FOLIATION DEVELOPMENT AND INTRUSIVE RELATIONSHIPS IN THE CRETACEOUS WRIGHTS LAKE AND JURASSIC PYRAMID PEAK PLUTONS OF THE NORTHERN SIERRA NEVADA BATHOLITH, DESOLATION WILDERNESS, CALIFORNIA

The structures of a Cretaceous and adjacent Jurassic pluton in the Sierra Nevada batholith near Lake Tahoe were studied to gain an understanding of the implications of magmatic foliation patterns and processes operating during pluton emplacement. The voluminous, poorly dated (ca. 106-92 Ma) Wrights Lake granodiorite is a generally homogeneous, medium- to coarse-grained hornblende-biotite granodiorite, with abundant enclaves and schlieren. It intrudes the Pyramid Peak granite (164±7 Ma) along a roughly N-S-trending contact. This older pluton is a coarse-grained quartz monzonite, but also features internal zones of diorite and gabbro, some of which form a sequence of gradational layers (Wiebe et al., 2002). Two overprinting, hornblende- and biotite-defined magmatic foliations and associated steeply plunging lineations are observed within the Wrights Lake granodiorite at the outcrop and map scale. A dominant, steeply dipping foliation of moderate to high intensity is parallel to the overall N-S trend of the pluton contact and bends to match a large recess in its eastern boundary, suggesting the influence of internal magma processes. A less intense fabric strikes ~E-W, discordant to both the pluton contact and to internal structures, and locally transposes the older foliation, associated enclaves, and schlieren. This orientation is similar to trends noted for the roughly coeval Tuolumne batholith (Žák et al., 2007), and is interpreted to reflect an interval of regional N-S shortening during final cooling, which is anomalous with respect to the rest of the Sierra Nevada batholith. Steep foliations within the Pyramid Peak granite, defined by coarse feldspar, strike consistently NNW and are interpreted to record WSW-ENE regional shortening in the Jurassic. The pluton contact is sharp and stepped in most places. Xenoliths occur only within 3m of the contact, and range from large, angular blocks to smaller, rounded pieces to zones of disaggregated feldspars. These suggest that brittle material transfer of the host helped accommodate emplacement. Solid-state deformation within the older pluton near the contact may indicate limited ductile flow also resulted from emplacement of the younger granodiorite.