The muscovite‐melt bathograd and low‐P isograd suites in north‐central Wopmay Orogen, Northwest Territories, Canada

Abstract

AbstractIn north‐central Wopmay Orogen, syntectonic low‐P(Buchan‐type) suites of mineral isograds outline regional metamorphic temperature culminations that are associated, at the higher structural levels, with emplacement of early Proterozoic plutons in the west part of a deformed and eastward transported continental margin prism. The mapped isograds mark the first occurrence of biotite, staurolite, andalusite, sillimanite, sillimanite‐K feldspar and K feldspar‐plagioclase‐quartz ± muscovite (granitic) pods in metapelites, with increasing proximity to the plutons.Microprobe analyses and field observations have resulted in the formulation of reactions for the ‘ideal’pelitic system K2O‐Na2O‐FeO‐MgO‐Al2O3‐SiO2‐H2O‐Al2O3‐SiO2‐H2O, to account for the various mineral assemblages of each metamorphic zone. A P‐T petrogenetic grid showing erosion surface P‐T curves for the northern Wopmay Orogen pelites, compiled on the basis of the mapped isograds and the inferred reaction(s) for each metamorphic zone, documents a variation in exposed metamorphic pressure ranging between 2 and 4 kbar.The configuration of a new bathograd, based on the invariant model reaction sillimanite + K feldspar + plagioclase + biotite + quartz + vapor ± muscovite + liquid and interpolated across three metamorphic suites, is consistent with a major regional structure culmination and with independently determined pressures obtained from anorthite‐grossular‐quartz‐Al2SiO5 geobarometry. The positive correlation between the configuration of the bathograd and the structural and pressure culmination points to the pressure‐dependence of anatectic‐granitic‐pod mineral associations.