THERMOMETRY AND CHEMOMETRY ON PLAGIOCLASE AND HORNBLENDE INCLUSIONS IN K-FELDSPAR MEGACRYSTS FROM THE TUOLUMNE INTRUSIVE COMPLEX, SIERRA NEVADA

U-Pb zircon geochronology of zircon inclusions in K-feldspar megacrysts (MKfs) from the 95-85 Ma, 1,100 km² Tuolumne intrusive complex (TIC) has shown that MKfs can preserve protracted histories of magma processes during their growth (Chambers et al., 2020). MKfs are present in the zoned porphyritic Half Dome granodiorite (3-4 cm) and Cathedral Peak units (3-12 cm) and the mixing and transitional units between them, providing a record of the most voluminous period of TIC growth during the maturation of the arc in the Cretaceous. Further, MKfs contain mineral inclusions that can be leveraged for thermometry and chemometry studies during MKfs growth, to track magma processes through time and the examination of MKfs growth environments. This study analyzed plagioclase and hornblende inclusions in MKfs along core to rim profiles in individual MKfs, between MKfs within the same unit, and across units and the southern and northern TIC.

Electron microprobe major oxide analyses on plagioclase and hornblende inclusions in MKfs are used in temperature and melt composition calculations in equilibrium with these minerals using methods by Caricchi and Blundy (2015), Scruggs and Putirka (2018), Zhang et al. (2016), and Putirka (2016).

Mineral inclusion temperatures vary from their core to rim across MKfs, many fluctuating between higher and lower temperatures. Several MKfs show higher temperatures toward the MKfs rims, which are generally hotter than the core temperatures. Elevated temperatures along core-to-rim profiles likely represent episodic magma recharge. The temperatures at which the inclusions were entrapped during MKfs growth are represented by the rim temperatures of the inclusions. Plagioclase and hornblende inclusions both show maximum temperatures of entrapment at around 750-770°C. MKfs mineral inclusions generally yield lower temperatures in northern Cathedral Peak unit than in the southern Cathedral Peak. LA-ICP-MS trace element analysis of the plagioclase and hornblende inclusions in progress will add more insights into the chemistry of the intruding magma and the interconnectivity of the TIC units with one another.