IN SITU U-PB ZIRCON GEOCHRONOLOGY OF XENOLITHS FROM THE GERONIMO VOLCANIC FIELD, SE ARIZONA: IMPLICATIONS FOR LOWER CRUSTAL DEVELOPMENT OF THE SOUTHERN BASIN AND RANGE

Granulite xenoliths from the Geronimo Volcanic Field (GVF), entrained in Quaternary-age alkaline volcanic rocks, provide insights into crustal development of the southern Basin and Range Province. Xenoliths are an important tool in this regard, because they can provide essential details on composition, age, and structure of the in situ lower crust that cannot otherwise be assessed directly. The GVF is located in the Cochise block of the Mazatzal terrane, accreted to North America at 1.69-1.65 Ga. It is located south of the predicted subduction path of the Shatsky Rise oceanic plateau correlative, where the slab is proposed to have torn and detached to create a slab window during the Laramide Orogeny. Approximately 20 m.y. afterwards, a second large oceanic plateau, a Hess Rise correlative, intersected the Mexican foreland. The GVF is located at the intersection of the two plateaus in the southern Basin and Range Province. In a preliminary study, zircons from four samples were analyzed by LA-ICP-MS, producing the first age constraints on the in situ lower crust of this region. Two quartzofeldspathic xenoliths yield concordant ages at 1.64 Ga, consistent with the age of the Mazatzal terrain, but two metadiorites yield younger ages of 76 to 1.7 Ma. These ages are much younger than the 1.7 Ga Mazatzal terrane through which GVF volcanics erupted, suggesting an additional and under-investigated influx of hot mantle that may be associated with asthenospheric upwelling through the slab window created when the less buoyant edges of the Farallon slab began to sink and break off. Further analysis of two samples, aided by cathodoluminescence images, reveal that zircons from both metadiorite and quartzofeldspathic xenoliths display distinct core and rim structures with quartzofeldspathic zircon recording a more recent complex history with oscillatory zoning in the rims. Zircon core and rim structures in both samples show distinct ages, comparable to those reported above, and one Archean age (2.4 Ga) core observed in a quartzofeldspathic zircon. New samples have recently been collected from GVF. The new sample set will be used to expand the compositional and geochronological data available, including zircon Hf isotopes to compliment the U-Pb geochronology, high precision ICP-MS trace element analyses, and new thermobarometry.