EVIDENCE FOR THE TIMING OF POST-LARAMIDE MAGMATIC UNDERPLATING IN THE SOUTHERN BASIN AND RANGE: NEW ZIRCON GEOCHRONOLOGY OF GRANULITE XENOLITHS FROM THE GERONIMO VOLCANIC FIELD, SE ARIZONA

Knowledge of the lower crust is essential for understanding the tectonic evolution of cratons. Yet its relative inaccessibility arguably makes it the part of the lithospheric column about which we know the least. Xenoliths brought to the surface in basaltic magmas, therefore, provide valuable insights into the composition, age, and structure of the deep crust that are otherwise difficult to obtain and provide a link between surface exposures and geophysical data. Here we use xenoliths entrained in Quaternary-age alkaline volcanics from the Geronimo Volcanic Field (GVF) of SE Arizona 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 crust as determined seismically. Three samples have been analyzed by in situ LA-ICPMS, producing the first age constraints on the lower crust of this region. One quartzofeldspathic xenolith yields concordant ages between 1.65 – 1.70 Ga, consistent with the inferred age of the Cochise block. Two metadiorite xenoliths yield much younger ages ranging from 54 to 5 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 our data do rule out delamination of continental lithosphere at this time, zircons in the xenoliths studied record no ages during this time period and instead 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).