THE CONSTRUCTION, EMPLACEMENT, AND STRUCTURE OF CRETACEOUS PLUTONS IN THE EBBETTS PASS REGION, NORTH-CENTRAL SIERRA NEVADA BATHOLITH

Cretaceous plutons ranging from ~122 to 88 Ma dominate a ~125-km-long segment of the Sierra Nevada batholith between Sonora Pass and Lake Tahoe. My field-based research in the Ebbetts Pass region focuses on the poorly known Lookout Peak tonalite (Klp), Stanislaus Meadow quartz monzonite (Ksm), ~117 Ma Ebbetts Pass granodiorite (Kep), and ~96-92 Ma Kinney Lakes granodiorite (Kkl). Abundant internal contacts and widespread schlieren imply that these plutons were likely constructed from multiple magma pulses. Magmatic features such as enclaves, enclave swarms, schlieren, and felsic dikes are common in the margins of all units and occur in the interior of the Klp and Kkl. The Kkl contains belts of schlieren m to 100s of m in length, commonly associated with abundant enclaves, and alternating mafic and felsic sheets with gradational to sharp contacts. The Ksm contains both a porphyritic and equigranular subunit with an ambiguous contact. The eastern margin of the Kep has abundant schlieren, internal contacts, and ductile shear zones. Multiple emplacement processes are recorded in the plutons. The Kep contains abundant cm-to-m-scale xenoliths of the Ksm which implies some stoping. Xenoliths are rare in the other units. Magmatic wedging is proposed for the Kkl based on local interfingering of felsic Kkl into the Ksm. Local solid-state structures oriented parallel to contacts at margins of the Kep, Ksm, and Kkl are compatible with ductile flow. Floor subsidence and roof uplift are difficult to determine because the floors and roofs of the plutons are not preserved. Magmatic foliations in the plutons dip 60° to vertical; some likely record internal magmatic processes and others regional NE-SW shortening. Steep magmatic lineation generally plunges SE. Solid-state deformation including foliation, ductile shear zones, and ductile slickensides, is variably developed in the plutons. The shear zones are ubiquitous in the Klp, which is presumed to be the oldest unit. E-W-trending ductile shear zones and felsic dikes cross-cut NW-trending shear zones. The NW-striking shear zones are presumably associated with NE-SW shortening recorded in other plutons in the central and southern Sierra Nevada batholith.