FIRST AGE CONSTRAINTS ON THE IN SITU LOWER CRUST OF SE ARIZONA: ZIRCON GEOCHRONOLOGY OF GRANULITE XENOLITHS FROM THE GERONIMO VOLCANIC FIELD YIELDS EVIDENCE FOR POST-LARAMIDE MAGMATIC UNDERPLATING IN THE SOUTHERN BASIN AND RANGE

Xenoliths entrained in Quaternary-age alkaline volcanics from the Geronimo Volcanic Field (GVF) of SE Arizona have been used to investigate the timing and mechanisms of lower crustal growth and evolution of the southern Basin and Range. The GVF is located within the Cochise block of the Mazatzal terrane, which was accreted to North America between 1.69 – 1.65 Ga. The lower crustal xenolith suite includes lithologies ranging from mafic meta-cumulates (pl + cpx + ol + sp) to more felsic metadiorites and quartzofeldspathic lithologies (plg + kspar + cpx + ox ± opx). Calculated temperatures of equilibration for the suite (836-960^oC) are consistent with pre-entrainment residence at depths of ~25-30km, or approximately the base of the seismically determined crust. Four samples have been analyzed by in situ LA-ICP-MS, producing the first age constraints on the lower crust of this region. Two quartzofeldspathic xenoliths yield concordant ages at 1.64 Ga, consistent with the inferred age of the Cochise block. Two metadiorite xenoliths yield much younger ages ranging from 76 to 1.7 Ma, suggesting a multi-stage process of replacement of this older Paleoproterozoic crust. Previous studies have proposed convective removal of continental lithosphere beneath the western U.S. during Laramide-age flat slab subduction (i.e. 80-40 Ma). While there may be some evidence for this within our data, uncertainties on these younger ages are currently large, making it hard to pinpoint discrete events at this time. Our data, however, do support a ca. 40-45 m.y. event of magmatic underplating that occurred predominantly post-Laramide (< 40 Ma), much of it during the Mid-Tertiary ignimbrite flare up event (ca. 40-25 Ma). The data also indicate the onset of Geronimo volcanism at 8-9 Ma. Previously reported Nd model ages of 1.4 – 1.2 Ga for the metadiorite xenoliths contrast with the younger ages reported here and require further investigation to reconcile with the new zircon geochronology. Further analysis of complex zoning observed in cathodoluminescence images may help to develop a more detailed history.