SILICIC MAGMAS AND TECTONICS: ZIRCON XENOCRYSTS HELP REVEAL THERMAL HISTORY AND RHEOLOGY OF THE CRUST ACROSS THE NORTHERN GREAT BASIN FROM THE CRETACEOUS TO THE MIOCENE

Miller and Bradfish (1980) linked Cordilleran Interior (CI) two-mica granite magmatism to Mesozoic deformation, positing that their peraluminous compositions reflected incorporation of thickened sialic crust. These low-T granitic magmas commonly contain xenocrystic zircons that archive the history of source regions, magmatic flux, temperatures and thus rheology of the crust through time.

Mid–Late Cretaceous CI granites in the Snake Range-Kern Mtns. all contain zircon xenocrysts with 1.0–1.8 Ga cores, matching age spectra of detrital zircons in Neoproterozoic strata. The addition and high proportion of zircons with composite 2.45/1.7 Ga cores in the 70 Ma Tungstonia granite pluton indicate partial melting of westward-underthrust basement had begun by this time. A subsequent 30 m.y. magmatic hiatus predates local onset of ignimbrite flare-up volcanism. Early, 40 Ma felsic porphyry dikes entrain Precambrian xenocrystic zircon populations similar to those of the Tungstonia but overgrowths yield ages spanning ca. 110–40 Ma. 70 to 40 Ma overgrowths suggest near zircon solidus conditions persisted at depth in both underthrust basement rocks and Neoproterozoic metasedimentary cover across the Laramide timespan. Scarcity of zircon xenocrysts in post–40 Ma intrusions/volcanic rocks is attributed to progressive heating of the crust by large volumes of mantle-derived magma during the ignimbrite flareup.

North, in the Albion-Raft River-Grouse Creek region, zircon xenocrysts suggest the 41–32 Ma Emigrant Pass plutonic suite and the 32–25 Ma Cassia plutonic complex (with no associated volcanism) have a common origin in a deep crustal “hot zone” in 2.5 Ga crust that remained above solidus temperatures for at least 16 m.y. Isotopic modeling suggests increasing hybridization of mantle-derived basalts with crustal melts through time. Melting led to flow of crust with rise of granite-cored gneiss domes to shallower levels (10-15 km). A similar history in the Ruby Mts.-East Humboldt core complex suggests northern Great Basin crust was sufficiently hot and “primed” to undergo extension but rapid BR faulting at ~ 17 Ma was therefore likely triggered by changes in boundary conditions.