Metamorphic evolution of granulites in the Minto Block, northern Québec: extraction of peak P‐T conditions taking account of late Fe‐Mg exchange

Abstract

ABSTRACTIn the central Minto Block of northern Québec, the Lake Minto and Goudalie domains are dominated, respectively, by orthopyroxene‐bearing plutonic suites (granite‐granodiorite and diatexite) and a tonalitic gneiss complex, both of which contain scattered remnant paragneisses. Two main granulite‐grade mineral assemblages are observed in the paragneiss: garnet (Grt)‐orthopyroxene (Opx)‐plagioclase‐quartz (GOPQ) and garnet (Grt)‐cordierite (Crd)‐sillimanite‐plagioclase‐quartz (GCSPQ). These show distinct lithological associations, with the GCSPQ assemblages occurring exclusively within the diatexite in the Lake Minto domain. Petrogenetic grid considerations demonstrate that the GOPQ rocks are higher grade than the GCSPQ rocks. Maximum temperatures for GOPQ rocks, obtained from equilibria based on Al solubility in orthopyroxene in equilibrium with garnet, range from 950 to 1000d̀ C, significantly higher than garnet‐orthopyroxene Fe‐Mg exchange temperatures of 700 ± 50d̀ C, the latter probably representing a closure temperature below peak conditions. The Al temperatures were corrected for late cation exchange by adjusting the Fe/(Fe + Mg) ratios in garnet and orthopyroxene, to achieve internal consistency between the GOPQ thermometers and barometers. Grt‐Crd thermometry records temperatures of 750±50d̀ C. Peak P‐T conditions range from 5‐6 kbar and 750‐800d̀ C in the Goudalie and eastern Lake Minto domains, to 7‐10 kbar and 950‐1000d̀ C in the western and central Lake Minto domain. This variability contrasts with the uniform crustal pressures of 5 ± 1 kbar recorded by the GCSPQ assemblages in the diatexites and the hornblende granodiorites (c. 4‐5 kbar) across the same area. The GOPQ rocks are inferred to record earlier P‐T conditions that prevailed before the formation of GCSPQ assemblages and the intrusion of the granodiorites. Partial P‐T paths in GOPQ rocks from both domains, based on net transfer equilibria corrected for Fe‐Mg resetting, document cooling of 100‐250d̀ C from thermal‐peak conditions, concomitant with a modest pressure decrease of 2‐3 kbar. Although textures diagnostic of isobaric cooling are not developed, the paths are consistent with a tectonic model in which granulite metamorphism and crustal thickening in the Minto Block were consequences of magmatic underplating. The progression from higher P‐T conditions recorded by GOPQ assemblages to lower P‐T conditions recorded by GCSPQ assemblages is attributed to variable amounts of synmagmatic uplift and cooling in a single, continuous thermal event in the Minto crust, associated with protracted crustal magmatism. In the Goudalie and eastern Lake Minto domains, where GOPQ and GCSPQ rocks and Hbl granodiorites have similar P‐T conditions of equilibration, the crust may not have been thickened as much as further west, where GOPQ P‐T conditions are significantly higher than those of the hornblende granodiorites and the GCSPQ rocks.