Granulite- and amphibolite-facies metamorphism in a convergent-plate-margin setting: Synthesis of the Quebec-Baffin segment of the Trans-Hudson Orogen

The tectonometamorphic histories of basement and cover units in the eastern Trans-Hudson Orogen attest to the importance of structural, geohydrologica l, and magmatic controls on the attainment of metamorphic conditions in a c onvergent-plate-margin setting. In northern Quebec, two metamorphic suites are recognized in the parautochthonous (lower-plate) Superior Province base ment. An older metamorphic suite comprises are-related, granulite-facies as semblages (less than 5 kbar and 860 to 920 degrees C) dated at ca. 2.70 Ga. A younger, overprinting, collision-related metamorphic suite comprises amp hibolite-facies assemblages (7.7 kbar at 640 degrees C to 9.8 kbar at 715 d egrees C) dated at ca. 1.80 Ga. Within the overlying Paleoproterozoic Cape Smith Belt (lower-plate, south-verging thrust belt), thermal-peak mineral g rowth is syn- to post-thrusting; a relatively high-pressure, greenschist- t o amphibolite-facies metamorphism (6.3 kbar at 400 degrees C to 9.1 kbar at 575 degrees C) is interpreted to be a result of ca. 1.80 Ga collision-rela ted thickening. At higher structural levels, upper-prate mid-crust plutonic and metasedimentary units of the 1.86-1.82 Ga Narsajuaq are contain granul ite-facies assemblages (7-10 kbar and 800-900 degrees C) retrograded to amp hibolite-facies conditions (7-9 kbar and 700-775 degrees C) during collisio n-related thrusting. At the structurally highest revel, granulite-facies me tamorphism of the (upper-plate) Lake Harbour Group and Blandford Bay assemb lage is related to emplacement of the 1.86-1.85 Ga Cumberland batholith. Be tween ca. 1820 and 1795 Ma, accretion and collision-related thickening resu lted in retrogression of the granulite-facies assemblages in the Narsajuaq are, Lake Harbour Group and Blandford Bay assemblage, thermal peak metamorp hism of lower-plate cover rocks, and re-equilibration of the parautochthono us Superior Province basement, all at greenschist- to amphibolite-facies co nditions. The integration of field, thermobarometric and U-Pb data establis hes a causal relationship between are plutonism and development of granulit e-facies assemblages in a convergent-plate-margin setting. In contrast, the attainment of greenschist- to amphibolite-facies conditions at mid-crust l evels during plate collision is primarily a function of structural and geoh ydrological controls.