THE REPULSE BAY BLOCK: A WINDOW INTO THE MIDDLE CRUST OF A PROTEROZOIC COLLISIONAL OROGEN

The Repulse Bay block of Nunavut, Canada lies at the southern edge of the Rae Province on Melville Peninsula. The crustal block is composed of a 2.8-2.6 Ga TTG magmatic suite that was subsequently deformed and metamorphosed at upper amphibolite to granulite facies during the ca. 1.88-1.76 Ga Trans-Hudson Orogeny. The Repulse Bay block consists of mainly tonalitic to granitic gneisses, stromatic metatexite migmatite, and local diatexites. Two segments of the middle crust are exposed within the Repulse Bay block; the western area represents a shallower amphibolite facies level whereas the eastern area represents a deeper granulite facies level. Both areas are structurally characterized by a subhorizontal, undulating gneissosity, consistent with mid crustal ductile deformation. Water assisted partial melting is recorded in the amphibolite facies orthogneisses to the west; it is recorded by melt segregation and the formation of peritectic hornblende in leucosomes. The source of water is likely derived from the injection of ubiquitous synkinematic granitic pegmatites. Granulite facies metamorphism within the eastern portion of the Repulse Bay block was synchronous with the formation of a regional gneissic fabric and is best preserved within rare metapelitic layers and orthogneisses that reach peak metamorphic temperatures (ca. 1824 Ma) above biotite dehydration melting. Subsequent high temperature isothermal decompression (ca. 1766 Ma) is recorded by cordierite and gahnite surrounding garnet. Abundant charnokitic intrusions are preserved within the high grade eastern portion of the block. We speculate that a large mantle anomaly was the cause of gabbro-diorite and charnokitic magmatism, driving medium-P granulite facies metamorphism. The derivative melts moved upwards through the crust and were emplaced as fluid rich granitic pegmatites in the middle crust. This drove the water-assisted partial melting and ductile deformation of higher level granitoid rocks. The Repulse Bay block is, therefore, a well preserved example of the mechanisms driving tectonometamorphism within two levels of the middle crust, during the amalgamation of Laurentia.