PRELIMINARY STUDIES OF EMPLACEMENT AND INTERNAL FABRICS OF THE GREEN LAKE PLUTON, SIERRA NEVADA BATHOLITH, CALIFORNIA

The Green Lake pluton is located along the northeastern boundary of Yosemite National Park on the eastern edge of the Sierra Nevada Batholith. This geographically peripheral Cretaceous granodioritic body of the Tuolumne Intrusive Suite lies within Mesozoic volcano-sedimentary units of the Saddlebag Lake pendant. Contacts with the host rocks provide good constraints on both its geometry and emplacement. It is thus a good candidate to test the different material transfer processes linked to pluton emplacement (e.g. faults, ductile flow, stoping). Preliminary mapping of this area shows high angle relationships between pluton boundaries and host rock with neither deflection of strain markers nor change of bedding thickness. This is especially true along the steep-sided walls of the intrusion. Very little deformation related to pluton emplacement can be observed and seems to be restricted to the roof in a narrow and discontinuous aureole (few meters) characterized by shallow-dipping foliation and concordant cleavage. Within the pluton one main NW-striking steeply dipping magmatic foliation (averaging 300°/80°) can be identified and sometimes associated with a second NNW-striking one. The main foliation is consistent regardless of the orientation of the pluton margin and is associated with a NW-trending lineation. Similar strike of cleavage and trend of stretching lineation also occur in host rock even if locally (southern margin, East Lake area), the magmatic foliation is at high angles (30-40°) to the regional NW-striking and steeply dipping S1. We thus believe that the magmatic foliation is tectonic. Moreover, the lack of late sub-solidus overprint suggests that the pluton, during its emplacement, recorded the last near solidus increment of deformation. Present observations in the Green Lake pluton rule out some of the suggested emplacement models such as ballooning or collapse of the pluton floor. Furthermore fault models implying roof uplift are not compatible with the presence of some interfingered margins. Thus, the common sharp and stepped margins of the intrusion imply that stoping is the most important process likely to explain described features and to solve (in part?) the space problem associated with the emplacement of the Green Lake pluton at this crustal level.