S-type granites in the western Superior Province: a marker of Archean collision zones

Detailed field observations indicate that Neoarchean S-type granites were emplaced along and (or) proximal to some terrane (tectonic) boundary zones in the western Superior Province, southeastern Manitoba. These S-type granites are characterized by the presence of at least one diagnostic indicator mineral, such as sillimanite, cordierite, muscovite, garnet, and tourmaline. They are medium- to high-K talc-alkaline, moderately to strongly peraluminous (ANKC >1.1), and contain >1% CIPW nonnative corundum. Compared with more voluminous, older I-type granitoids in tonalite-trondhjemite-granodiorite suites in the region, the S-type granites occur as relatively small intrusions and have high (SiO2 >72 wt.%) contents with a small silica range (SiO2 = 72.2-81.2 wt.%), but a large range of Zr/Flf (17.1-43.8) and Nb/Ta (0.3-16.0) ratios. These geochemical characteristics suggest that the S-type granites were derived from partial melting of heterogeneous sedimentary rocks deposited as synorogenic flysch that underwent burial and crustal thickening during terrane collision. Although the S-type granites display geochemical variations in individual and between different plutons, their low Sr (<400 ppm) and Yb (<2 ppm) contents and low Sr/Y (<40) and La/Yb (<20) ratios are consistent with a partial melting process that left a granulite-facies residue consisting of plagioclase, pyroxene, and +/- garnet. The S-type granites display low zircon saturation temperatures (mostly <800 degrees C) and low emplacement pressures (<300 MPa), similar to strongly peraluminous leucogranites formed in the Himalayas. Therefore, we propose that the Neoarchean S-type granites in the western Superior Province, whose source rocks were deposited between colliding tectonic blocks between 2720 and 2680 Ma, may serve as a geological marker of some Archean terrane boundary zones.