STRUCTURAL AND PETROLOGIC IMPLICATIONS OF MAGMATIC UNDERPLATING IN A CONTINENTAL ARC: OBSERVATIONS FROM THE ATHABASCA GRANULITE TERRANE

The Athabasca granulite terrane, Saskatchewan, CA, provides a field example of the heterogeneous (in space and time) effects of magmatic underplating and intraplating on lower continental crust during orogenesis. A wide variety of processes associated with extreme heating and deformation occur in the heterogeneous host rocks, including: high-volume melting, crustal flow, tonalite and granite genesis, magma assimilation and contamination, metamorphic strengthening and weakening, and crustal densification. Limited melting or weakening occurred in the relatively strong tonalite domains. Instead, mafic dikes and sills in the tonalite were partially melted to Grt + tonalitic leucosome. In contrast, juvenile arc-related felsic plutonic rocks and migmatitic metasediments underwent extensive melting and weakening and flowed laterally, possibly in deep crustal channels. During flow, metamorphic reactions served to strengthen the rocks, and subsequent shear zones tend to be steep and more localized. Other granitoids in the region were apparently heated and deformed but not melted. One common reaction involves Opx + Pla = Grt + Cpx. Extensive Grt production led to increased density, up to 3.0 g/cm3 even in felsic rocks and may play a role in stabilization and possibly delamination of lower continental crust. Within the Fehr granite/gneiss, partial melting was particularly extensive. Leucosome veins, pods, and pools apparently represent the products of several melting reactions, including hydrous modal melting and both biotite and hornblende dehydration melting. Granitic partial melt apparently accumulated in an early (S1) foliation and then was mobilized during formation of a “megacrenulation” into S2. This “pumping” mechanism provides one model for melt segregation in the deep crust. Subsequent mafic dikes mingled and mixed with the new granitic magma, producing a wide range of hybrid compositions. With increasing extent of granite melting, mafic dikes became more irregular in geometry, apparently trapped by the granite mush. Based on the Athabasca example, underplating can initiate a number of positive and negative feedbacks that ultimately result in dramatic and probably irreversible changes in character, rheology, and composition of the overlying crust.