Joint 118th Annual Cordilleran/72nd Annual Rocky Mountain Section Meeting - 2022

Paper No. 39-7
Presentation Time: 8:30 AM-6:00 PM


NOURSE, Jonathan, Department of Geological Sciences, California State Polytechnic University, Pomona, 3801 W Temple Ave, Pomona, CA 91768 and THOMPSON, Mark A., EnGEN Corporation, 36330 Grimaud Ct., Murietta, CA 92562

We present a 1:6000 scale geologic map with new U-Pb zircon geochronology to document a previously unrecognized Permo-Triassic intrusive complex in the hanging wall of the Sierra Madre thrust in the southern San Gabriel Mountains. The Claremont Wilderness block is separated from Paleoproterozoic basement and Neoproterozoic metasediments of the Potato Mountain block to the north by an east-trending fault. Proterozoic framework rocks include isolated xenoliths of biotite granite augen gneiss (1380±9 Ma) and ubiquitous screens of bio±hbld-qtz-feldsp gneiss that have suffered Mesozoic lead loss linked to upper intercept ages of 1408±17, 1435±20, and 1694±21Ma. The Permo-Triassic intrusions all contain inherited Proterozoic zircon marked by discordia arrays with upper intercepts between 1.40 and 1.74 Ga. Crystallization ages correspond to lower intercepts constrained by near-concordant zircon populations as follows: 298±14 Ma granite gneiss, 284±8 Ma bio-hbld diorite gneiss, 281±15 Ma felsite, 280±9 Ma bio monzogranite, 257±11Ma widespread metapegmatite with aplite, 250±8 Ma sphene-hbld qtz monzodiorite, and 211±11 Ma alkali granite. These intrusions are foliated, folded, and recrystallized; two contain discrete populations of near-concordant, high U/Th zircon that record Late Cretaceous metamorphism at 75±4 Ma (N = 11) and 88±4 Ma (N =8). A foliated biotite granite dike (82±1 Ma; N=42) preserves minor 1.6 Ga inheritance. A culminating Late Cretaceous crustal melting event is manifested by unfoliated biotite granite plutons and dikes (75±1 Ma; N=11 and 76±0.4 Ma; N=10) that contain >60% inherited Proterozoic zircon with significant age modes at 1.4, 1.65, and 1.75 Ga.

The Claremont Wilderness block records important Permo-Triassic arc magmatism distinctly older than the 218 Ma Mt. Lowe intrusive suite. Restoration along the San Gabriel and San Andreas fault systems places these rocks just north of the Mexican border, on strike with Permo-Triassic plutons of northwest Sonora that intrude similar Proterozoic basement (Arvizu and Iriondo, 2015). These areas mark sites of early Permian Cordilleran arc initiation. A NE-younging age distribution compared with similar age patterns in Mojave Desert-Sierra Nevada plutons suggests a double arc paleogeography, or at least a pronounced right-step in the arc.