Metamorphism in the Prince Albert Group, Churchill Province, District of Keewatin, N.W.T.

Abstract

<p> A sequence of metasedimentary rocks comprising the Prince Albert Group, within and to the southwest of the Ellice Hills, District of Keewatin, N.W.T., was studied. Petrographic examination of the four major facies present -- quartzites, greywacke-paragneisses, metaultrabasics and iron formation was carried out and geochemical whole rock data was obtained using X.R.F. methods. </p> <p> Metamorphism occurred during the Hudsonian orogeny and came in three distinct pulses. These pulses are evident in thin section. The first pulse is characterized by the fonnation of garnet poikiloblasts and a biotite foliation; the second by a stronger biotite and hornblende foliation accompanied by quartz and muscovite porphyroblasts, and the third pulse is characterized by the growth of fibrolite needles. The last pulse of metamorphism shows that fibrolite and orthoclase formed from the dehydration of muscovite in the presence of quartz. Thus, a pressure and temperature regime for this event can be inferred from published experimental studies. These indicate that PM2O ranged from 2.0 to 3.5 Kbars and that temperature ranged from 640° ± l0°C to 670° ± l0°C. Previous pulses may have had higher pressure ranges but certainly lower temperature ranges prevailed. The present metamorphic grade of the Prince Albert Group displays mineral assemblages indicative of the Sillimanite-orthoclase-almandine Subfacies of the Almandineamphibolite Facies as defined by Winkler (1967). </p> <p> Structural deformation is closely associated with metamorphism. At least three periods of deformation have occurred. The first is evident in thin section by the s1 foliation and parallel trains of sialic material in garnet poikiloblasts. The second period of deformation caused the rotation of the above garnets, plus formation of the F2 isoclinal folds, s2 biotite foliation, crenulation of the s1 foliation and the formation of muscovite-quartz porphyroblasts. The third period of deformation is responsible for the F3 folding, warping of the F2 axial trace and the antisotropic growth of fibrolite.</p>