FIELD AND PETROFABRIC RELATIONS OF THE LONG LAKE SHEAR ZONE AND THE TREASURE LAKES PHASE, LAMARCK GRANODIORITE, CENTRAL SIERRA NEVADA, CALIFORNIA

Contact and fabric relations between the Long Lake Shear zone (LLSZ) and the Treasure Lakes phase of the Lamarck Granodiorite (Klt) suggest that the emplacement of the pluton and deformation in the shear zone are genetically related. Fabric relations of the deformation zone indicate two distinct periods of contraction across the subvertical LLSZ, the second of which overlapped in time with the emplacement of the Klt. Mapping (1:10,000), kinematic, and petrofabric analysis begin to elucidate the deformation history of the LLSZ and the emplacement history of the Klt, which is the oldest phase of the Lamarck pluton (94.5 Ma, Gracely, UNC MS thesis, 2007). The Klt contains numerous wall rock screens, many of which were not previously mapped (Bateman, 1965; Hathaway 2002, UCLA PhD diss.). The screens appear to have been isolated in situ, similar to relations in the nearby McDoogle pluton (Mahan et al., 2003; Stearns and Bartley, this volume), and thus the screens are interpreted to mark contacts between growth increments. Sheets of Klt < 1-25 m thick were injected lit-par-lit along the pre-existing foliation (S1). The dikes and wall rocks were then folded and boudined together (F2) during reactivation of the LLSZ. Klt dikes appear to have been more competent than schistose wall rock based on boudinage , yet probably still contained pore melt because they lack a solid-state fabric related to this deformation. Sense of shear is dominantly dextral and reverse (east-side-up) based on offsets of intrusive contacts and asymmetry of partially annealed porphyroclasts in thin section. These observations, combined with the U-Pb age of the Inconsolable pluton (Ki, 95.4 Ma, Gracely, 2007) which bounds the LLSZ to the east, narrowly limit the age of reactivation of the LLSZ and indicate the following history: 1) a period of pre-Lamarck (late J and/or early K?) contractional mylonitic shearing across the LLSZ which involved Jurassic and older rocks; 2) quiescence of the shear zone and emplacement of the adjacent, nearly undeformed Ki; and 3) dextral and east-side-up reactivation of the LLSZ, during emplacement of the highly composite Klt which concluded before emplacement of the undeformed Marie Louise phase of the Lamarck (~ 92 Ma, Coleman et al., 1995).