MISSISSIPPIAN ASSEMBLY OF THE NISUTLIN ASSEMBLAGE - EVIDENCE FROM PRIMARY CONTACT RELATIONSHIPS AND MISSISSIPPIAN MAGMATISM IN THE TESLIN TECTONIC ZONE, PART OF THE YUKON-TANANA TERRANE OF SOUTH-CENTRAL YUKON

Metamorphosed and ductilely deformed sedimentary, plutonic, and volcan ic rocks of the Nisutlin and Anvil assemblages make up the Yukon-Tanan a terrane in the Teslin tectonic zone study area. The Nisutlin assembl age consists of siliceous schist - quartzite and graphitic phyllite th at share a primary depositional contact, and Early Mississippian tonal ite to quartz diorite that intrudes the siliceous schist - quartzite a nd possibly the graphitic phyllite. The Anvil assemblage includes meta gabbro and mafic schist - greenstone that share an intrusive contact r elationship. Tonalite to quartz diorite of the Nisutlin assemblage is characterized by minor zircon inheritance with an average Proterozoic age, epsilon(Nd)(350 Ma) values of -2.5 to -6.2, and Nd model ages of 1.50-1.79 Ga. These data suggest that the magmatic bodies have inherit ed a component of continentally derived material. Primary contact rela tionships and age data indicate that the Nisutlin assemblage had forme d by Mississippian time, and regional correlations show that this asse mblage makes up a large part of the Yukon-Tanana terrane of southern Y ukon. Assembly of the Nisutlin assemblage by Mississippian time indica tes that it did not form as a late Paleozoic and early Mesozoic subduc tion melange, and it suggests that its tectonic fabrics did not result from the progressive growth of a Permo-Triassic subduction complex. W e suggest that the Nisutlin assemblage was part of a crustal block tha t lay outboard of North America in Mississippian time, and that it tay in the hanging-wall plate of a Permo-Triassic subduction zone as a re latively coherent assemblage, rather than forming within the zone as a subduction complex.