Petrological studies in southern California have revealed middle to upper crustal portions of two juxtaposed Mesozoic magmatic arcs one being a native terrane of the Cordilleran orogen and the other "exotic" Tujunga or San Gabriel terrane. Crystallization thermobarometry of successively intruded and dated plutons and their thermal aureoles have provided "crustal nails" to track the variable depth history of orogenic crust in both terranes. Deep Mesozoic crust is now exposed at the surface by virtue of two contrasting tectonic settings - lower plates of extensional detachment faults in Tertiary metamorphic core complexes and upper plates of Mesozoic basement-cored thrust faults. The goal of this proposal is to document the nature of Mesozoic are development as a function of crustal depth across the terrane boundary and to utilize this information to constrain the accretionary history of this region of the Cordillera. This proposal outlines a petrological investigation of the plutons and host assemblages in both terranes. The approach is field based but includes petrologic, chemical, isotopic, and mineralogic studies to evaluate evolution of the plutonic suites from their inception at the source region to the condition of their ascent and emplacement. Work to suggests that the two terranes were accreted by mid-Cretaceous, if not by the middle Jurassic. Much of the apparent "exotic" nature of the Tujunga terrane stems from its partial derivation from the middle crust. Work to date has identified crustal sections that one resided at depths to 16 to 30 km. These middle crustal rocks exhibit several distinct characteristic differences from that at shallower crustal levels including the occurrence of Proterozoic anorthosite + charnockite and an abundance of Mesozoic plutons ranging from unusally calcic two-mica granitoids to a variety of intermediate igneous suites including diorite, quartz diorite, and tonalite.
