GEOCHRONOLOGY AND THERMOCHRONOLOGY OF LOWER CRUSTAL XENOLITHS FROM MONTANA: A TEMPORAL RECORD OF ASSEMBLY, STABILIZATION, AND REACTIVATION

Geologic interpretation of velocity models for the North American lithosphere obtained by EarthScope will depend upon an understanding of the thermal histories from the crust and underlying lithospheric mantle. The geologic age of imaged velocity structures in the lithospheric mantle can in part, be constrained by the age and thermal history of the overlying lower crust. Lower crustal rocks often contain U-bearing accessory minerals (titanite, apatite, rutile) that are dateable by U-Pb methods and whose range of Pb closure temperatures (~650-400 °C) can place time constraints on the lithosphere history; from assembly to establishment of stable cratonic geotherms.

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.