IGNITING A MAGMATIC FLARE-UP: THE SALINIAN ARC, CENTRAL CALIFORNIA

Continental arcs are the primary locations where continental crust is distilled to an intermediate, calc-alkaline composition. The root zones of continental arcs are thought to be the primary sites of continental crust formation, yet few deeply exhumed batholiths are available for direct study. Consequently, geologic constraints on the tectono-magmatic processes by which continental arcs produce continental crust are lacking. The Coast Ridge belt of central coastal California provides an exceptional opportunity to directly observe the cumulative effects of lower crustal melting, mixing, assimilation, and homogenization related to construction of the Late Cretaceous California arc. We present new major and trace element chemistry, as well as Sr and Nd isotopic ratios determined on Coast Ridge belt assemblages representative of 20 to 30 km crustal levels. Mid-Cretaceous orthogneisses and Late Cretaceous gabbroids of the Coast Ridge belt are predominantly calc-alkaline, some of which exhibit cumulate characteristics, and all show enriched isotopic compositions (Sr_i=0.7076±0.0010, εNd= -2.2±2.3). REE patterns in igneous and metaigneous rocks of the Coast Ridge belt suggest that they are sourced somewhat deeper than the exposed section, but probably not significantly below 40 km paleodepth. A compilation of age and isotopic compositions of the California arc shows time- and space-dependent shifts in δ¹⁸O, Sr_i, ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb and εNd. These variations require that supracrustal input increased with time between paleo-latitudes of ~35.5 and 33.5˚ N. We posit that subduction accretion assemblages were rapidly emplaced beneath the arc at the base of the upper plate. Dehydration melting in underplated material is hypothesized to have taken place within the magmatic source regime to produce isotopically enriched granitoids in the southern California arc. If tectonic erosion and underplating of accretionary material was responsible for high-flux magmatism in the California arc, then this process contributed greatly to the growth of North America and to the evolution of its crust toward an intermediate composition.