SOURCES OF THE EARLY CRETACEOUS PLUTONS IN THE TURTLE AND WEST RIVERSIDE MOUNTAINS, CALIFORNIA - ANOMALOUS CORDILLERAN INTERIOR INTRUSIONS

Ages and initial isotopic ratios of Early Cretaceous (similar to 100 M a) plutons of the Cordilleran Interior in the southern Turtle and West Riverside mountains distinguish then from Late Cretaceous plutons in surrounding ranges in the eastern Mojave Desert. Furthermore, the stud ied plutons have isotopic and geochemical characteristics more similar to plutons of Cretaceous age in the coastal batholiths (Peninsular Ra nges and Sierra Nevada) than to most Mesozoic plutons in the Cordiller an Interior. The studied plutons are calcic, in contrast to the mostly calc-alkaline Mesozoic plutons of the eastern Mojave Desert. Distinct ive isotopic signatures of the granitoids include lower initial Sr-87/ Sr-86 of 0.705-0.710, delta(18)O of + 6.3 to + 77 parts per thousand, Pb-208/Pb-204 of 38.3-39.5, and higher epsilon(Nd) of - 3.86 to - 9.60 than the Late Cretaceous plutons in the region. The distinctive chara cteristics of these Early Cretaceous plutons are probably both locatio n and time specific and result from: (I) emplacement in a cold, untapp ed 'Mojave-type' Proterozoic upper crust, (2) a significant component of basaltic magmas partially melted from the asthenosphere or subconti nental lithosphere and (3) a magmatic component derived from Proterozo ic, mafic, lower crust. They interacted less with their crustal hosts than did the later, more voluminous Late Cretaceous plutons.