UTILIZING SINGLE-GRAIN ZIRCON ISOTOPE DATA TO INVESTIGATE THE SIGNIFICANCE OF THE MEDICINE HAT BLOCK (SOUTHERN ALBERTA, NORTHERN MONTANA) IN THE ASSEMBLY OF LAURENTIA

Located on the border between Montana in the USA, and Alberta Canada, the Medicine Hat Block (MHB) is one of the core cratonic elements that amalgamated in the Paleoproterozoic to form Laurentia. Little is known about the role the MHB played during the formation of Laurentia, or its relationships to surrounding geologic elements. This is partly the result of limited data collected from this craton due to the younger supracrustal sequences that cover the majority of the MHB. The primary source of samples definitively from the MHB comes from two locations: 1) xenoliths of variably metamorphosed gneisses, amphibolites, and meta-plutonic rocks collected from Eocene volcanic rock in the Sweetgrass Hills, and 2) similar lithologies recovered from boreholes that penetrate to the MHB basement. Recent zircon single-grain LA-ICPMS U-Pb ages revealed a range of Archean ages, 3.27 Ga to 2.63 Ga, two samples yielding Paleoproterozoic ages at 1.78 and 1.82 Ga, and one detrital sample yielding a concentration of age peaks ranging from ~ 1200 Ma to 1600 Ma. Paird zircon Hf isotopic results revealed that Archean-aged zircon are generally suprachondritic, with Hf_t values between 8.3 and -8.7. However, the Paleoproterozoic grains yielded negative Hf_t values ranging from -6.8 to -21.2, suggestive of a reworked Archean crustal component in their genesis. In particular, the Sweetgrass Hill xenolith suite is characterized solely by Paleoproterozoic ages, with evolved Hf_t suggesting that any older U-Pb ages were reset by granulite facies metamorphism and zircon recrystallization. The combined U-Pb and Hf isotopic data from these samples helps illuminate the character of the MHB and its relationships to other cratonic elements in the region, such as the Wyoming and Hearne cratons; as well as the Great Falls Tectonic Zone (GFTZ). U-Pb ages overlap between these elements; however, the abundance of positive Hf_t values of the ~2.8 Ga ages suggests the MHB is distinct from the Wyoming Craton, and that the GFTZ must indeed be a collisional zone as previously proposed. Paleoproterozoic ages observed in the granulite xenolith samples supports this distinctness as well, and supports proposed models of a Paleoproterozoic underplating event observed in other published xenolith data, as well as interpreted in seismic sections.