POSTCOLLISION MAGMATISM AND TECTONICS IN NORTHWEST ANATOLIA

A suite of biotite-hornblende granodiorite intrusions has been emplace d into blueschist-facies metasediments in northwest Anatolia, followin g collision between two continental margins, now represented by the Ta vsanli and Sakarya zones. The Ar-40/Ar-39 ages of phengites and glauco phanes from the blueschists, metamorphosed under unusually high P-low T conditions (P = 20 +/- 2 kbar, T = 430 +/- 30-degrees-C), suggest th at metamorphism apparently occurred over a period spanning at least 20 Ma from 108 to 88 Ma. Post-tectonic granodiorites were emplaced durin g the Eocene (53 to 48 Ma) resulting in a cordierite and andalusite-be aring thermal aureole, indicative of pressures of approximately 3 kbar . Trace-element systematics of the granodiorites are consistent with a derivation either from mantle-derived magmas by fractional crystallis ation in shallow magma chambers, or from anatexis of crustal lithologi es of intermediate composition at pressures < 10 kbar. The preservatio n of high P-low T assemblages in the blueschists together with the ran ge of ages determined for blueschist-facies metamorphism are indicativ e of rapid exhumation of delaminated fragments from a subducted contin ental margin. However decompression melting of the crust is unlikely t o have been a significant cause of magmatism, both because exhumation of the blueschists from deep crustal levels predated magmatism by at l east 25 Ma, and because of the small melt fraction (< 0.1) that may be generated in crustal lithologies by this process. Melting in the mant le wedge is required either to generate a primary melt for the derivat ion of magmas of intermediate composition or to provide an advective h eat source for crustal melting. The cause of melt formation in the upp er mantle may be related to the termination of subduction following co llision during the Mid-Eocene.