Paper No. 2
Presentation Time: 9:15 AM
GEOTHERMOBAROMETRY OF THE CASTLE CREEK QUARTZ MONZODIORITE SUPPORTS RAPID CRETACEOUS SUBSIDENCE OF THE MINERAL KING METAMORPHIC PENDANT, SIERRA NEVADA, CALIFORNIA
The ca. 98 ± 1.3 Ma Castle Creek quartz monzodiorite intruded on the western margin of the Triassic to Cretaceous Mineral King metamorphic pendant, located in the south-central part of the Sierra Nevada batholith. The intrusive relationship of the pendant and adjacent pluton are important because paleodepths in this part of the batholith vary significantly, supporting a complex structural history. Major, minor, and trace element compositions (by EMPA) of hornblende, plagioclase and quartz for the Castle Creek pluton were evaluated for thermobarometric conditions of crystallization. Al-in-hornblende barometry corrected for plagioclase compositions indicates 1.7-4.0 kbar or pressure equivalent to an average crystallization depth of 10.5-10.9 km. This contrasts with the ~106 Ma Empire Mountain quartz diorite, located in the western part of the pendant, that was emplaced at ~3.3 km based on Al-in-hornblende pressure calculations and pervasive low-δ18O in skarns around the pluton (D’Errico et al., 2012). Pressure and temperature in the Castle Creek pluton were calculated using multiple barometers and thermometers to compare the results across multiple methodologies (e.g., Anderson and Smith, 1995; Ridolfi et al., 2010, 2012). The preferred crystallization pressure and temperature for the Castle Creek is 2.84 ± 0.6 kbar and 736 ± 35 °C, derived from the Al-in-hornblende barometer and the plagioclase geothermometer of Holland and Blundy (1994). Additional data from the Ti-in-quartz thermometer (Wark and Watson, 2006) indicates temperatures of 735 ± 23.7 °C for quartz included in other phases. Based on these results, we propose a three-stage model for the Mineral King pendant: (1) initial surface deposition of rhyolite ending ~130 Ma; (2) subsidence to ~3.3 km by 106 Ma and (3) subsidence to ~11 km by 98 Ma. This suggests a minimum rate of subsidence of ~1 km/m.y. between 106-98 Ma, likely evidence of downward host rock transport to accommodate the intrusions.