Correlation and petrogenesis of the Miocene volcanic rocks in the San Emigdio and San Juan Bautista areas, California

The San Emigdio (SE) area in the southern San Joaquin Valley and the San Juan Bautista (SJB) area in the northern Gabilan Range lie approximately 295 km apart on opposite sides of the San Andreas fault. The geochemistry of the lower Miocene volcanic rocks from these two areas was studied ·to determine: 1) whether they could be correlated across the fault, and 2> their petrogenesis. The Miocene volcanic rocks from the SE and SJB areas are the same age (approximately 22.5 Ma). Close similarities in major- and trace-element, and isotopic composition strongly suggest that the two areas were in close proximity at the time of eruption and have since been offset by movement along the San Andreas fault. The geochemical data suggest that basalt located in the SE area is not compositionally primitive and probably interacted with the crust. Trace-element and isotopic data from the SE-SJB intermediate to felsic volcanic rocks represent melted crustal rocks, perhaps greywacke from the Franciscan Formation. The SE-SJB volcanic rocks fit well with a group of Neogene volcanic rocks in California that correlate closely with the temporal and spatial position of the Mendocino triple junction. Geochemical comparisons with other volcanic rocks of this group support a similar origin of triple-junction magmatism. One possible model involves decoupling of the subducted slab and consequent upwelling of mantle material. The rising mantle induces melting, which produces basalt. The basalt then induces melting of crustal rocks to produce the more siliceous volcanic rocks.