LATE CRETACEOUS INTRUSIVE MIGMATITES OF THE LITTLE SAN BERNARDINO MOUNTAINS, SOUTHERN CALIFORNIA

Geologic maps of the Little San Bernardino and southeast San Bernardino Mountains show a band of Proterozoic rocks immediately south and west of the Mesozoic intrusive rocks that dominate the exposures in this region. These rocks have been considered to be Proterozoic because they are strongly deformed and often lithologically variable at the few meter scale. Field and geochronologic studies of these rocks indicate that they are largely Late Cretaceous intrusive rocks that form a migmatitic complex. U-Pb ages on zircon, monazite, and sphene and Ar-Ar ages on hornblende and biotite are concentrated between 72 and 80 Ma with U-Pb sphene and Ar-Ar ages lying between 72 and 76 Ma, the probable time of peak metamorphism and deformation. A few late Jurassic (150 Ma), pluton-sized bodies have been identified by ion microprobe U-Pb dating, but all have had their conventional U-Pb zircon and sphene and Ar-Ar ages completely to partially reset to the late Cretaceous. Two suites of late Cretaceous intrusives are present: (1) biotite granites with monazite and (2) hornblende-biotite diorites to granodiorites and biotite granites with sphene. Leucogranitic dikes intrude both suites and contribute strongly to the heterogeneous appearance of the exposures. Limited hornblende barometry indicates pressures of 4.5-6 kb. Field studies suggest that there is a gradational transition from these highly deformed rocks to the undeformed Mesozoic intrusives that lie immediately to the north and east in Joshua Tree National Park. Crystallization ages for the volumetrically dominant, undeformed Cretaceous intrusives, including the White Tank granite, are 74-80 Ma and the two migmatite compositional suites are present also in the undeformed intrusives. All intrusive rocks have abundant inherited zircon cores indicating derivation from the regionally extensive, Proterozoic lower crust. The migmatitic rocks therefore appear to be the deeper level, mid-crustal, equivalents of the undeformed intrusives. Their formation preceded that of the Orocopia-Pelona schists and is probably related to pre-Laramide deformation in the magmatically heated and softened middle crust.