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 degrees C, 3.7-3.9 kbar, isograd (2) 625-640 degrees C, 3.0-3.9 kbar, and isograd (3) 640-660 degrees 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 degrees 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 degrees C over 420 m. The metastable persistence of staurolite and andalusite could result in an actual gradient closer to 30 degrees C.
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