Metamorphic evolution of blueschists of the Alt?nekin Complex, Konya area, south central Turkey

Abstract

The Altinekin Complex in south central Turkey forms part of the south-easterly extension of the Tavsanli Zone, a Cretaceous subduction complex formed during the closure of the Neo-Tethys ocean. The protoliths of metamorphic rocks within the Altinekin Complex include peridotites, chromitites, basalts, ferruginous cherts and flysch-facies impure carbonate sediments. Structurally, the complex consists of a stack of thrust slices, with massive ophiolite tectonically overlying a Cretaceous sediment-hosted ophiolitic melange, in turn overlying a sequence of Mesozoic sediments. Rocks within the two lower structural units have undergone blueschist–facies metamorphism. Petrographic, mineral–chemical and thermobarometric studies were undertaken on selected samples of metasedimentary and metabasic rock in order to establish the time relations of deformation and metamorphism and to constrain metamorphic conditions. Microstructures record two phases of plastic deformation, one predating the metamorphic peak, and one postdating it. Estimated peak metamorphic pressures mostly fall in the range 9–11 kbar, corresponding to burial depths of 31–38 km, equivalent to the base of a continental crust of normal thickness. Best-fit peak metamorphic temperatures range from 375 to 450°C. Metamorphic fluids had high H2O:CO2 ratios. Peak metamorphic temperature/depth ratios (T/d values) were low (c. 10–14°C/km), consistent with metamorphism in a subduction zone. Lawsonite-bearing rocks in the southern part of the ophiolitic melange record lower peak temperatures and T/d values than epidote blueschists elsewhere in the unit, hinting that the latter may consist of two or more thrust slices with different metamorphic histories. Differences in peak metamorphic conditions also exist between the ophiolitic melange and the underlying metasediments. Rocks of the Altinekin Complex were subducted to much shallower depths, and experienced higher geothermal gradients, than those of the NW Tavsanli Zone, possibly indicating dramatic lateral variation in subduction style. Retrograde P–T paths in the Altinekin Complex were strongly decompressive, resulting in localized overprinting of epidote blueschists by greenschist–facies assemblages, and of lawsonite blueschists by pumpellyite–facies assemblages. The observation that the second deformation was associated with decompression is consistent with, but not proof of, exhumation by a process that involved deformation of the hanging-wall wedge, such as gravitational spreading, corner flow or buoyancy-driven shallowing of the subduction zone. Copyright © 2005 John Wiley & Sons, Ltd.