GEOCHRONOLOGY AND THERMOCHRONOLOGY OF LOWER CRUSTAL XENOLITHS FROM MONTANA: A TEMPORAL RECORD OF ASSEMBLY, STABILIZATION, AND REACTIVATION
In central Montana, Eocene volcanics have exhumed lower crustal xenoliths along a N-S trending transect. Largely beneath Phanerozoic sedimentary cover lie two Archean terranes; the Medicine Hat Block (MBH) and the Wyoming Province (WP). A NE-SW trending geophysical anomaly between these terranes, the Great Falls Tectonic Zone (GFTZ), is interpreted as a Proterozoic suture. U-Pb dating of zircon core/rims from MHB felsic granulites record crystallization/overgrowth at ~2.7/2.0 Ga. Rutile U-Pb dates indicate that lower crustal T's of ~400 °C were attained in the MHB by ~1.8 Ga but were re-set at ~0.8 Ga. Xenoliths from the same locale were found by the Lithoprobe study to have equilibrated at 950°C and 11.3-13.5 Kbar. Zircon from mafic granulite xenoliths within the GFTZ yield U-Pb dates of ~1.76 Ga; identical to published dates from exposed GFTZ crustal rocks. Rutile dates suggest the lower crust beneath the GFTZ cooled to ~400 °C by ~1.2 Ga but were also partially reset ca. 0.8 Ga. Further south within the WP, mafic granulite xenoliths yield zircon core/rim ages of ~2.0/1.76 Ga. Apatite U-Pb analyses suggest the northern, WP had not cooled to ~400 °C until ca. 1.0 Ga. Garnet bearing two-pyroxene xenoliths from the GFTZ and WP are interpreted as lower crustal (>8 Kbar).
Lower crustal xenoliths from Montana record a protracted history from assembly at ~2.7-2.0 Ga, to metamorphism during collision of the MHB and WP at ~1.76 Ga, and stabilization at ~1.75-1.0 Ga followed by a final thermal perturbation at ca. 800 Ma. Meaningful geologic interpretation of imaged lithospheric structures will hinge on understanding the thermal history and the age of geophysical features.