U-PB GEOCHRONOLOGY OF A NESTED PLUTON: MOUNT WHITNEY INTRUSIVE SUITE, SIERRA NEVADA, CALIFORNIA

The Sierra Nevada batholith, composed of compositionally diverse arc-related plutons, provides valuable insight into the temporal and spatial evolution of a magmatic arc. The composite Mount Whitney Intrusive Suite (MWIS) in the southwestern Sierra Nevada, California is composed of the Paradise Granodiorite and the Mount Whitney Granodiorite. Current hypotheses suggest the Paradise pluton was intruded by the Whitney pluton as both cooled from the walls inward. Biotite K-Ar ages suggest the Paradise Granodiorite cooled below biotite closure temperatures 5 Ma after the Whitney Granodiorite, contradicting the prevailing model of solidification. Eleven samples collected along a transect of the two plutons were dated using U-Pb geochronology of igneous zircon and titanite grains. Zircon grains from the Paradise pluton range from to 85.2 ± 0.8 Ma to 88.4 ± 1.0 Ma, while zircon grains from the Whitney pluton range from 84.6 ± 0.4 Ma to 85.2 ± 2.1 Ma. Results from titanite grains mimic the age trends observed in the zircon data with the expection of the sample from the pluton contact, recording an age of 86.8 ± 2.8 Ma. We interpret this age to indicate a time when titanite grains reached resetting temperatures during intrusion by the Whitney pluton. These results offer new insight into the complex cooling history of this nested pluton and temporal constraints on a model where the Paradise pluton was intruded by Whitney pluton as both cooled from the walls inward.