Resolving arc processes through detrital zircon U-Pb geochronology and geochemistry: a case study from the southern California Mesozoic convergent margin

Abstract:

Detrital zircon geochronology has been widely exploited to establish temporal characteristics in sedimentary source terranes. Detrital zircon geochemistry, however, has been largely overlooked given results from continentally derived igneous zircon that show subtle intersample variation in trace-element concentrations, and which make correlation between detrital zircon and their host terrane difficult. Nevertheless, recent studies suggest systematically variable geochemistry in McCoy Mountain detrital zircons derived from the southern California Mesozoic arc, and our preliminary data from the Peninsular Ranges batholith indicates strong correlations between whole-rock and zircon geochemistry. Here, we present coupled U–Pb geochronology and geochemistry measured by laser ablation split stream ICPMS on detrital zircons from Nacimiento block forearc sediments in Central California to characterize temporal and geochemical trends in the adjacent Mesozoic arc terrane.

1098 grains of Mesozoic age analyzed from 22 samples in the Nacimiento block define three periods of high magmatic flux in the Permian (270–250 Ma), Jurassic (170–140 Ma), and late Cretaceous (115–90 Ma). Zircon from the Permian arc is the least abundant of the three magmatic pulses, although they consistently display elevated Yb/Gd and U/Yb. Jurassic zircons display consistently low U/Yb, variably elevated Yb/Gd, abruptly higher Th/U and LREE from 155–145, and abruptly lower REE concentrations from 145–140 Ma. Zircon from the Cretaceous arc displays gradually increasing U/Yb, Th/U and LREE, with abruptly decreasing Yb/Gd at 95 Ma. The geochemical trends observed in the Nacimiento block detrital zircons of Cretaceous age are strikingly similar to temporal changes in geochemistry known from Cretaceous arc rocks of the Mojave and Peninsular Ranges, and strongly suggest a southern California provenance for Nacimiento block sediments. Furthermore, the similarity of geochemical trends between Cretaceous detrital grains and known whole-rock analyses suggests that our results can be used to make inferences regarding Jurassic and Permian arc processes, and that with sufficient sampling, detrital zircon geochemistry can be used as a powerful tool to characterize regional-scale geochemical trends over time.