Paper No. 39-2
Presentation Time: 8:30 AM-6:00 PM
CONSTRUCTION, EMPLACEMENT, AND STRUCTURE OF TWO MAJOR INTRUSIVE SUITES IN THE NORTHERN SIERRA NEVADA BATHOLITH
Our field-based study focused on the construction, emplacement and structure of the northernmost component of the Fine Gold Intrusive Suite, the ~117 Ma Ebbetts Pass granodiorite, and the marginal unit of the Sonora Pass Intrusive Suite, the ~96-92 Ma Kinney Lakes granodiorite. This study area also includes two smaller Cretaceous host plutons to the suites: the Lookout Peak tonalite and Stanislaus Meadow quartz monzonite. Magmatic foliations in the interior of the Stanislaus Meadow quartz monzonite strike NNW, subparallel to the regional strike. Magmatic foliations formed by internal processes in the Ebbetts Pass granodiorite dip steeply and generally strike SW. A second NW-striking magmatic foliation that is subparallel to regional strain is common. Xenoliths of the Stanislaus Meadow occur in the Ebbetts Pass granodiorite within ~100 m of the contact, implying stoping and substantial cooling of the quartz monzonite before emplacement of the granodiorite. The West of Elbow granodiorite, which contains many elongate enclaves, schlieren-rich zones, and internal contacts, was thought to be a host pluton, but it has similar lithology to the Ebbetts Pass granodiorite and their contact is gradational. Thus, the West of Elbow rocks are inferred to be a marginal phase of the pluton. The Lookout Peak tonalite is presumed to be the oldest host unit due to the solid-state deformation, strong foliation, and later injections of felsic material. Foliations in the Lookout Peak tonalite strike NNW and SSW. The heterogenous Kinney Lakes hornblende-biotite granodiorite is the youngest unit in the area. Foliations in Kinney Lakes granodiorite strike from WNW to NNE and become subparallel to the NNW-trending contacts with its host rocks. There are also zones of weakly foliated, intermediate to felsic rock. There are schlieren-rich zones in the pluton up to ~5 m in width. The variation in lithology and abundant internal contacts suggest that this pluton formed from multiple magmatic pulses.