PRESSURE TEMPERATURE CONDITIONS OF EARLY PROTEROZOIC METAMORPHISM DURING THE TRANS-HUDSON OROGEN AS DETERMINED FROM ROCKS STRADDLING THE FLIN-FLON - KISSEYNEW BOUNDARY AT NIBLOCK AND FILE LAKES, MANITOBA

The Niblock Lake and File Lake areas straddle the boundary between the Kisseynew gneiss belt and the Flin Flon belt, near the town of Snow Lake, Manitoba. The region contains pelitic schists metamorphosed to lower to middle almandine-amphibolite facies. Metamorphic conditions were studied by examining relative timing of growth of metamorphic minerals by geothermobarometry of selected samples. Calculated temperatures and pressures are compared with those estimated from mineral assemblages and reactions on a petrogenetic grid. Two metamorphic (M1 and M2) and deformation (D1 and D2) phases have been recognized in the Niblock Lake and File Lake areas. M1 (contemporaneous with D1 folding) was characterized by growth of micas but no higher grade minerals. Temperatures and pressures of M1, therefore, were probably less than about 475-degrees-C and 3.5 kbar (1 kbar = 100 MPa). Peak conditions, reached during M2 (late- to post-D2 folding), resulted in growth of garnet, staurolite, sillimanite, and (in the Niblock Lake area) andalusite. In the Niblock Lake area, M2 temperatures range from 525 to 625-degrees-C, with most samples between 550 and 600-degrees-C; pressures range from 2.5 to 5 kbar, with most samples between 2.5 and 4 kbar. In the File Lake area, M2 temperatures range from approximately 560 to 625-degrees-C; pressures range from 3.3 to 4.6 kbar.