U-PB ZIRCON GEOCHRONOLOGY OF CRUSTAL XENOLITHS CONFIRMS PRESENCE OF ARCHEAN BASEMENT BENEATH THE CENTRAL AND EASTERN SNAKE RIVER PLAIN

Knowledge of the age and composition of the crust is important to understanding tectonics and lithospheric evolution of the Northern Rocky Mountains, as well as petrogenesis of magmatism in the region. Crustal architecture beneath much of southern Idaho is poorly known due to limited surface exposures and extensive blanketing by Miocene-Recent Snake River Plain (SRP) rhyolite-basalt volcanism. Archean basement is exposed as far west as SW Montana, S-central Idaho and NW Utah. Its presence has been inferred beneath the eastern SRP from the occurrence of granulite xenoliths in certain SRP lavas, but geochronology is based only on whole rock Pb-Pb and Sm-Nd isotopic data. To better constrain the age and composition of the basement beneath the SRP, and its possible influence on Phanerozoic magmatic and tectonic processes, we determined U-Pb zircon ages for four mafic to felsic granulite xenoliths from three localities along the northern margin of the SRP: Square Mountain in the west (DM103, opdalite, 73.8% SiO2), Craters of the Moon National Monument (COM9A, opdalite, 73.1%; COM22, enderbite, 68.5%), and Spencer/Kilgore volcanic field in the east (SK3, norite, 54.9%). The host lavas for the xenoliths have ages of ca. 4 Ma, 2-10 Ka, and Holocene, respectively.

Zircons were separated from the xenolith samples using standard procedures and analyzed by LA-ICPMS using a Nd-YAG 213nm laser and ThermoFinnigan© Element2 HR-ICPMS. All xenoliths contain zircons yielding Archean ages, with a general trend of decreasing age from east to west. Most samples exhibit complex Archean histories with multiple age populations, most likely from different magmatic episodes: zircons from SK3 yield ages clustering around 3.10, 2.90 and 2.76 Ga; COM9A has ages clustering at 2.76, but ranging between 2.60 and 2.50 Ga; COM22 has a bimodal population with age peaks at 3.20 and 2.75 Ga, with intermediate ages and one core > 3.5 Ga; DM103 zircons yield a consistent age of 2.57 Ga. Zircons from COM22 and SK3 have complex low-U overgrowths that yield 206Pb/238U ages close to 0 Ma. No evidence is seen for zircon growth between late Archean and recent times. All samples have ubiquitous lower concordia intercepts near 0 Ma indicating recent Pb loss in the zircons, probably related to modern SRP magmatism.