PROVENANCE OF VOLCANIC CLASTS FROM LATE CRETACEOUS FOREARC BASIN CONGLOMERATES, NORTHERN PENINSULAR RANGES OF SOUTHERN CALIFORNIA

Volcanic clasts from the Campanian-Maastrichtian Cabrillo Formation conglomerates in San Diego and the late Turonian Baker Canyon conglomerates in the Santa Ana Mountains are interpreted as locally derived forearc basin detritus shed from the adjacent Peninsular Ranges batholith (PRB) magmatic arc. Based on proximity and hand sample characteristics, prior workers have correlated the volcanic clasts to the Early Cretaceous Santiago Peak Volcanics (SPV) exposed along the western side of the PRB. Whole rock major- and trace-element compositions of thirty-two volcanic clasts were determined by X-ray fluorescence analysis. The clasts define a high-K calc-alkaline trend on a plot of SiO₂ versus K₂O that is distinct from arc tholeiite and calc-alkaline trends defined by SPV compositions. The volcanic clasts also have higher trace-element contents of Ba, Zr, Nb, Sr, and Rb relative to the SPV, consistent with the higher K nature of the conglomerate suites. A Baker Canyon clast has an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70425, which is at the high end of values determined from the SPV. The volcanic clasts also differ petrographically from the SPV; the clasts contain fresh plagioclase in contrast to SPV plagioclase which is typically albitized in response to static greenschist facies recrystallization. Further, sanidine present in Baker Canyon clasts is unknown from the SPV. Ten new zircon U-Pb laser ablation ICPMS ages from volcanic clasts of the Cabrillo Formation cluster from 96.6 ± 2.5 to 103.7 ± 2.5 Ma, while four Baker Canyon volcanic clasts yielded zircon U-Pb TIMS ages that range from 105.6 ± 2.0 to 107.7 ± 2.0 Ma. These clast ages are distinctly younger than U-Pb zircon ages determined from the SPV which range from ~110-135 Ma. Based on the differences in chemistry, petrography, and age we conclude that it is unlikely the conglomerate clasts were derived from in situ SPV outcrops. Rather, the ages and high-K calc-alkaline affinity of the clasts suggest that these volcanics may represent supracrustal volcanic cover of the PRB farther east from the SPV which has been completely removed by erosion.