SEVIER-LARAMIDE EVOLUTION OF GREAT BASIN DEEP CRUST ENCAPSULATED IN GROWTH HISTORIES OF ZIRCON MICROXENOLITHS FROM MEGACRYSTIC MUSCOVITE GRANITES

The Kern Mountains and Deep Creek Range (39-40° N; Nevada-Utah border) expose variably deformed and metamorphosed Neoproterozoic to Paleozoic rocks of the Cordilleran passive margin intruded by Mesozoic and Cenozoic granitic plutons and dikes. Notably, some of the Late Cretaceous to Eocene intrusions contain up to 20% modal euhedral, 1-5 cm muscovite books. The Tungstonia granite (~ 70 Ma) of the Kern Mountains is the youngest Cretaceous intrusion, the final and most peraluminous phase of a ~50 m.y. history of Cretaceous magmatism that incorporated increasing crustal components through time (Barton, 1990). About 30 Ma separates this intrusive event from younger Eocene magmatism which represents the local onset of the southward sweep of magmatism across the future Basin and Range province (Armstrong and Ward, 1991). The younger magmatic event began with intrusion of mafic and felsic dikes ~40Ma. The felsic dikes in the Kern Mountains and adjacent Deep Creek Range have euhedral muscovite that is often kinked and may be xenocrystic. The zircons from both suites of muscovite-bearing intrusions are complexly zoned and are interpreted as partly xenocrystic as well. They normally contain three chemically distinct age domains: (1) Precambrian cores that are predominantly 1.7 Ga overgrowing 2.45 Ga), (2) mantled by metamorphic/magmatic zircon (~120-70 Ma in Tungstonia and 110-40 Ma in the Eocene dikes), and (3) syn-emplacement 40 Ma magmatic rims. A minor percentage of zircons in the 40 Ma dikes are entirely Jurassic age, or contain Cretaceous magmatic cores. Given the interpretation that muscovite-bearing granitoids represent local melting/remobilization of deep basement and its cover (Best et al. 1974), these data imply that zircon growth conditions were at least locally present from mid-Cretaceous to Eocene time in the ≥1.7 Ga basement that underlies the region.