Zoned Poikiloblastic Garnets: P-T Paths and Syn-Metamorphic Uplift through 30 km of Structural Depth, Wopmay Orogen, Canada

Abstract

In the early Proterozoic Wopmay Orogen (Northwest Territories, Canada), the occurrence of garnet-biotite-sillimanite/kyanite-plagioclase-quartz assemblages in pelitic schists at a variety of obliquely exposed structural levels enables the use of calibrated geothermometers and geobarometers through 30 km of composite structural relief. Direct derivation of multipoint P-T paths from single garnets is attained from core-to-rim microprobe analyses of zoned poikiloblastic garnets, which contain biotite, plagioclase, quartz, and less commonly Al2SiO5 inclusions. The documented garnet zoning and the entrapment of the mineral inclusions is compatible with partial-equilibrium growth models. The lack of significant diffusion re-equilibration in the garnet interiors is favored by sample restriction to medium-grade schists and is attested by the preservation of normal-zoning profiles, the lack of garnet diffusion babes around biotite inclusions, the matching composition trends of garnet-core to -rim plagioclase inclusions with those of zoned matrix plagioclase grains, and the systematic variation of the derived P-T data. Only the garnet rims, which are characterized by a reversal of compositional trends and by textural resorption, are interpreted to indicate local post-thermal-peak re-equilibration. The calculated P-T paths quantify segments of uplift trajectories corresponding to pressure drops of 2·5–1·5 kb from maxima of 9·3–5·0 kb depending on structural level. This is concurrent with initial increases of 25–75°C to peak-temperature conditions and is followed by variable drops in temperature during continued decompression. Individual paths are consistent with modelled variations of metamorphic conditions as a function of loading, uplift, and erosion in overthrust terrains. Considered with U-Pb zircon geochronological data the P-T paths, studied as a set, indicate an average uplift rate that varies spatially from 1·5–2·7 mm y−1. This variation can be related to late cross folding of the orogenic internal zone, suggesting that the syn-metamorphic uplift was structurally controlled. The distribution of peak-temperature conditions attained during decompression is independent of structural depth. This, and the inverted metamorphism documented in the Wopmay Orogen, require that final variations in temperature result from thermal relaxation of isotherms in, and away from, a hot crustal allochthon composed in part of high-T rift-fill units and a syntectonic granitic batholith.