SUBMICRON U-PB AGE ANALYSIS OF CONTACT METAMORPHOSED ZIRCON: IMPLICATIONS FOR THE UNDERTHRUSTING HISTORY OF THE RAND SCHIST

Ion microbe U-Pb age depth profiling was performed on <1 micron overgrowths on contact metamorphosed detrital zircons from the Rand Schist. Similar schist was underplated beneath southern Californian during Late Cretaceous low-angle subduction. Although underplating of the schist has been widely thought to have marked extinction of the mid-Cretaceous arc, this model has been contradicted by a reported 79 Ma U-Pb zircon age for the Randsburg Granodiorite that implied Late Cretaceous contact metamorphism of the Rand Schist. However, submicron rims from detrital zircon at the contact with the Randsburg Granodiorite yield 18-20 Ma U-Pb ages that are identical to zircon U-Pb ages we have measured from an informally designated Yellow Aster Porphyry that pervasively intrudes the coarser grained Randsburg Granodiorite. Dikes into the schist and volcanic rocks of Red Mountain yield similar U-Pb ages. Similarly, Ar-Ar biotite ages from the pluton and the contact aureole are also 18-20 Ma. The youngest regional detrital zircon U-Pb and Ar-Ar cooling ages from the Rand Schist indicate that it was accreted beneath the Sierran arc at 77-80 Ma. Field observations and single crystal ion probe and TIMS U-Pb analysis reveal that the coarse-grained Late Cretaceous “Randsburg Granodiorite” are xenoliths of an 87 Ma intrusion from the upper plate of the Rand fault (Atolia Granodiorite) that foundered into the Yellow Aster Porphyry. Hence the 79 Ma multi-zircon U-Pb age previously reported for the Randsburg Granodiorite is a mixing age with no implications for the timing of emplacement of the schist.