A ZONED LOW P-HIGH T-COMPLEX AT THE LEVEL OF ANATEXIS - STRUCTURAL AND PLUTONIC PATTERNS IN METASEDIMENTS OF THE ARCHEAN-YELLOWKNIFE-SUPERGROUP, NEAR BATHURST INLET, NWT, CANADA

Within metagreywackes and schists of the Archean Yellowknife Supergroup the development of an "anatectic stratigraphy" during the late stages of regional deformation has resulted in the formation of a complex with a metamorphicplutonic-structural zoning similar to that occurring in Phanerozoic low P-high T terrains. From bottom to top the "stratigraphy" consists of: (1) small-folded migmatite, grading with decreasing partial melting to (2) sillimanite schist, strongly strained by the emplacement of two-mica granites; (3) andalusite-cordierite schist, the site of large folds related to the emplacement of the complex. Each zone is characterized by a different structural style resulting from the interplay between metamorphic grade and deformation. For instance in the sillimanite zone, the structure was influenced by the extraction of syntectonic melt and its emplacement into fractures at low effective pressure. This zone was also the ductile envelope into which sills and plutons of two-mica granites derived from the lower migmatites migrated. The time taken for evolution of the complex has been constrained by U-Pb zircon and monazite dating of a member of the early tonalite-diorite suite and members of the two-mica granite suite. This shows the evolution of the complex was rapid, ca. 10 Ma, but possibly polyphase. The formation of a distinctively styled complex accompanied by syn-tectonic anatexis and plutonism, may be typical of lower crustal levels of the Yellowknife Supergroup and other low P-high T terrains. © 1990.