PALEOARCHEAN CRUST OF THE MINNESOTA-MICHIGAN CORRIDOR: EVIDENCE FROM THE WATERSMEET DOME, NORTHERN MICHIGAN

Paleoarchean crust was recognized in the Minnesota River valley (MRV) of SW Minnesota in the 1970s and in the upper peninsula of Michigan (UPM) in the early 1980s by Z. Peterman and colleagues of the U.S.G.S., who used multi-grain U-Pb zircon data to show that the protolith of a tonalite gneiss exposed in the core of the Watersmeet gneiss dome was ~3.5 Ga, and that late Archean and Paleoproterozoic crust was also present. Subsequent geophysical studies were interpreted to suggest continuity of this crust from the MRV into the UPM. The antiquity and high metamorphic grade of the MRV and UPM crust contrasts strongly with the Neoarchean greenstone belts of the southern Superior Province (Wawa sub-province). Paleoarchean crust of the MRV has been affected by Neoarchean events and to a lesser extent by Paleoproterozoic tectonism. Similarly, the Paleoarchean of the UPM also experienced Neoarchean events, but with a much stronger Paleoproterozoic overprint (Penokean orogeny). In order to refine our understanding of the evolution of the Archean crust of the UPM, we report new U-Pb ages coupled with Hf isotopic compositions of zircons from eight samples from the Watersmeet dome. Ages determined by multi-collector LA-ICP-MS range from 1.8 Ga to 3.6 Ga. Initial eHf in these zircons is overwhelmingly negative with a maximum value of +3, but are as low as -32, and each age-group (~1.8, ~2.7, and ~3.6 Ga) shows closed system evolution in age-Hf space. Samples with the most negative initial eHf yield single stage depleted mantle model ages >4.0 Ga, suggesting that Eoarchean and/or Hadean sources were involved in their petrogenesis. A single 3.82 Ga xenocryst in the 3.6 Ga tonalite gneiss provides direct evidence for this older crust. In addition, the Neoarchean Carney Lake granite (UPM) also contains >3.8 Ga xenocrysts. Detrital zircons of comparable age are also present in Huronian sedimentary rocks. Collectively these data show that the antiquity of the crust along the southern boundary of the Superior Province is comparable to that of the Nuvvuagittuq supracrustal belt along the northern margin of the Superior Province. Both terranes also experienced Neoarchean (~2.7 Ga) overprints coeval with the majority of ages associated with the aggregation of the greenstone belts of the Superior Province.