THE LUCKY 5 AND ORIFLAMME CANYON PLUTONS: KEY TEMPORAL MARKERS IN THE STRUCTURAL AND TECTONIC EVOLUTION OF THE PENINSULAR RANGES BATHOLITH, SE CALIFORNIA

The Lucky 5 pluton truncates S_1a, a continuous foliation, and associated folds in the Triassic Julian Schist. Peraluminous muscovite granite/granodiorite along the margin of the Lucky 5 grades into a core of amphibole-biotite granite/granodiorite. Most Lucky 5 samples exhibit hypidomorphic granular textures. However, along its easternmost margin a discontinuous ~2-3 m thick zone of weakly foliated rock has been mapped. Laser ablation ICP-MS U-Pb analyses of 30 zircon grains yield a crystallization age of 130.3 ± 2.6 Ma for the Lucky 5. To the east of the thin foliated zone in the Lucky 5, an ~2.5 km wide screen of the Julian Schist separates the Lucky 5 from the Oriflamme Canyon pluton. Within the screen a continuous foliation (S1b) in the Julian Schist is defined by the aligned (001) facies of phyllosilicates and stretched and flattened quartzofeldspathic aggregates. S1b can be traced eastward into the Oriflamme Canyon pluton where it is represented by a mylonitic foliation defined by elongated and flattened quartz ribbons that are transected by west verging shear bands dipping ~30-40^o eastward. Map-scale kink bands fold both S_1b in the Julian Schist and the mylonitic foliation in the Oriflamme Canyon pluton. Laser ablation ICP-MS U-Pb analyses of 27 zircon grains yield a crystallization age of 116.1 ± 2.4 Ma for the Oriflamme Canyon pluton. The above data suggest the following. (1) S_1a and associated folds in the Julian Schist are older than ~130 Ma, and therefore represent pre- to early batholithic deformation associated with the development of the Cuyamaca Laguna Mountains shear zone during the Late Jurassic or earliest Cretaceous. (2) The Oriflamme Canyon pluton was intruded during NE-SW shortening. As a result, the thermally softened region in and around the intruding pluton yielded resulting in the SW emplacement of a segment of the Julian Schist over the Lucky 5 pluton and the formation of S_1b ~116 Ma. This phase of range-perpendicular shortening occurred during the orthogonal subduction between the Farallon and North American plates 110-120 Ma. (3) Map-scale NW trending kink bands fold both S_1b in the Julian Schist and the mylonitic foliation in the Oriflamme Canyon, and are therefore post-116 Ma. They are tentatively interpreted to represent range parallel shortening that may be associated with strike-slip faulting.