GEOLOGIC CONSTRAINTS ON MID-CRUSTAL MAGMATIC PROCESSES IN THE MOJAVE SEGMENT OF THE CRETACEOUS CORDILLERAN ARC

In continental arc settings, rheological boundaries in over-thickened crust prevent the ascension of hot buoyant magma, creating voluminous magma bodies that are home to abundant magmatic processes such as assimilation-fractional crystallization (AFC). Though the effects of magmatic processes are seen in erupted products at the surface, dispute surrounds the depth at which these processes occur and the role they play in altering the geochemical signature of a magma. Models of the vertical architecture of magmatic systems make conflicting predictions about whether and to what extent mid-crustal magmatic processing occurs in continental arcs. The Little San Bernardino Mountains of the Eastern Transverse Ranges of Southern California lie in a tilted crustal section with many analogous characteristics with the modern Andean arc. The section preserves granitic plutons of predominantly Cretaceous age, intruding Proterozoic metamorphic and metasedimentary host rock from shallow to ~20 km paleodepths. Wide Canyon, which transects the deepest exposed mid-crustal zone of the section, is an approximately 16 km long north/south trending canyon that crosscuts the tilted crustal section at approximately 18.65 km projected paleodepth (Needy et al., 2009), extending into a zone hosting apparent AFC processes. Canyon transects were conducted focusing on the present magmatic and potential assimilant components and structural evidence for interactions between components. The top of the canyon is a fine-coarse grained syenogranite to monzogranite, with an increasing abundance of amphibolite and gneissic blocks towards the bottom of the sheeted complex as it transitions into a zone of migmatization. Gneiss becomes the dominant component in this zone. An unaltered, km scale garnetiferous gneissic unit marks the middle of the canyon. Plutonic abundances increase towards the bottom of this unit, yielding various degrees of assimilation and stoping, ultimately yielding the pristine equigranular biotite syenogranite near the bottom of the canyon. Field data suggests extensive mid-crustal magmatic processes such as assimilation and fractional crystallization. This field work will serve as the foundation for geochemical analysis and U-Pb zircon geochronology, to better quantify the observed magmatic processes.