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

• Published in: Journal of Petrology
• Main Author: ST-ONGE, MARC R.
• Format: Text
• Language: English
• Published: Oxford University Press 1987
• Subjects: Articles; Northwest Territories; Canada
• Online Access: http://petrology.oxfordjournals.org/cgi/content/short/28/1/1; https://doi.org/10.1093/petrology/28.1.1

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 Al 2 SiO 5 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, ...