RECONSTRUCTION OF THE NEOPROTEROZOIC PLACERITA FORMATION OF SAN GABRIEL MOUNTAINS, CALIFORNIA IN CONTEXT OF RODINIA RIFT MODELS

The Placerita Formation is an amphibolite facies assemblage of pelitic gneiss, quartzite, calcsilicate gneiss, and marble exposed along the southwest edge of the San Gabriel Mountains. Inferred protoliths, facies patterns, and MDAs of ca. 795-768 Ma are consistent with deposition in an offshore environment at the southwest margin of Laurentia during rifting of the Rodinia supercontinent. We utilize the 48 Ma reconstruction of Bird and Ingersoll (2022) to restore Placerita Formation to a corrected position along the margin, with effects of Cenozoic extension and dextral shear removed. Nine scattered localities on opposite sides of the San Gabriel Fault include Placerita Cyn, Limerock Cyn, Fish Creek-N. Icehouse Cyn, Verdugo Hills, Potato Mountain, Ontario Ridge, Cucamonga Peak, and Penstock Ridge. 26 quartzite / meta-arkose samples yielded 2269 detrital zircon ages (adjusted for Late Cretaceous lead loss) that display common characteristics on Detrital Py plots. Age maxima are typically diffuse and overlapping, suggesting a complex mixture of proximal and distal bedrock sources. Common age modes of 2.7-2.5, 1.79-1.58, and 1.5-1.35 Ga likely represent provenances in Wyoming and Proterozoic southwest Laurentia. Age populations between 1.28 and 1.0 Ga are reasonably derived from Grenville sources of Texas and southeast Laurentia. However, distinct age modes of 2.0-1.8, 1.58-1.5, 1.35-1.28 Ga are interpreted as exotic detritus shed from a “mystery” Rodina rift conjugate.

A larger scale reconstruction restores various strands of the Late Jurassic sinistral Mojave-Sonora megashear transform system. This model brings central and western Sonora outcrops of Neoproterozoic strata into proximity with the San Gabriel and San Bernardino Mountains. An additional provocative feature is restoration of Grenville gneisses of Oaxaca and Puebla, Mexico to a position much closer to the southern California margin. These rocks offer a convenient source for 0.98-0.93 Ga detritus in the Placerita Formation that otherwise would require transcontinental transport from very limited exposures in the southern Appalachian Mountains.

Ongoing Lu-Hf and Sm-Nd isotopic studies of the Placerita Formation and associated Proterozoic basement aim to develop a more robust data base essential for matching the San Gabriel Mountains to a unique conjugate rift pair whose identity remains elusive.