Cordilleran Section - 115th Annual Meeting - 2019

Paper No. 30-1
Presentation Time: 1:30 PM


ACKERSON, Michael R., Department of Mineral Sciences, Smithsonian National Museum of Natural History, Smithsonian Institution, MRC 0119, PO BOX 37012, Washington, DC 20013-7012

Extracting crystallization temperatures and thermal histories recorded within minerals is critical to understanding how granitic magmas assemble, and to better elucidating the relationship between silicic plutonic and volcanic systems. Recent work has suggested that magmas may be stored at temperatures cooler than previously thought, and that minerals in granitic systems record crystallization temperatures below the granitic wet solidus. These observations complicate conventional interpretations of late-stage magma evolution and require more work to constrain the extent of low-temperature processes and the thermal and chemical evolution of silicic magmas.

This study builds on previous research on granitic rocks of the Tuolumne Intrusive Suite (TIS)— a compositionally-zoned group of calc-alkaline granitoids in the Sierra Nevada, California. Ti-in-quartz thermobarometry combined with diffusion modeling of Ti concentration profiles in quartz in these rocks demonstrated the quartz crystallized at temperatures significantly below the granitic wet solidus. However, uncertainties in cooling rates and estimates of titanium activity during quartz crystallization make it difficult to put precise constraints on the temperatures and cooling rates recorded within these crystals.

Here, a novel diffusion modeling technique is utilized to place better constraints on the temperatures and cooling histories of quartz in granitic rocks of the TIS. This technique also enables more accurate determination of titanium activity during granitic mineral crystallization. The results of this work demonstrate that granitic minerals record complex crystallization histories between super- and sub-solidus processes. The temperatures and cooling histories recorded by quartz within the TIS vary from the margins to the center of the Suite, demonstrating that the ~10 million years of assembly of this large-volume of granitic material was facilitated by an evolving thermal regime.