GEOLOGY AND U-PB GEOCHRONOLOGY OF PLACERITA CANYON, WESTERN SAN GABRIEL MOUNTAINS, CALIFORNIA

We present a geologic map and preliminary U-Pb geochronology of Placerita Canyon in the western San Gabriel Mountains, recently exposed by the 2016 Sand Canyon wildfire. The area contains distinct metasedimentary rocks of the Placerita Formation (Miller, 1934; Oakshott, 1958), previously interpreted to be a Precambrian or Paleozoic marine assemblage, and more recently recognized as a key paleogeographic element situated at the rifted margin of Laurentia (Zylstra et al., 2017). We also targeted this area to test proposed correlations with metasedimentary strata of Limerock Canyon and Ontario Ridge that provide potential constraints on right-lateral displacement across the south branch San Gabriel Fault. Field relations show that the Placerita Formation is composed of four distinct metasedimentary units repeated by folds and intruded by sill-like bodies of Mesozoic granitoids. The folded metasedimentary package records upper amphibolite facies metamorphism and includes (from most abundant to least): (1) sill-gt-bio-qtz-feldsp gneiss with rare graphitic schist, (2) gross-diop-calcsilicate gneiss, (3) quartzite, and (4) white marble. Average foliation is N65W/55NE over an exposed structural thickness of 665 m. The Mesozoic intrusives include mylonitized biotite granite and weakly foliated quartz diorite that preserve greenschist facies fabrics. 104 detrital zircons from a quartzite collected at a structurally deep part of the section were analyzed on the ICP-MS at CSUN. The ²⁰⁶Pb/²⁰⁷Pb age spectrum reveals major peaks at ~1.1 Ga, ~1.45 Ga, ~1.6 Ga, ~1.8 Ga, and ~2.03 Ga, with a minor Late Archean component (2.5-3.0 Ga). Two concordant grains (947+/-84Ma) constrain the maximum depositional age. Compared to 5 samples from Ontario Ridge, this population is most similar to a quartzite collected high in the section, but lacks a 1.2 Ga peak. The biotite granite yielded 24 highly discordant grains with a complex age spectrum: major peaks at 122.1±7.4 Ma (N=3) and 171.4±7.2 (N=11), with minor peaks at ~ 215Ma, ~255 Ma, and ~350-400 Ma. This rock may be an Early Cretaceous intrusive with complex inheritance, or a Mid Jurassic intrusive affected by Early Cretaceous metamorphism. The Placerita Formation provides important clues for Neoproterozic and Cenozoic reconstructions involving the San Gabriel Mountains.