A REFINED U-Pb AND Sm-Nd FRAMEWORK FOR PALEOPROTEROZOIC ORTHOGNEISSES IN THE SAN GABRIEL MOUNTAINS AT THE RIFTED MARGIN OF LAURENTIA

The Rodinia supercontinent concept is nearly 35 years old, yet debate remains on the configuration of continental fragments prior to rifting. The western Laurentian margin was one side of a ca. 750 Ma rift zone, but identity of the material removed from Laurentia remains elusive. Rodinia models hinge on correlations with southwest Laurentia, in which the San Gabriel Mountains (SGM) of southern CA are the furthest western exposure. If southwestern Laurentia is a true tie-point, then the age and geochemical fingerprint of Proterozoic basement in the SGM should match the western conjugate rift-pair. Thus, we aim to fully characterize the protolith ages and magma sources of SGM gneisses to facilitate correlations across the rifted margin. We present zircon U/Pb geochronology and Sm-Nd analyses ofstrongly deformed Paleoproterozoic orthogneisses from the Glendora Ridge area. Five gneisses analyzed so far yield calculated upper intercept ages of 1661 ± 12, 1677 ± 21, 1690 ±12, 1766 ± 10, and 1812 ± 14 Ma. Of particular interest is the 1812 Ma fine-grained banded meta-rhyolite(?), one of the oldest igneous bodies in southern CA. Orthogneisses > 1.8 Ga are sparse but have recently been identified in the Liebre Mountain block. We suggest that previously unrecognized ~1.8-1.9 Ga igneous basement is key to elucidating the Rodinia supercontinent configuration. Weakly deformed granite plutons intrude the gneisses and give a lower age bound of 1405 Ma for upper amphibolite-facies metamorphism. A few zircons in the orthogneisses preserve cryptic evidence for metamorphic disturbance at 1620-1543 Ma, whereas one sample shows clear metamorphic overgrowths at 1383 ± 13 Ma. Paleoproterozoic magmatism and tectonism was longer-lived than previously thought, spanning an age range comparable to the Mojave province. Our ongoing Sm-Nd and Lu-Hf studies aim to characterize the Paleoproterozoic magma sources. Eleven samples analyzed by Premo et al. (2023) from Glendora Ridge and elsewhere in the SGM have ϵNdi values between 0.53 and -2.64. These results indicate more enriched sources compared to contemporaneous arc magmatism of the Yavapai and Mazatzal provinces. Three 1.4 Ga granites yield ϵNdi values between -0.71 and -1.53. Identification of an older basement (>1.8 Ga) and the strategic far west paleogeographic location of the SGM make this an important locality to focus future Rodinia studies.