RECONNAISSANCE GEOLOGY AND U-PB ZIRCON GEOCHRONOLOGY OF CLAREMONT WILDERNESS PARK, EASTERN SAN GABRIEL MOUNTAINS

Claremont Wilderness is part of the Potato Mountain Block, sinistrally offset from the Ontario Ridge by the San Antonio Canyon fault and uplifted in the hanging wall of the Sierra Madre thrust. We report new mapping and U-Pb geochronology that builds on previous results of Premo et al., (2007) and Nourse and Premo (2016). The study area is composed of a northwest dipping section of orthogneisses and pelitic-quartzose metasedimentary rocks intruded by Late Cretaceous granites. Mineral assemblages in the pre-Cretaceous rocks record upper amphibolite facies metamorphic conditions.

U-Pb analyses were conducted on the SHRIMP-RG at Stanford and the LA-ICP-MS at CSU Northridge. The oldest basement rocks are isolated xenoliths within leucogranite. A biotite granite gneiss in lower Cobal Cyn yields an upper intercept age 1767±10 Ma (N = 25), and an augen gneiss has a mean ²⁰⁷Pb/²⁰⁶Pb age of 1380±9 Ma. (N=12). We analyzed 4 samples of quartzite and meta-arkose to constrain provenance of metasediments that overlie the basement. Potato Mountain quartzite shows Proterozoic provenance with peaks at 1.1, 1.2, 1.4, and 1.8 Ga (N=25). Cobal Canyon quartzite (N=100) preserves a prominent 1.6-1.8 Ga peak, with minor peaks at 1.1 and 1.4 Ga and evidence for strong Late Cretaceous overprint. Two additional samples (N=200) display comparable Proterozoic age distributions plus prominent Permo-Triassic populations.

Three Late Cretaceous granites yield additional information. A biotite granite dike with a Concordia age of 81.7±0.2 Ma crosscuts the metasedimentary section but displays a weak foliation. This sample preserves minor 1.7 Ga inheritance, in contrast to two unfoliated granite dikes that contain >50% inherited grains with recurring peaks at 1.4, 1.65, and 1.75 Ga. The younger populations yield weighted mean ²⁰⁶Pb/²³⁸U ages of 76.1±0.4 Ma and 75.2±0.9 Ma for this culminating crustal melting event.

Claremont Wilderness provides important constraints on the Proterozoic and Mesozoic evolution of the eastern San Gabriel Mountains. Despite obfuscation by Late Cretaceous granitic intrusions, the area appears to preserve an unconformity between previously undescribed Paleoproterozoic basement rocks and a continental margin sedimentary sequence. The Permo-Triassic detritus is especially intriguing and begs further study.