SYNTECTONIC MAGMATISM AND THE DEVELOPMENT OF COMPOSITIONAL LAYERING, UNGAVA OROGEN (NORTHERN QUEBEC, CANADA)

Layer- and foliation-parallel emplacement of granitic veins was an imp ortant process in the regional development of compositional layering i n now-exhumed middle crustal sections of both Archean and Paleoprotero zoic age in the northern Ungava peninsula, Quebec (Canada). In the Arc hean Superior Province, diorite and tonalite plutons were penetrativel y deformed and metamorphosed at granulite-facies conditions coeval wit h voluminous granitic magmatism. The Paleoproterozoic Narsajuaq are co ntains evidence for contemporaneous magmatism, transpressional deforma tion and granulite-facies metamorphism prior to its collision with the Superior Province basement. In both plutonic domains, regional compos itional layering is defined by (1) metre to kilometre-scale alternatio n of generally well foliated tonalite and quartz diorite bodies; and ( 2) centimetre to kilometre-scale, variably deformed granitic veins and sheets that lie parallel to layering/tectonic foliation in the host r ocks. Syn-tectonic intrusion of a substantial portion of the veins alo ng extension fractures sub-parallel to layering/foliation (i.e. at hig h angle to the regional shortening direction) is interpreted to have o ccurred due to the combination of a strong anisotropy and high magma p ressures. Compositional layering generated and/or enhanced by this pro cess may contribute to the overall seismic reflectivity of the middle and lower crust.