INVESTIGATING THE KUNA CREST AND SENTINEL GRANODIORITE CONTACT ON THE WEST SIDE OF THE TUOLUMNE INTRUSIVE COMPLEX, SIERRA NEVADA BATHOLITH, CALIFORNIA

The 1,100 km², 95-85 Ma Tuolumne intrusive complex (TIC) in the Sierra Nevada batholith is a composite intrusion of four main granodiorite units. Its outer rim is composed of the oldest Kuna Crest unit (and associated rocks). U-Pb zircon geochronology on the SE Kuna Crest lobe revealed the oldest zircon ages for the Kuna Crest unit at 95 Ma. This age overlaps with an U-Pb zircon age of 95 Ma for the ~200 km² Sentinel granodiorite (Miller et al., 2014), which is in contact with the Kuna Crest diorite to granodiorite on the west side of the TIC. While Calkins and Peck (1930) suggested that the Sentinel unit may be part of the TIC, and Petsche (2008) reported a local gradational contact, Miller et al. (2014) concluded that the originally >1 myr age gap between the old Kuna Crest ages and the Sentinel pluton warrant that the Sentinel pluton represent a separate magma pulse.

We revisited the Kuna Crest-Sentinel contact west of the TIC near Hwy 120 and mapped it at 1:10,000 scale. The ~N-S striking contact between the two plutons is gradational over <2.3 km. It is lined by highly deformed blocks of the Yosemite Valley Intrusive suite. The medium-grained Kuna Crest diorite to granodiorite is biotite- and hornblende-rich with up to 50% mafic content and contains plagioclase phenocrysts locally. Mafic enclaves are very elongated and flattened parallel to the contact and the magmatic to locally subsolidus mineral fabric. Schlieren layers are more common closer to the contact. The Sentinel granodiorite is medium- to coarse-grained with much lower color index. Characteristic euhedral hornblende grains range in size <1-3 cm. Enclaves in the Sentinel pluton have much lower aspect ratios than the Kuna Crest enclaves. Schlieren layers are found across the Sentinel and are largely parallel to the contact. The mineral fabrics are magmatic.

The gradational contact between the Kuna Crest and the Sentinel plutons suggest that the two units were co-magmatic at one point and were able to hybridize along the contact. We speculate that the Kuna Crest unit with its higher biotite content was able to deform more strongly against the stronger feldspar-rich Sentinel mineralogy as the interior TIC units were emplaced. Petrography and XRF whole rock element analyses in progress will further test this hypothesis and the hybridization model, respectively.