K-FELDSPAR MEGACRYST AND MINERAL INCLUSION THERMOMETRY AND CHEMOMETRY RECORD A LARGE, HOT, FELSIC MAGMA CHAMBER FOR THE TUOLUMNE INTRUSIVE COMPLEX, SIERRA NEVADA, CALIFORNIA

The construction and thermal conditions of magma chambers are key to understanding large volcanic eruptions. The 1,100 km² large and 10 myr long-lived Tuolumne Intrusive Complex (TIC) in the Sierra Nevada was a large, long-lived intrusive system capable of feeding volcanic eruptions. The interior gradational units reveal a bathtub-ring crystallization pattern of the TIC, preserving a large, pluton-wide magma chamber that remained active via multiple injections of magma over 3-5 myrs. This study uses the evidence of thermal histories preserved in potassium feldspar megacrysts (MKfs) and their mineral inclusions to test conditions of the magma chamber and the mechanisms that kept it active for millions of years.

Thermometry on MKfs and their hornblende and plagioclase inclusions indicate fluctuating magmatic temperatures from near a solidus of 670°C up to 966°C throughout the growth of the MKfs implying frequent thermal episodes of injecting magma, with most of their growth in temperatures above 750°C. Magma injections are supported by episodic increases of trace and rare earth element concentrations that are in concert with temperature increases. U-Pb ages and the thermal results of this study reveal that a large, interconnected magma chamber existed for 3-5 myrs under hot storage.

Large, long-lived magma systems such as the TIC are constructed through incremental magma injections sustaining high temperature over millions of years. The formation and persistence of such a voluminous magma chamber implies that magma can be stored in hot systems capable of feeding volcanic eruptions on the order of 10³ km³ for extended periods.