P-T EVOLUTION OF THE TESLIN SUTURE ZONE AND CASSIAR TECTONITES, YUKON, CANADA - EVIDENCE FOR A-TYPE AND B-TYPE SUBDUCTION

Mineral composition and quantitative thermobarometric studies indicate that the Teslin-Taylor Mountain and Nisutlin terranes within the Teslin suture zone (TSZ), Yukon, record widespread high-P/T metamorphic conditions consistent with subduction zone dynamothermal metamorphism. The highest P-T conditions (575-750-degrees-C and 9-17 kbar) are preserved in tectonites formed during normal dip-slip ductile shear. Dextral strike-slip tectonites record lower P-T conditions (400-550-degrees-C and 5-8 kbar), and tectonites which show reverse shear have peak temperatures of c. 420-degrees-C and a minimum peak pressure of 3 kbar. Dynamothermal metamorphism took place in a west-dipping B-type subduction zone outboard of western North America in Permo-Triassic time. TSZ tectonites were underplated against the hangingwall plate of the subduction zone. Following subduction of the ocean basin which separated North America from the hangingwall plate, TSZ tectonites were overthrust eastward as a coherent structural package as a result of A-type subduction of Cassiar strata in early Jurassic time. (Par)autochthonous Cassiar tectonites, which comprised the leading edge of the western North American margin, record prograde moderate-P, high-T metamorphism (550-750-degrees-C and 7-13 kbar) synchronous with top-to-the-east ductile shear. Metamorphism occurred as a result of subduction of the North American margin into the TSZ subduction zone in early Jurassic time. Following metamorphism Cassiar tectonites cooled slowly from 500 to 300-degrees-C during the period middle Jurassic to middle Cretaceous. TSZ and Cassiar tectonites were deformed during changing P-T conditions. Data from each of these tectonite packages indicate that grain-scale strain partitioning may have allowed local recrystallization of individual minerals by the addition of mechanical energy. The composition of the new grains reflects the P-T conditions under which that particular grain was deformed.