The Ospwagan Group supracrustal sequence of the Thompson Nickel Belt, Manitoba, includes semipelitic and pelitic schist, iron formation, and mafic volcanic rocks that were deformed and metamorphosed during the Trans-Hudson orogeny (ca. 1.8 Ga). North–south-trending metamorphic isograds at the Pipe II mine in the central portion of the belt show the following sequence from west to east: (1) andalusite + staurolite-out, in metapelites, (2) orthopyroxene-in, in meta-iron formations, (3) sillimanite + K-feldspar-in, in metapelites and semipelites. A roughly NE–SW-trending migmatite-in isograd (4), in semipelites, transects isograd (3) in the south portion of the study area. Equilibrium-assemblage diagrams for representative bulk-compositions were calculated using the Theriak–Domino software to provide pressure and temperature constraints for each of the metamorphic isograds. Isograds developed at the following metamorphic conditions: isograd (1) 585–600°C, 3.7–3.9 kbar, isograd (2) 625–640°C, 3.0–3.9 kbar, and isograd (3) 640–660°C at 3.0–3.6 kbar. The position of isograd (2), orthopyroxene-in for iron formation, occurs down-grade of isograd (3), sillimanite + K-feldspar-in for pelites, roughly 125°C below temperatures of granulite-facies metamorphism. The appearance of orthopyroxene at the middle amphibolite-facies to upper-amphibolite-facies transition is favored by the Fe- and Mn-rich bulk composition of the iron formation. The oblique orientation of isograd (4) to isograds (1–3) may be the result of an increasing pressure gradient toward the south, or the infiltration of melt-inducing fluids, or both. The isograd sequence suggests an apparent thermal gradient of at least 65°C over 420 m. The metastable persistence of staurolite and andalusite could result in an actual gradient closer to 30°C.
- © 2011 Mineralogical Association of Canada