STRUCTURE AND EMPLACEMENT OF THE BUENA VISTA CREST INTRUSIVE SUITE, SIERRA NEVADA BATHOLITH, CALIFORNIA

The normally zoned Buena Vista Crest Intrusive Suite (BVCS) was emplaced from ~100 to 95 Ma into slightly older plutons of the central Sierra Nevada batholith. It is one of the least studied intrusive suites in this part of the batholith, and thus a structural study of the northern BVCS and its host rocks was conducted to evaluate foliation patterns and emplacement mechanisms. The northern BVCS is comprised largely of the marginal granodiorite of Illilouette Creek and internal granodiorite of Ostrander Lake. Other units include the older granodiorite of Turner Ridge, which occurs as a small body and dikes, and the Bridalveil Granodiorite, the youngest BVCS unit, which mostly forms < 1.5 km wide sheets. Steep magmatic foliation and lineation are present in all of the units. The BVCS intrudes ~102 Ma El Capitan Granite and younger Taft Granite of the Yosemite Valley Intrusive Suite and contains metasedimentary xenoliths up to 2 m across. Previous mapping suggested the BVCS intruded Jurassic or Early Cretaceous diorite; however, the diorite is mingled with both Taft Granite and 99 ± 1 Ma Illilouette Creek granodiorite implying emplacement of the Yosemite Valley suite and BVCS may overlap. The Illilouette Creek granodiorite is characterized by two foliations, a dominant N-S striking fabric and weaker E-W striking fabric. The E-W foliation is concordant to generally sharp host rock contacts, and is probably related to emplacement, whereas the N-S foliation may reflect regional strain. A similar pattern is found in the younger Tuolumne Intrusive Suite to the NE, but there the E-W foliation is attributed to regional strain and the N-S foliation to internal processes (Paterson et al 2004). Regardless, the N-S and E-W pattern of foliation formed over a span of ≥ 10 Ma. Foliation in hornblende-biotite granodiorite of the Ostrander Lake unit varies from NE-SW striking in the south to N-S in the north and is bent into concordance with sharp, E-W contacts with Illilouette Creek. Host rock contacts and magmatic foliations are typically N-S trending and are cut discordantly by the BVCS; thus, little to no structural aureole is present and ductile flow did not play a key role in the emplacement of the BVCS. The discordant relations and isolated metasedimentary xenoliths suggest that stoping was an important material transfer process during emplacement.