REGIONAL AND EMPLACEMENT-RELATED TECTONISM DURING INTRUSION OF THE TUOLUMNE BATHOLITH: RESULTS FROM THE BENSON LAKE AND MAY LAKE PENDANTS, SIERRA NEVADA, CALIFORNIA

A number of Paleozoic metasedimentary pendants occur along the western margin of the 93-85 m.y. Tuolumne Batholith (TB) and the eastern boundary of the 102 m.y. El Capitan granite (ECG). We have completed detailed mapping of the Benson Lake and May Lake pendants and the surrounding TB and ECG. The metasediments in the pendants range from quartzites to metapelites interbedded with calcsilicate and marble layers. The metapelites are locally migmatitic, and rarely sillimanite-bearing. Structures in both pendants suggest at least 3 folding phases: i) early isoclinal folds, ii) main, cm to 100s of m wide, open, upright folds, iii) late local kink folds. Axial planes of the second fold generation are often associated with steep, melt-present shears resulting from regional deformation. Magmatic folds of ECG dikes indicate syntectonic intrusion of the ECG. The ECG is medium grained, porphyritic and locally mingled with a dioritic phase. East of the pendants the TB consists of medium grained granodiorite to tonalite of the Glen Aulin (GA) unit. The ECG and GA both have narrow subsolidus, dip-slip shear zones, but are dominated by magmatic fabrics. A large number of rafts and stoped blocks occur along the pluton-host rock contacts. They disintegrate mechanically, and only rarely melt. The structures in both pendants indicate pre-ECG intrusion, regional deformation and some minor emplacement related ductile strain. ECG and GA emplacement mainly occurred by stoping. Partial melting could be regional or related to the ECG and TB intrusion; it is strongly controlled by the mineralogy. Both pendants reveal a similar succession of deformation phases, but the orientations of the main structures differ by about 30-40˚ reflecting rotation of at least one of the pendants towards the margins of the plutons during magma emplacement. We conclude based on field evidence and interpretation of structures that regional deformation resulted from transpression dominated tectonics during which the ECG and the TB were emplaced and underwent some subsolidus deformation. The fact that the pendants are only preserved along the western margin of the TB (no pendants in the center of the plutons) argues against a diking model proposed by Coleman et al. (2004) and supports an emplacement model where younger pulses ascend internally in a large magma chamber.