MESOZOIC TECTONOMAGMATIC OVERPRINT OF A NEOPROTEROZOIC LAURENTIA MARGIN SEDIMENTARY ASSEMBLAGE AT PLACERITA CANYON, WESTERN SAN GABRIEL MOUNTAINS, CALIFORNIA

The western San Gabriel Mountains host distinct metasedimentary rocks of the Placerita Formation (Miller, 1934; Oakshott, 1958), situated at the southwest edge of known Proterozoic Laurentia basement. Graphite-bearing pelitic gneiss and subordinate quartzite, calcsilicate gneiss and marble are intensely folded and intruded by sill-like bodies of Mesozoic granitoids prominently exposed in Placerita and Los Pinitos Canyons. Upper amphibolite facies metamorphism of the sediments is evident from prograde sill-gt-bio-qtz-feldsp and woll-gross-diop-ep mineral assemblages. To assess provenance, we analyzed 314 zircons from four quartzite samples on the LA-ICPMS at CSUN. Resulting U-Pb ages were mostly discordant and plot on discordia arrays with Jurassic to Late Cretaceous lower intercepts. Filtered data (77 analyses with <20% normal discordance and <5% reverse discordance) yield a composite probability density plot of 207Pb/206Pb ages with maxima at ~1.1, 1.35, 1.65, and 2.0 Ga, and a minor late Archean component. The nine youngest analyses have 206Pb/238U ages between 967 and 680 Ma, indicating a Neoproterozoic maximum depositional age. SHRIMP-RG weighted mean 206Pb/238U ages of Jurassic and Cretaceous intrusives, coupled with crosscutting field relations constrain two episodes of deformation. An unfoliated biotite granite pluton that intrudes folds and amphibolite facies fabrics of the Placerita Formation is 147±3 Ma. Two foliated sills of granite with statically annealed mylonitic fabric were dated at 148±4 Ma and 145±4 Ma. Unfoliated quartz diorite in Los Pinitos Canyon (78±2 Ma) intrudes the sheared Late Jurassic granites and likely caused recrystallization of fabrics, and Pb loss in Placerita detrital zircons. The provenance signature of the metamorphosed Placerita Formation resembles detrital zircon results from the Cordilleran miogeocline, but also shares certain characteristics with the underlying Pahrump (Death Valley) and Big Bear (San Bernardino Mountains) groups. However, the greater abundance of shale and siltstone protoliths implies a more distal marginal setting. Despite the strong Mesozoic tectono-thermal overprint, Neoproterozoic marine strata in Placerita Canyon remain important to study as they preserve a key paleogeographic element at the rifted margin of Laurentia.