TECTONIC SETTING, PETROLOGY AND GEOCHRONOLOGY OF JADEITE PLUS GLAUCOPHANE AND CHLORITOID PLUS GLAUCOPHANE SCHISTS FROM NORTH-WEST TURKEY

The north-west Turkish blueschists of represent a subducted passive co ntinental margin sequence dominated by metaclastic rocks and marble. T . depositional age of the blueschist protoliths are probably Palaeozoi c to Mesozoic, while the age of the high-pressure/low-temperature meta morphism is Late Cretaceous. Blueschists are tectonically overlain by a volcanosedimentary sequence made up of accreted oceanic crustal mate rial that locally shows incipient blueschist metamorphism and by spine l peridotite slices. The metaclastic rocks with regional jadeite and g laucophane, which comprise the lower part of the blueschist unit, make up an over 1000-m-thick coherent sequence in the Kocasu region of nor th-west Turkey. Rare metabasic horizons in the upper parts of the meta clastic sequence with sodic amphibole + lawsonite but no garnet indica te lawsonite blueschist facies metamorphism. The blueschist metaclasti cs in the Kocasu region are practically free of calcium and ferric iro n and closely approximate the NFMASH system in bulk composition. Two l ow-variance mineral assemblages (with quartz and phengite) are jadeite + glaucophane + chlorite + paragonite and chloritoid + glaucophane paragonite. The metaclastics comprise up to several-metres-thick layer s of jadeite schist with quartz, phengite and nearly 100 mol% jadeite. Phase relations in the metaclastics show that the chloritoid + glauco phane assemblage, even in Fe2+-rich compositions, is stable in the jad eite stability field. In the NFASH system the above assemblage without the accompanying garnet has a narrow thermal stability field. Mineral equilibria in the metaclastics involving chloritoid, glaucophane, jad eite, paragonite and chlorite indicate metamorphic P-T conditions of 2 0 +/- 2 kbar and 430 +/- 30-degrees-C, yielding geothermal gradients c lose to 5-degrees-C km-1, one of the lowest geotherms recorded. Bluesc hists in the Kocasu region, which have been buried to 70 km depth, are tectonically overlain by the volcanosedimentary sequence and by perid otite buried not deeper than 30 km. Phengites from two jadeite schists were dated by Ar/Ar laser probe; they give an age of 88.5 +/- 0.5 Ma, interpreted as the age of metamorphism. Blueschists and the overlying peridotite bodies are intruded by 48-53-Ma-old granodiorite bodies th at were emplaced at 10 km depth. This suggests that the exhumation of blueschists by underplating of cold continental crust, and normal faul ting at the blueschist-peridotite, interface occurred during the Late Cretaceous to Palaeocene (88-53 Ma).