Formation of an Archean tectonic mélange in the Schreiber‐Hemlo greenstone belt, Superior Province, Canada: Implications for Archean subduction‐accretion process

Abstract

The late Archean (circa 2750–2670 Ma) Schreiber‐Hemlo greenstone belt, Superior Province, Canada, is composed of tectonically juxtaposed fragments of oceanic plateaus (circa 2750–2700 Ma), oceanic island arcs (circa 2720–2695 Ma), and siliciclastic trench turbidites (circa 2705–2697 Ma). Following juxtaposition, these lithotectonic assemblages were collectively intruded by synkinematic tonalite‐trondhjemite‐granodiorite (TTG) plutons (circa 2720–2690 Ma) and ultramafic to felsic dikes and sills (circa 2690–2680 Ma), with subduction zone geochemical signatures. Overprinting relations between different sequences of structures suggest that the belt underwent at least three phases of deformation. During D1 (circa 2695–2685 Ma), oceanic plateau basalts and associated komatiites, arc‐derived trench turbidites, and oceanic island arc sequences were all tectonically juxtaposed as they were incorporated into an accretionary complex. Fragmentation of these sequences resulted in broken formations and a tectonic mélange in the Schreiber assemblage of the belt. D2 (circa 2685–2680 Ma) is consistent with an intra‐arc, right‐lateral transpressional deformation. Fragmentation and mixing of D2 synkinematic dikes and sills suggest that mélange formation continued during D2. The D1 to D2 transition is interpreted in terms of a trenchward migration of the magmatic arc axis due to continued accretion and underplating. The D2 intra‐arc strike‐slip faults may have provided conduits for uprising melts from the descending slab, and they may have induced decompressional partial 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 Phanerozoic counterparts, orogen‐parallel strike‐slip faulting in the Schreiber‐Hemlo greenstone belt played an important role in magma emplacement.