Paper No. 96-11
Presentation Time: 9:00 AM-1:00 PM
IN SITU U-PB ZIRCON GEOCHRONOLOGY AND HF ISOTOPE ANALYSIS OF XENOLITHS FROM THE GERONIMO VOLCANIC FIELD, SE ARIZONA: IMPLICATIONS FOR LOWER CRUSTAL EVOLUTION SINCE 2.4 GA
New zircon U-Pb geochronology and Hf-isotope analyses of granulite xenoliths from the Geronimo Volcanic Field (GVF), SE Arizona, constrain the age of formation of GVF lower crust and provide evidence for subsequent tectonic events that impacted its composition. The GVF, located in the Mazatzal Province, accreted to the southern margin of Laurentia at ~1.69-1.65 Ga Zircon U-Pb ages for quartzofeldspathic lithologies present evidence of an event at ~1.65 Ga, consistent with the age of the Mazatzal terrane. One quartzofeldspathic zircon core was dated at 2.38 Ga, suggesting the presence of an older component during formation of the Mazatzal crust, possibly originating as cratonic Laurentian sediment. Two quartzofeldspathic xenoliths contain zircon cores with discordant U-Pb ages that trend toward ~1.48-1.42 Ga, suggesting modification of the Mazatzal terrane during the Picuris Orogeny (~1.48 Ga) or associated with the 1.48-1.35 Ga A-type granite plutonism widespread across the southwestern US. These same quartzofeldspathic samples also present young ages ranging from ~50-15 Ma, suggesting later alteration of the Proterozoic terrane by Tertiary magmatism associated with Farallon slab subduction. Metadiorites, previously inferred to be ~1.4 Ga on the basis of whole rock Nd model ages, yield zircon U-Pb ages ranging from ~76 to 2 Ma, with most concentrated between 35-25 Ma. These Eo-Oligocene ages overlap the timing of rhyolitic ignimbrite emplacement in the nearby Chiricahua Mountains, suggesting an origin through recrystallization of zircons in older Paleoproterozoic crust or addition of new juvenile melts that assimilated Paleoproterozoic crust during magmatic underplating in the Cenozoic. Zircon Hf isotope data from both granulite lithologies are consistent with mantle separation at 2.4 Ga, following an intermediate (176Lu/177Hf = 0.018) crustal isotope evolution path. Positive εHf(t) values in Mesoproterozoic zircon cores of some quartzofeldspathic xenoliths suggest reworking of this older Paleoproterozoic crust between the time of Mazatzal formation and the 1.4 Ga Picuris Orogeny and/or the widespread A-type granite event. Eocene to Oligocene age zircons have ɛHf(t) values that range from -17 to 0, suggesting significant lower crustal modification during Farallon slab subduction.