ZIRCON GEOCHRONOLOGY OF GRANULITE XENOLITHS FROM THE GERONIMO VOLCANIC FIELD AND IMPLICATIONS FOR TECTONIC EVOLUTION OF THE LOWER CRUST OF THE SW UNITED STATES

One model to explain shallowing of the Farallon plate during the Late Cretaceous is subduction of buoyant oceanic plateau crust [1]. Plate reconstructions predict the Shatsky conjugate intersected North America at ~90 Ma in So California and the Hess Rise conjugate intersected northern Mexico about 20 m.y. later. Granulite xenoliths entrained in Quaternary-age alkaline volcanic rocks from the Geronimo Volcanic Field (GVF) of SE Arizona provide an opportunity to ground truth some of the predicted consequences of flat slab subduction in the lower crust. GVF is located in the Cochise block of the Mazatzal terrane, which accreted to North America at ~1.7 Ga. It lies at the intersection between the predicted subduction paths for the Shatsky and Hess conjugates, i.e. where a slab window is predicted to have occurred. Zircons extracted from GVF granulite xenoliths have produced some of the first age constraints for in situ lower crust in this region. Preliminary analysis by in situ LA-ICP-MS for two quartzofeldspathic xenoliths produced concordant ages at 1.64 Ga, whereas two metadiorite xenoliths yielded much younger ages, ranging from 76 to 1.7 Ma. While the older ages are consistent with the age of the Mazatzal terrane, the younger ages provide evidence for relatively recent underplating of the lower crust and replacement of Paleoproterozoic crust in a multi-stage process that broadly corresponds to the time when slab rollback was underway beneath western North America. Cathodoluminescence images show that zircons from metadiorite sample GN22-3 are typically elongate, homogeneous crystals. Zircons from quartzofeldspathic sample GN22-5 record a more complex history with core and rim structures, and some oscillatory zoning in the rims. Th-U ratios for metadiorite samples are higher than those for quartzofeldspathic samples (median values of 0.6 vs. 0.2, respectively), consistent with the observation that felsic rocks tend to have lower Th/U than mafic and intermediate rocks. Further LA-ICP-MS analysis will target distinct zircon growth zones and collect trace element data to better constrain crustal events.

[1] Liu et al, 2010, The role of oceanic plateau subduction in the Laramide orogeny. Nature Geoscience 3, 353.