THE NEOPROTEROZOIC LIMEROCK CANYON ASSEMBLAGE: A PIECE OF THE RIFTED LAURENTIA MARGIN IN THE WESTERN SAN GABRIEL MOUNTAINS, CALIFORNIA, INTENSELY DEFORMED DURING JURASSIC TIME

The Limerock Canyon assemblage of Powell (1993) is a distinct package of marine metasedimentary rocks fringing the southwest edge of Laurentian Proterozoic basement in southern California. We present new geological mapping and U-Pb zircon analyses from the type locality where a December 2017 wildfire and subsequent winter rains enabled access to spectacular exposures. The metasedimentary section is composed of upper amphibolite facies graphite-bearing pelitic gneiss, interlayered with ~20% quartzite and ~5% marble marker units. Pre-metamorphic sills of fine-grained diorite (~5%) share a deformational fabric with the host sediments. Tight folds with subhorizontal N25W trending hinges duplicate the section throughout the map area. Two bodies of biotite granite augen gneiss exposed in the cores of anticlines are overlain by granule conglomerate and meta-arkose units that mark the base of the section. We analyzed zircons from key lithologies on the ICP-MS at CSUN to constrain the geologic history. Presence of Paleoproterozoic basement is confirmed by two samples of augen gneiss that yield upper intercept ages of 1741±13 Ma and 1731±8 Ma, with discordia arrays suggesting Late Jurassic lead loss. Detrital zircons (N=545) from five samples of quartzite in the overlying sedimentary section are generally discordant; 213 grains with <20% normal discordance and <5% reverse discordance give a meaningful assessment of provenance. A composite probability density plot of 207Pb/206Pb ages displays a prominent maximum at 1.3-1.4 Ma, with lesser peaks at 1.0-1.1, 1.65, and 1.8-2.0 Ga. Eleven grains with 206Pb/238U ages between 998 and 670 Ma demonstrate a Neoproterozoic maximum depositional age. Jurassic deformation of the Limerock Canyon assemblage is bracketed by Concordia ages of 180±1 Ma on a strongly foliated diorite sill and 152±2 Ma on a sharply crosscutting biotite granite dike. The Late Jurassic dike preserves Proterozoic inherited components and Late Cretaceous lead loss, while a granite pluton (84±1 Ma) underlying Limerock Peak records both Jurassic and Proterozoic inheritance. The Limerock Canyon assemblage shares a common geologic history with the Placerita Formation ~12 km to the northwest. These Neoproterozoic continental margin strata provide important clues on the breakup and dispersal of Rodinia.