Evolution of regional metamorphism during back-arc stretching and subsequent crustal shortening in the 1.9 Ga Wopmay Orogen, Canada

Abstract

The stratigraphic units, structural elements and metamorphic mineral assemblages of a regional metamorphic culmination in the 1.9 Ga Wopmay Orogen are exposed over greater than 30 km of composite structural depth, in a series of oblique sections produced by cross folding. Regional metamorphism developed continuously in three sequential, rapidly changing thermo-tectonic régimes within an evolving continental magmatic arc. At ca . 1900 Ma, stretching of intra-arc crust resulted in the accumulation of clastic sediment and bimodal volcanic rift-fill deposits. The onset (first stage) of regional metamorphism is marked by high- T low P mineral assemblages, condensed metamorphic zonal sequences and extensive areas of high-grade gneisses devoid of associated plutons. These features are interpreted in terms of a high thermal gradient related to stretching and thinning of the continental lithosphere. Five to ten million years after stretching, following deposition of a west-facing sedimentary prism, a suite of 1896—1878 Ma plutons was emplaced into the rift and margin deposits as they underwent subhorizontal shortening and deformation during the Calderian Orogeny. Thrusted and folded syn-orogenic foredeep deposits are also intruded by the syn-tectonic plutons. At high and intermediate structural levels, syn-tectonic metamorphic mineral growth and metamorphic zonal sequences which are spatially related to the plutons, document heat advection into the deforming marginal prism and mark a second stage of regional metamorphism related to the emplacement of the plutonic bodies. Inverted mineral isograds in autochthonous Proterozoic units beneath a basal décollement record downward thermal relaxation of isotherms following east-directed Calderian transport of the deformed, thickened, and still hot marginal prism over a relatively cold basement. Derivation of multi-point P - T trajectories from post-tectonic, poikiloblastic garnets charts metamorphic mineral growth during uplift and erosion of the internal zone, documenting the third (final) stage of regional metamorphism in Wopmay Orogen. The short erosional time interval (less than 11 Ma) between tectonic thickening and the end of uplift constrains the heat required for this last metamorphic stage to be inherited from the two preceding thermo-tectonic régimes: epicontinental stretching and the emplacement of the syn-tectonic plutonic suite.