LATE CRETACEOUS PLUTONIC AND METAMORPHIC OVERPRINT OF PROTEROZOIC METASEDIMENTS OF ONTARIO RIDGE, EASTERN SAN GABRIEL MOUNTAINS, CA

The rifting of Rodinia, particularly in western Laurentia, has been a longstanding geologic problem, as Proterozoic rocks in the western U.S. are few and far between. The eastern San Gabriel Mountains contain a large metasedimentary package that has never been dated or thoroughly mapped. We present U-Pb detrital zircon geochronology results of 296 grains dated at CSUN’s LA-ICPMS; also 24 plutonic grains and 19 metamorphic rims of detrital grains analyzed on Stanford’s SHRIMP-RG, along with a detailed map and stratigraphic column for these Ontario Ridge metasediments. Probability plots of ²⁰⁶Pb/²⁰⁷Pb ages for two quartzite samples show three distinct peaks at ~1200, 1380-1470 and 1740-1780 Ma. A third sample shares the first two peaks and displays additional peaks at 1590-1620 and 1840-1880 Ma, but lacks the 1740 Ma peak. Maximum depositional age is constrained by 3 grains between 906 to 934 Ma; 11-28% discordant. The ~1200 Ma peak distinguishes the Ontario Ridge quartzites, and is rarely seen in the western U.S. Possible sources include the San Gabriel anorthosite complex, the Llano arc of west Texas (Marsaglia, 2002), or perhaps even Rodinia’s conjugate rift pair with western Laurentia. Our composite data set bears striking similarities to detrital zircons from the Big Bear group of the San Bernardino Mountains (Barth and Wooden, 2009). The detrital age signature also appears to match preliminary data from the Potato Mountain block (Premo et al., 2007), displaced ~8 km from the main metasediment body by the left-lateral San Antonio Canyon fault. The metasediments are intruded by pre-metamorphic granodiorite of Icehouse Canyon (85.9±0.6 Ma) and post-metamorphic quartz diorite of Ontario Peak (75.8±0.9 Ma). Two quartzites, metamorphosed to upper amphibolite facies, contain zircons with metamict rims (75.7±1.2 Ma; 12 grains and 76.7±2.4 Ma; 7 grains), consistent with the last documented metamorphic event in the San Gabriels. These new data provide important keys for future research into the Proterozoic and Mesozoic history of North America, including the breakup of Rodinia.