Paper No. 39-3
Presentation Time: 8:30 AM-6:00 PM
CONSTRUCTION AND STRUCTURE OF THE NORTHERN MARGIN OF THE LATE CRETACEOUS SONORA PASS INTRUSIVE SUITE, NORTH-CENTRAL SIERRA NEVADA BATHOLITH, CALIFORNIA
Understanding of the tectonic and magmatic processes of the Sierra Nevada batholith is insightful to the reconstruction of arcs globally. Relatively little research has been conducted in the northern portion of the batholith, particularly in the Bear Valley - Lake Alpine area. This study focuses on the northern margin of the ~96-92 Ma marginal unit of the normally zoned Sonora Pass Intrusive Suite, the equigranular Kinney Lakes granodiorite. This granodiorite is characterized by heterogenous subunits with relatively high color indices, increased abundance of hornblende relative to the dominant granodiorite, and more intense magmatic foliations. Internal contacts between subunits are gradational. Overall, field relations imply construction by multiple pulses, which interacted in the magmatic/mush state. Foliation strikes in the Kinney Lakes granodiorite range from E-W to NE, and dips are steep, with foliation intensity increasing slightly towards the E-W trending contact with the ~94.8 Ma Lake Alpine granodiorite. Schlieren and mafic enclave swarms also increase in abundance towards the contact. The contact between the Kinney Lakes granodiorite and Lake Alpine granodiorite near Bear Lake is marked by reverse ductile shear zones, xenoliths of stoped metasedimentary blocks, enclaves, felsic dikes, schlieren, and intense foliations. The Lake Alpine granodiorite is characterized by a higher abundance of biotite than the Kinney Lakes granodiorite, which is hornblende-rich. The Lake Alpine granodiorite displays E-W foliations near the contact with the Kinney Lakes granodiorite, which rotate to NW farther from the contact. The unit also locally contains internal structures such as mafic enclaves, schlieren, stoped metavolcanic blocks, and ladder dikes. Near the Bear Valley Ski Area, a previously unrecognized and undated host rock granite contains K-feldspar phenocrysts, is well-foliated, and has been intruded by andesitic bodies. Future planned geochronological work will better determine the age of this granite.