AGE CONSTRAINTS AND CORRELATION OF “SUR SERIES” METASEDIMENTARY FRAMEWORK ROCKS: IMPLICATIONS FOR THE ORIGIN OF THE SALINIAN TERRANE, CENTRAL CALIFORNIA

The Salinian Terrane of central California is an allochthonous assemblage of chiefly Late Cretaceous granitic rocks (the “Salinian Arc”) and metasedimentary framework lithologies. Salinian metasedimentary rocks, a suite of pelitic, psammitic, and carbonaceous protoliths collectively referred to as the “Sur Series,” are believed to be part of the Neoproterozoic to late Paleozoic SW U.S. Cordilleran miogeocline and Mesozoic cover strata. However, the correlation of Sur Series metasediments across and outside of the Salinian Terrane is greatly complicated by 1) high-grade metamorphism and partial melting, 2) extensive Late Cretaceous to recent faulting, and 3) limited and poor exposure due to vegetation and pervasive intrusion by the Salinian Arc. We address these complexities through analysis of ~2400 zircon U-Pb ages from the Sur Series framework rocks collected along the ~300 km length of the Salinian Arc.

Our new geochronologic work indicates that maximum depositional ages, based on the youngest well-defined group of detrital zircon U-Pb ages, range from Neoproterozoic to early Mesozoic. Paleozoic and older protoliths exhibit some heterogeneity, but commonly contain prominent pre-Cordilleran detrital zircon age peaks at ca. 0.7, 1.1, 1.4, and 1.7 Ga, whereas early Mesozoic rocks contain significant quantities of Permian and Triassic grains with lower proportions of pre-Cordilleran grains. These relations: 1) confirm that the Sur Series likely represents a sequence of Paleozoic and older miogeoclinal rocks and early Mesozoic cover rocks, and 2) point to the Mojave Desert region as the most likely original site of deposition of Sur Series strata. Finally, targeted analysis of zircon grains and grain domains exhibiting convolute zonation patterns in high-resolution cathodoluminescence images yields a population of 85-110 Ma ages commonly associated with elevated U/Th ratios. These relations suggest that Late Cretaceous zircon ages were grown in-situ after deposition and hence record the timing of Sur Series metamorphism.