocks (quartz and feldspars) can crystallize together at temperatures as low as 330 °C. It has yet to be demonstrated whether granitic mineral assemblages exist in nature at such low temperatures.

Advances in trace-element thermobarometry and diffusion modeling in crustal minerals (e.g., zircon and quartz) have enabled research into the thermal histories recorded in granitic mineral assemblages. In this study we combine Ti-in-quartz thermobarometry, cathodoluminescence (CL) imaging and diffusion modeling of Ti concentration gradients in quartz to investigate the petrogenesis of the Tuolumne Intrusive Suite (TIS) of the Sierra Nevada Batholith. The results indicate that quartz crystallized at temperatures of 474-561 °C, which is several hundred degrees below the canonical granodiorite wet solidus (~690˚C at 2 kbar). This conclusion agrees with feldspar and amphibole chemistry of the TIS and demonstrates that for some granitic systems, the traditional wet solidus is not the low-temperature limit of granitic mineral crystallization.

1. Huang, F. et al. Chemical and isotopic fractionation of wet andesite in a temperature gradient: Experiments and models suggesting a new mechanism of magma differentiation. Geochim. Cosmochim. Acta 73, 729–749 (2009).

2. Schairer, J. F. & Bowen, N. L. Melting relations in the systems Na₂O-Al₂O₃-SiO₂ and K₂O-Al₂O₃-SiO₂. Am. J. Sci. 245, 193–204 (1947).