The petrogenesis of the paragneisses, Quesnel Lake, Manitoba

Abstract

Detailed mapping and sampling were completed over an area of two square miles, on the north-west end of Quesnel Lake, Manitoba. In all, 175 samples were collected, and classified into; 1a) quartz-oligoclase-biotite gneiss, 1b) quartz gneiss, 2) migmatite, 3) amphibolite, 4) pegmatite, and 5) granite. Thirty-two thin sections were studied, and modal and chemical analyses were carried out on fifteen samples. Biotite separates from these fifteen samples were analysed chemically. The gneisses consist of quartz, oligoclase, biotite, and sillimanite with minor amounts of potash feldspar and garnet; accessory minerals are chlorite, muscovite, magnetite, zircon,and hematite. They represent equilibriuim assemblages of the sillimanite-almandite-orthoclase subfacies of the amphibolite facies. The chemical compositions of the gneisses are similar to that of greywacke-type sedimentary rocks. The gneisses display relic bedding structures, and they are therefore interpreted to be metamorphosed sedimentary rocks. Temperature and pressure conditions were uniform across the area. Temperatures reached 680-700* C and pressures exceeded 4 kilobars, conditions which are related to a depth of 15-20 kilometers. The gneisses can therefore be interpreted as part of a uplifted block exposed by erosion. Under the conditions of metamorphism, the three gneissic units may represent differing degrees of anatexis or partial melting of original sedimentary rocks, which is now shown by differing amounts of quartz-feldspar layers.