Date of Award
M.S. in Chemistry
University of Mississippi
The Medicine Hat Block (MHB) is one of the core cratonic elements that amalgamated in the Paleoproterozoic to form Laurentia. However the role of the MHB in the formation of Laurentia is poorly constrained due to virtually all of the MHB being concealed by Proterozoic and younger supracrustal sequences limiting availability of data. The primary sources of samples from the MHB are 1) xenoliths of variable metamorphic grade including gneisses amphibolites and meta-plutonic rocks collected from Eocene volcanic rock; and 2) similar lithologies recovered from boreholes that penetrate to the MHB basement. Previous work on samples from the MHB yielded Paleoproterozoic ages (1.78 – 1.82 Ga) and Archean ages (2.5 – 3.3 Ga). New zircon single-grain LA-ICP-MS U-Pb analyses on MHB xenoliths expand on previous ages with: one Neoarchean age (2.79 Ga) nine Paleoarchean ages (3.45 – 3.54 Ga) and one younger detrital sample with multiple age peaks. Samples with > 3.3 Ga ages indicate an earlier origin for the MHB than was indicated by previous data. 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 Paleoproterozoic grains yielded εHfT values ranging from -6.8 to -21.2 suggestive of juvenile and evolved crustal mixing. The combined U-Pb and Hf isotopic data help characterize 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 for the 2.8 – 3.54 Ga aged MHB xenoliths suggests the MHB is unique. These values show that the Great Falls Tectonic Zone must indeed be a Paleoproterozoic collisional zone and supports proposed models of a Paleoproterozoic underplating event observed in other xenoliths and in seismic sections.
LaDouceur, Blake Oswell, "The Medicine Hat block and the assembly of laurentia: new interpretations from single-grain zircon analyses" (2019). Electronic Theses and Dissertations. 1770.
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