Syn-tectonic magmatism and the development of compositional layering, Ungava Orogen (northern Quebec, Canada)

Layer- and foliation-parallel emplacement of granitic veins was an important process in the regional development of compositional layering in now-exhumed middle crustal sections of both Archean and Paleoproterozoic age in the northern Ungava peninsula, Quebec (Canada). In the Archean Superior Province, diorite and tonalite plutons were penetratively deformed and metamorphosed at granulite-facies conditions coeval with voluminous granitic magmatism. The Paleoproterozoic Narsajuaq arc contains evidence for contemporaneous magmatism, transpressional deformation and granulite-facies metamorphism prior to its collision with the Superior Province basement. In both plutonic domains, regional compositional layering is defined by (1) metre to kilometre-scale alternation 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 rocks. Syn-tectonic intrusion of a substantial portion of the veins along extension fractures sub-parallel to layering/foliation (i.e. at high angle to the regional shortening direction) is interpreted to have occurred due to the combination of a strong anisotropy and high magma pressures. Compositional layering generated and/or enhanced by this process may contribute to the overall seismic reflectivity of the middle and lower crust.