Archaean crustal growth processes in southern West Greenland and the southern Superior Province: geodynamic and magmatic constraints

Abstract

AbstractEo- to Mesoarchaean greenstone belts (e.g. 3800–3700 Ma Isua,c. 3075 Ma Ivisaartoq, 3071 Ma Qussuk) occur within orthogneisses of the southern West Greenland Craton. Greenstone belts are composed mainly of metavolcanic rocks with minor ultramafic and sedimentary schists. Compositionally, volcanic rocks are dominantly tholeiitic basalts, boninites, and picrites, with minor intermediate to felsic volcanic rocks. These greenstone belts appear to have formed in convergent margin geodynamic settings. Detailed field observations, contrasting ages, and metamorphic and structural histories suggest that this craton was assembled in several accretionary tectonothermal events, involving accretion of arcs, back-arcs, forearcs, and continental fragments by horizontal tectonics. The Superior Province of Canada was also built by the amalgamation of oceanic and continental fragments ranging in age from 3700 to 2650 Ma, during five discrete tectonothermal events over 40 Ma between 2720 and 2680 Ma. The Neoarchaean (2750–2670 Ma) Wawa greenstone belts are composed of tectonically juxtaposed fragments of oceanic plateaux, oceanic island arcs, back-arcs, and siliciclastic trench turbidites. Following juxtaposition, these diverse lithologies were collectively intruded by syn- to post-kinematic granitoids with subduction zone geochemical signatures. Oceanic island arc lavas are easily distinguished from oceanic plateau counterparts because they possess positively fractionated rare earth element (La/Smcn> 1 and Gd/Ybcn> 1) and high field strength element depleted (Nb/Thpm< 1; Nb/Lapm< 1) patterns. In addition, the island arc association includes pyroclastic rocks that are rare to absent in the oceanic plateau volcanic association. Structural studies indicate that the Wawa greenstone belts underwent a complex history of deformation including thrusting, strike-slip faulting, and asymmetric folding. These belts constitute part of ac. 1000 km scale subduction–accretion complex that formed along an intra-oceanic convergent plate margin during trenchward migration of the magmatic arc axis. Several first-order geological observations on Archaean greenstone belts of SW Greenland and the Superior Province suggest that Phanerozoic-style plate-tectonic models can provide an elegant explanation for their structural, lithological, metamorphic and geochemical characteristics.