"COBBLING” TOGETHER THE AGE AND COMPOSITION OF MAFIC FOREARC BASEMENT IN COASTAL SOUTHERN CALIFORNIA PRIOR TO NEOGENE BORDERLAND RIFTING

An important lithotectonic element of the Mesozoic convergent margin along the coast of western North America is a belt of mafic, ophiolitic rocks extending for several thousand kilometers. This forearc basement structurally overlies the Cretaceous subduction complex and includes the well-studied Coast Range Ophiolite and equivalent exposures in west central Baja CA. Unfortunately, these relationships have been obscured in the Southern California Borderland (SCB) by profound Neogene deformation involving rifting and transrotation. Later subsidence submerged the SCB, making direct study difficult. Remnants of this mafic forearc have been observed as the Mid-Jurassic Willows Plutonic Complex (WPC) on Santa Cruz Island (SCI) and in boreholes throughout the borderland. The WPC on SCI is comprised of the Willows diorite (163-169 Ma) and Alamos plagiogranite (141 Ma). The goal of this study is to provide an alternative and statistically more meaningful approach for sampling the forearc basement, permitted by analysis of mafic clasts preserved within the San Onofre breccia (SOB). The SOB is a syn-tectonic unit that formed during the 18-12 Ma extension and uplift of the SCB. Its clast assemblage contains material eroded from the Catalina Schist Subduction Complex and mafic clasts similar in composition to the WPC and Mid-Cretaceous Peninsular Ranges Batholith (PRB) gabbro/diorite. Clasts like those from the WPC can be differentiated from PRB clasts by their saussurite mineral assemblage. Mafic SOB clasts deposited along the coast from San Onofre to Laguna Beach and on SCI have been petrographically selected on the basis of saussuritization. These samples are being analyzed for U-Pb zircon age, zircon trace element concentrations and whole rock chemical composition. Preliminary data from a saussurite clast (San Onofre) yielded a U-Pb zircon crystallization age of 148.6±1.7 Ma, broadly overlapping with other reported forearc basement ages. Its source region is further supported by zircon trace element data showing that the Ti, Yb, U and Hf signature plot within the continental arc field near the transition into evolved mafic compositions from Iceland and Hawaii. Additional zircon age/trace element and whole rock concentration work is underway with SOB mafic clasts to more completely assess their tectonic affinity with the WPC and other forearc rocks in coastal southern CA.