ABSTRACT The Quetico subprovince of the Superior Province is among the largest metasedimentary belts globally and provides evidence of tectonic processes at play during the final stages of Archean cratonization. Although previous studies have shown that deformation and metamorphism of the 2.7‐Ga turbiditic protolith resulted in regional subvertical tectonic fabrics and symmetrical N–S field gradients with the highest conditions in the east‐central part of the belt, constraints on the spatiotemporal evolution of metamorphism and deformation in the Quetico belt are underdeveloped. To obtain new insights into the tectonometamorphic history of the belt, 12 samples from garnet‐in to orthopyroxene‐in were subjected to garnet Lu–Hf chronology and trace‐element mapping. Garnet Lu–Hf ages from the garnet, staurolite–andalusite, and cordierite zones in the northwestern and southwestern Quetico overlap between 2664 and 2657 Ma, demonstrating that subsolidus metamorphism was coeval in these parts of the belt. In the eastern Quetico, ages are younger in the staurolite–andalusite, sillimanite, melt, K‐feldspar, and orthopyroxene zones (2651–2643 Ma). Subsolidus garnet features Cr, V, Ti, Y, and HREE zoning showing a ghost phyllosilicate fabric that is continuous and parallel, or slightly rotated relative to the external foliation, implying syn to post‐tectonic garnet growth. Suprasolidus garnet features more homogenized Cr, V, Ti, Y, and HREE distributions and higher Zr and P concentrations indicative of prograde suprasolidus growth. Prominent HREE‐rich annuli around embayed garnet boundaries in the orthopyroxene zone are attributed to garnet resorption near peak temperatures in the belt. These observations indicate a single, protracted tectonothermal event that extended across the Quetico, rather than a series of discrete events involving syn‐tectonic medium‐ P / T metamorphism followed by low‐ P / T overprinting and plutonism. The data are interpreted to record melt‐enhanced advective heat transport and exhumation within the upper crust during transpressive shortening, characteristic of ultra‐hot orogens (UHO). East–west diachroneity in garnet growth and differences in peak metamorphic grade indicate two loci where the sequence of heating and extrusion occurred at different times. This model provides a testable hypothesis for the tectonic evolution of UHO belts in Archean terranes.
Rehm et al. (Fri,) studied this question.
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