THE MEDICINE HAT BLOCK AND THE ASSEMBLY OF LAURENTIA: NEW INTERPRETATIONS FROM SINGLE-GRAIN ZIRCON ANALYSES

The Medicine Hat Block (MHB) is one of the core cratonic elements that amalgamated in the Paleoproterozoic to form Laurentia. However, unlike other notable crustal blocks in the region, such as the Wyoming and Hearne, the role of the MHB in the formation of Laurentia is poorly constrained due to limited data availability. Investigating the underlying basement rock below the MHB is difficult due to expansive Proterozoic and younger supracrustal sequences that conceal virtually the entire craton. The primary source of samples from the MHB come from 1) xenoliths of variable metamorphic grade including gneisses, granulites, amphibolites, and meta-plutonic rocks collected from Eocene volcanic rock; and 2) similar lithologies recovered from boreholes that penetrate to the MHB basement. Established U-Pb ages for the MHB include a mixture of Paleoproterozoic (1.70 – 1.87 Ga) and Archean (2.60 Ga – 3.30 Ga) signatures. Recent zircon single-grain LA-ICP-MS U-Pb analyses on xenoliths reveal a range of ages, one Neoarchean (2.79 Ga), and nine Paleoarchean (3.45 Ga - 3.54 Ga). Ages greater than ~3.30 Ga have not yet been recognized in the MHB and indicate an earlier origin for the craton than was previously suggested. In-situ zircon Lu-Hf isotopic results revealed that Archean-aged zircon are generally suprachondritic, with εHfT values between 8.3 and -8.7. However, previously analyzed Paleoproterozoic grains yielded εHfT values ranging from -6.8 to -21.2, suggestive of a reworked Archean crustal component. The combined U-Pb and Hf isotopic data from these samples illuminates the character of the MHB and its relationships to the Wyoming and Hearne cratons. U-Pb ages for the MHB overlap with those of the Wyoming and Hearne; however, the abundance of evolved εHfT values of the 2.8 Ga to 3.54 Ga aged MHB xenoliths suggests the block is a distinct craton. These values show that the GFTZ must indeed be a Paleoproterozoic collisional zone as proposed by others and supports proposed models of a Paleoproterozoic underplating event observed in other xenoliths and in seismic sections.