SOURCE OF EOCENE SEDIMENT TRANSPORTED THROUGH THE BREACHED MOJAVE SEGMENT OF THE MID-CRETACEOUS CALIFORNIA BATHOLITH: CONSTRAINTS FROM COUPLED PB ISOTOPIC AND AR-AR AGE RESULTS FROM DETRITAL K-FELDSPAR

Geologic and detrital zircon data indicate that a major breach in the Mojave-Salinia segment of the mid-Cretaceous California batholith, formed as a result of Laramide deformation, allowed extraregional sediment from the retro-arc to be transported into the forearc during the Eocene. Exactly how far northeast this sediment was sourced relative to the Sierra Nevada (SNB) and Peninsular Ranges (PRB) batholiths remains controversial, particularly in light of the Late Paleocene–Early Eocene “California River” that is believed to have contemporaneously transported batholithic sediment in the opposite direction from southern California into Utah. Basement rocks of the greater Mojave region have distinctive Pb isotopic values that can be used to address this question. We have measured Pb isotope compositions via LA-ICP-MS analysis of K-feldspar from 145 predominantly Late Cretaceous and Triassic-Paleocene basement samples from southernmost SNB, Mojave Desert, and Salinia to supplement existing data. Most SNB and Mojave Desert K-feldspar Pb isotope values plot on a ca. 100 Ma geochron with ²⁰⁷Pb/²⁰⁶Pb between 0.80 and 0.86 and ²⁰⁸Pb/²⁰⁶Pb between 1.95 and 2.13. Salinia plots at the low end of this trend with ²⁰⁷Pb/²⁰⁶Pb ~0.80 and ²⁰⁸Pb/²⁰⁶Pb ~1.98. The more primitive western zone of the PRB also falls upon this array (²⁰⁷Pb/²⁰⁶Pb 0.82-0.85 and ²⁰⁸Pb/²⁰⁶Pb 2.03-2.09) while the cratonal eastern PRB yield lower values (²⁰⁷Pb/²⁰⁶Pb 0.80-0.82 and ²⁰⁸Pb/²⁰⁶Pb 1.99-2.03). In contrast basement rocks of the eastern Mojave yields much more diverse Pb isotopic compositions (²⁰⁷Pb/²⁰⁶Pb 0.81-0.89 and ²⁰⁸Pb/²⁰⁶Pb 2.01-2.21). Eocene strata from the southernmost SNB (San Emigdio Mtns.) and correlative rocks within Salinia (La Honda Basin) yield a range of Pb isotopic values that require contributions from southeast of the SNB including the eastern Mojave region. Similarly, coupled detrital K-feldspar Ar-Ar closure ages and age spectra that constrain a major cooling event centered upon 80±5 Ma is also characteristic of the broader Mojave region. Based upon these results, we conclude that the NE-flowing California River is unlikely to have also sourced the broader Mojave region during the Eocene and may instead have tapped eastern portions of SNB further north.