NEW MAPPING AND GEOCHRONOLOGY OF BASEMENT ROCKS SOUTHWEST OF THE SAN ANDREAS FAULT IN THE LIEBRE MOUNTAIN 7.5’ QUADRANGLE, SOUTHERN CALIFORNIA

New geologic mapping of the Liebre Mountain 7.5’ Quadrangle under the STATEMAP program by the California Geological Survey and U/Pb dating of zircons from basement rocks by the CSUN Laser Lab has revealed new insights into the geologic history of the Liebre Mountain block of the NW San Gabriel Mountains (SGM), SW of the San Andreas fault (SAF). For context, recent U/Pb dating of zircons in the adjacent Burnt Peak 7.5’ Quadrangle revealed upper intercept Paleoproterozoic ages for gneissic rocks ranging from about 1.72 to 1.78 Ga and locally up to 1.95 Ga. U/Pb dating of zircons in the Liebre Mountain Quadrangle revealed new Early Proterozoic ages for quartzofeldspathic orthogneisses ranging from about 1.9 to 2.0 Ga, which may be the oldest known for rocks exposed in the Transverse Ranges. On-going U/Pb dating south of the Clearwater fault indicated the presence of both 1.99 Ga and 1.33 to 1.47 Ga gneisses. Dating of paragneiss in the southeast portion of the Liebre Mountain Quadrangle and nearby SW portion of the Burnt Peak Quadrangle, which contains bands of marble and quartzite adjacent to Paleoproterozoic gneisses, yielded clusters of detrital zircon ages revealing a preliminary maximum depositional age of about 1.1 Ga. The large span of time between the formation of the Paleoproterozoic gneisses and deposition of the adjacent paragneiss protolith supports the presence of a significant, previously unrecognized unconformity in the Liebre Mountain area; no evidence of large-scale faulting has yet been recognized along the contact. Porphyritic granite that intrudes the Paleoproterozoic gneisses at Redrock Mountain was previously speculated to be Jurassic in age, but new U/Pb dating indicates the pluton is about 315 Ma, which is an unusual age in this area. A sample of Triassic megaporphyry obtained just S of the SAF yielded an age of about 236 Ma, similar to previous reported ages of this unit SW of Liebre Mountain, and across the SAF at Mill Creek, to support 160 km of right-lateral slip on the Mojave strand. A sample of dacite from the Neenach Volcanics NE of the SAF yielded an age of about 27 Ma, which is more in line with K/Ar ages of 23-24 Ma initially reported for both the Neenach and Pinnacles Volcanics to support their correlation across the SAF; younger Ar/Ar dates published for the Neenach Volcanics may be inaccurate due to argon loss related to alteration processes.