Metamorphism and intrusion in the Benjamin Lake map-area, Northwest Territories
Creators: name: Davidson, Anthony
Publisher: name: University of British Columbia
Year: 2011

description: Benjamin Lake map-area lies within the Archaean Slave province of the Canadian Shields. It is underlain by metamorphosed volcanic and sedimentary rocks of the Yellowknife Group that are tightly folded and have been intruded by a series of granodiorite plutons. Assessment of the relations between metamorphism and intrusion is based mainly on field observations, on the results of a petrographic study of over 300 thin sections of the metasedimentary and granitic rocks, and on mineral compositions determined by optical methods. The reactions that produced the various metamorphic minerals are discussed. Regional metamorphic grade in the metasediments ranges from the upper part of the greenschist facies to the middle part of the cordierite-amphibolite facies of the low pressure andalusite-sillimanite facies series. Pelites and semipelites of amphibolite grade are knotted schists characterized by large porphyroblasts of cordierite and andalusite. Metamorphism, in part synkinematic, has outlasted regional deformation. Increase in the andalusite content of the knotted schists towards the east-southeast is considered to have been controlled by conditions of metamorphism rather than by changes in rock composition. Stability relations of observed minerals and their assemblages lead to estimates of conditions during regional metamorphism between 450° and 600°C at between 2 and 3½ Kb. Both pressure and temperature increased in an east-southeasterly direction. The cordierite isograd represents an isothermal surface of about 525°C. Granodioritic magma invaded the metasediments during the later stage of tectonism. Time relations between the resultant plutons have been established. The earliest plutons were emplaced during regional metamorphism and involved the bordering metasediments in the formation of migmatites. Later intrusions have sharp contacts and display a range in form from narrow, dike-like stocks to plutons with oval plan. The latter appear to have developed from the former by swelling in place and have accommodated themselves by pushing aside the previously deformed metasediments. Later still are larger plutons that are concordant in parts and discordant in others. Introduction of pegmatites preceded this latest phase of intrusion but succeeded the earlier phases. The plutonic series shows a gross compositional change with time from equigranular hornblende-biotite quartz diorite through biotite granodiorite to porphyritic granodiorite and quartz monzonite with megacrysts of K-feldspar. Some stocks of granodiorite show a certain amount of internal differentiation, with the K-feldspar content increasing from margin to core. Granodiorite stocks are surrounded by narrow aureoles in which the rocks are characterized by higher proportion of biotite and andalusite than in the regional knotted schists. Pressure-temperature relations between cordierite-muscovite and biotite-andalusite assemblages suggest that pressures in the aureoles of the stocks were slightly higher than those that prevailed during regional metamorphism, which in turn suggests forceful intrusion. The presence of high quartz pseudomorphs in some granodiorites places an upper pressure limit of about 4 Kb on granodiorite crystallization.