DO BACK-ARC SIERRAN-LIKE PLUTONS IN THE SOUTHWEST USA SIGNAL MID-CRETACEOUS SLAB ROLLBACK?

Mid-Cretaceous 100-90-Ma granodiorites intrude old continental crust in the eastern Mojave Desert of SE California and western Arizona and represent moderate-flux back-arc magmatism inboard from the contemporaneous high-flux arc in the Sierra Nevada (SN) and Peninsula Ranges (PRB) batholiths. The back-arc rocks tend to be calcic, in contrast to typically alkalic modern back-arc rocks. The isotopic signatures signal involvement of old continental crust but also source components more primitive than for neighboring Jurassic and latest Cretaceous batholiths. Age, composition, and isotopes link these mid-Cretaceous back-arc rocks to the SN–PRB arc. They record an isotopic pull-up event (in Hf and Nd isotopes) relative to high-flux Jurassic and latest Cretaceous events that produced neighboring east Mojave batholiths. A likely asthenosphere Hf isotopic signal in these rocks, together with a contemporaneous thick sedimentary basin (McCoy Mountains Formation), suggests to us that crustal extension occurred in the back arc at 100-90 Ma. Slab rollback may have caused the extension.