Structural and metamorphic evidence for Mesoarchaean subduction in the Finlayson Lake greenstone belt, Superior Province, Ontario

Abstract

The unique structural architecture of Archaean terranes has generated competing models for early earth tectonics. Understanding the structural and metamorphic history of an individual terrane allows us to compare the deformation path to that predicted by tectonic models, and determine the best model for matching field observations. The Finlayson Lake greenstone belt is a Mesoarchaean terrane lying between three different gneiss terranes in the south-central Wabigoon subprovince in Canada. The belt has been interpreted as either a synformal keel sagducted between rising gneiss diapirs or as three fault-bounded allochtonous sub-belts of different ages. We present a detailed structural field study to define the deformation history of the Finlayson Lake greenstone belt and show that it is not consistent with either previous hypothesis. The Finlayson Lake greenstone belt is a single volcanic package that incorporates detritus from exposed felsic terranes similar in age and composition to the adjacent 3.0Ga Marmion tonalites. The geometry of the greenstone belt is defined by two outward-facing palaeo way-up orientations (anticlinal) and geothermobarometry records a clockwise metamorphic path with a deep prograde event at 820 ± 40MPa (27–30km) and 600 ± 45°C, followed by peak metamorphism at 635 ± 165MPa (21–23km) and 625 ± 25°C. The deformation history records sinistral transpression during peak metamorphism and continued flattening during retrogression and exhumation from ductile to brittle regimes. The structural and metamorphic results are comparable with modern subduction-accretion style settings. The intensity of both retrogressive and brittle deformation fabrics during exhumation decrease from east to west away from the eastern boundary shear zone, the Marmion Shear Zone. Stronger deformation along the eastern margin of the Finlayson Lake greenstone belt, adjacent to the Marmion Shear Zone, suggest reactivation during exhumation and is likely related to the 2.7Ga amalgamation of the Superior Province.