SYNTECTONIC INTRUSION AND EXHUMATION OF A MESOZOIC PLUTONIC COMPLEX IN THE LATE CRETACEOUS, GRANITE MOUNTAINS, SOUTHEASTERN CALIFORNIA

New thermochronologic data and geobarometry indicate rapid exhumation of mid-crustal rocks from the Granite Mountains, SE California, during the late Cretaceous extensional collapse of the Sevier Orogen. Ion microprobe ²⁰⁶Pb/²³⁸U ages range from ~72-76 Ma for Granite Mountains plutons indicating Cretaceous emplacement, whereas a Providence Mountains pluton yielded a Jurassic age of 151 Ma. Emplacement depths determined by Al-in-hornblende geobarometry were mid-crustal (14-17 km) for Granite Mtns. plutons and upper crustal (6-9 km) for a southern Providence Mtns. pluton. ⁴⁰Ar/³⁹Ar hornblende ages of ~75-77 Ma for Granite Mtns. samples are indistinguishable from U/Pb zircon ages suggesting rapid initial cooling from ~750-500 °C by conductive heat loss to country rock. Multi-domain K-feldspar ⁴⁰Ar/³⁹Ar models indicate continued rapid cooling below 500 °C to initial K-feldspar closure at ~300-250 °C at rates up to 100 °C/m.y., followed by continued rapid cooling to final K-feldspar closure temperatures of ~150 °C at ~66-69 Ma, suggesting rapid tectonic exhumation to depths of ~5 km. Intrusion and subsequent tectonic denudation appear to have been synchronous. Reconstructed cooling histories show that the NE Granite Mtns. were below 252 °C by 73 Ma while plutons to the SW were >375 °C suggesting proximity to an active normal fault with NE plutons in the footwall. A northeast normal fault is also suggested by the juxtaposition of mid-crustal Granite Mtns. plutons (footwall) with shallow crustal Providence Mtns. plutons (hangingwall). K-feldspar models indicate that cooling rates from ~300-150 °C were fastest in the SW (63 °C/m.y.), and progressively slower to the NE (16 °C/m.y.), suggesting the Granite Mtns. plutons were exhumed by a major low-angle top-to-the-southwest normal fault, which is consistent with other Late Cretaceous extensional structures in the region. U-Th/He apatite ages range from ~40-21 Ma indicating slow cooling and regional tectonic quiescence following rapid exhumation in the Late Cretaceous. He ages exhibit a positive correlation with elevation suggesting the Granite Mtns. and Providence Mtns. cooled slowly through ~70 °C as a single block during regional erosional denudation in the mid-Tertiary. Miocene extension beginning at ~25 Ma resulted in final unroofing and exposure of the plutons.