METAMORPHISM IN THE NORTHERN TALTSON MAGMATIC ZONE, NORTHWEST-TERRITORIES

The Taltson Magmatic Zone (TMZ) forms the southern part of the Thelon- Taltson orogen, in the Northwest Territories. It is separated from its northern continuation by the Great Slave Lake shear zone-Mcdonald fau lt zone, a major transform-fault system along which the Slave Province indented the Churchill Province. Within the TMZ, fine-grained metased imentary and minor mafic volcanic rocks of the Rutledge River Basin (s imilar to 2.3-2.1 Ga) postdate mixed gneisses and plutonic rocks (larg ely 2.4-2.3 Ga) that form the Churchill Province to the east. These me tasedimentary rocks have been intruded by three Taltson batholiths: th e Deskenatlata granodiorite (1.99 Ga), the S-type, Slave monzogranite (1.96 Ga), and the S-type, Konth syenogranite (1.94 Ga). Widespread oc currence of orthopyroxene and the assemblage garnet-cordierite-K-felds par indicates granulite-facies conditions during emplacement of the Sl ave and Konth plutons, whereas the Deskanatlata plutonism was accompan ied by lower temperatures. Garnet-orthopyroxene thermometry based on t he equilibrium: Alm = 3 Fs + Al2O3 (in orthopyroxene) yields near-peak temperatures between 920 and 1045 degrees C for most granite and para gneiss samples, whereas recorded pressures are roughly constant at 6.9 +/- 0.9 kbar. The distribution of samples with spinel and quartz in c ontact is centered on the Konth granite, suggesting an increasing ther mal regime from similar to 1.99 to 1.93 Ga, with the highest temperatu res reached during the generation and emplacement of the Konth granite . Temperatures recorded by TMZ samples exceed those predicted in publi shed thermal models of thickened continental crust, and appear to be h igher than those attending metamorphism in the Thelon orogen. A possib le mechanism to account for elevated thermal conditions is increased f lux of mantle heat resulting from ocean-ridge subduction following sub duction of oceanic crust that produced the Deskenatlata plutonic suite .