Formation of an Archean tectonic melange in the Schreiber-Hemlo greenstonebelt, Superior Province, Canada: Implications for Archean subduction-accretion process

The late Archean (circa 2750-2670 Ma) Schreiber-Hemlo greenstone belt, Supe rior Province, Canada, is composed of tectonically juxtaposed fragments of oceanic plateaus (circa 2750-2700 Ma), oceanic island tires (circa 2720-269 5 Ma), and siliciclastic trench turbidites (circa 2705-2697 Ma). Following juxtaposition, these lithotectonic assemblages were collectively intruded b y synkinematic tonalite-trondhjemite-granodiorite (TTG) plutons (circa 2720 -2690 Ma) and ultramafic to felsic dikes and sills (circa 2690-2680 Ma),wit h subduction zone geochemical signatures. Overprinting relations between di fferent sequences of structures suggest that the belt underwent at least th ree phases of deformation. During D-1 (circa 2695-2685 Ma), oceanic plateau basalts and associated komatiites, arc-derived trench turbidites, and ocea nic island are sequences were all tectonically juxtaposed as they were inco rporated into an accretionary complex. Fragmentation of these sequences res ulted in broken formations and a tectonic melange in the Schreiber assembla ge of the belt. D-2 (circa 2685-2680 Ma) is consistent with an intra-arc, r ight-lateral transpressional deformation. Fragmentation and mixing of D2 sy nkinematic dikes and sills suggest that melange formation continued during D2. The D1 to D2 transition is interpreted in terms of a trenchward migrati on of the magmatic are axis due to continued accretion and underplating. Th e D2 intra-are strike-slip faults may have provided conduits for uprising m elts from the descending slab, and they may have induced decompressional pa rtial melting in the subarc mantle wedge, to yield synkinematic ultramafic to felsic intrusions. A similar close relationship between orogen-parallel strike-slip faulting and magmatism has recently been recognized in several Phanerozoic transpressional orogenic belts, suggesting that as in Phanerozo ic counterparts, orogen-parallel strike-slip faulting in the Schreiber-Heml o greenstone belt played an important role in magma emplacement.