DETRITAL ZIRCON GEOCHRONOLOGY AND GEOCHEMISTRY OF THE NACIMIENTO BLOCK, CENTRAL CALIFORNIA COAST

The California Mesozoic arc is commonly cited as an archetypal example of a convergent margin tectonic setting, although the juxtaposition of Nacimiento Block subduction mélanges and Salinian Block arc rocks across the enigmatic Sur–Nacimiento Fault in the central California coast serves as a reminder of the complexity of the Cordilleran Mesozoic forearc. Here, we present our initial geochronologic/geochemical results from detrital zircons in Nacimiento Block forearc sediments as part of a broader study designed to place paleogeographic constraints on the evolution of the Sur–Nacimiento Fault through investigations of Nacimiento Block provenance and the evolution of adjacent Mesozoic arc terranes.

Nacimiento Block forearc sediments consist of Late Jurassic–Early Cretaceous mudstones and greywacke interlayers of the Toro Fm overlain by Late Cretaceous arkosic sandstones and mudstones of the Atascadero Fm. 6 detrital zircon geochronology samples from the Toro Fm and one sample from the Atascadero Fm were measured by LA-ICPMS at UC Santa Barbara; split-stream analyses were completed on one sample from each formation to supplement zircon ages with REE geochemistry. Toro Fm analyses display a Cordilleran probability maxima of 140–160 Ma that yields maximum depositional ages consistent with Tithonian–Valanginian fossils in adjacent outcrops, although ~60% of the analyzed grains are pre-Cordilleran with broad peaks at 0.3–0.6, 1.0–1.2, ~1.45, and 1.6–1.8 Ga, and indicate significant input from recycled Triassic/Jurassic and Neoproterozoic–Early Paleozoic strata. Atascadero Fm detrital zircons are dominated by 90–110 Ma grains with <5% of analyzed grains from pre-Cordilleran terranes. Trace element concentrations in Mesozoic zircons from both formations display significant variability within given age ranges, although bulk averages are characterized by higher Yb/Gd during Permian–Triassic arc magmatism, and higher Th/U with lower U/Yb during Late Jurassic magmatism. These initial results are broadly consistent with previously published detrital zircon geochemical trends from the McCoy Mountains of Southern California suggesting a Southern Sierra or Mojave provenance for Nacimiento Block sediments, while also adding further insight into regional arc processes through time.