DISTINCTIVE JURASSIC AND LATE CRETACEOUS THERMAL DISTURBANCE OF ZIRCONS FROM THE PLACERITA FORMATION, WESTERN SAN GABRIEL MOUNTAINS, CALIFORNIA

The Placerita Formation is a metamorphosed Neoproterozoic continental margin assemblage associated with Jurassic (180-147 Ma) and Late Cretaceous (84-78 Ma) intrusions. The formation is exhumed along Late Cenozoic faults in Placerita, Los Pinetos, and Limerock Canyons. We present comprehensive data plots of U-Pb zircon analyses (LA-ICP-MS and SHRIMP-RG) from 19 samples to constrain basement age, provenance signature, and superimposed tectonomagmatic events. Within the data are pervasive, profound discordance patterns associated with Jurassic and Late K thermal disturbances. For example, 3 Paleoproterozoic augen gneiss samples preserve discordia arrays with 1.74-1.68 Ga upper intercepts and Late Jurassic lower intercepts. 8 quartzite samples display discordia trajectories with late Mesozoic lower intercepts. Probability density plots of ²⁰⁷Pb/²⁰⁶Pb ages (<20% normal discordance) vary between samples, but preserve significant maxima at 1.0-1.1, 1.35-1.45, 1.6-1.65, 1.7-1.75, and 1.85-2.0 Ga. The youngest detrital zircon population (N=15) has recalculated upper intercept ages between 980 and 750 Ma when anchored at an 80 Ma lower intercept that presumes Late K lead loss. A crosscutting 152±1 Ma granite dike in Limerock Canyon preserves Proterozoic inheritance similar to ages of local metasedimentary and basement host rocks; this also contains high U/Th Late K zircons consistent with lead loss. Microfabrics in 4 analyzed Late Jurassic intrusives record recrystallization of quartz and feldspar likely caused by adjacent Late K plutons. The 84±1 Ma Limerock Peak porphyritic granodiorite preserves 57% Jurassic and Proterozoic inherited grains suggestive of local crustal melting, whereas the 78±1 Ma Los Pinetos quartz diorite lacks inheritance. Greenschist facies Laramide(?) foliation is locally developed in the Los Pinetos quartz diorite, but not present in the Limerock granodiorite. Placerita and Limerock Canyons share geologic histories that record synkinematic Late Jurassic magmatism and metamorphism, similar structural orientations, and Late K overprint. Farther east, likely correlative rocks at Claremont Wilderness Park and Ontario Ridge contain fragments of 1.77-1.75 Ga basement and preserve a similar Late K thermal disturbance, but Jurassic intrusions are not recognized.