TECTONIC SIGNIFICANCE OF THE ORIGIN AND DISPERSAL OF OPHIOLITIC METAGABBRO IN CENTRAL ANATOLIA

The origin of gabbroic outcrops in the Niğde metamorphic massif of Central Anatolia and of gabbro cobbles in conglomerates surrounding the massif has implications for the processes of subduction, collision and post-collisional unroofing related to Anatolian continental assembly. Metagabbro outcrops along the N margin of the Niğde Massif have been interpreted by some as intrusions and others as part of the Central Anatolian Ophiolite (CAO). Geochemical features of the metagabbro such as Ti/V ratios of < 10 and depleted LREE (La_N/Yb_N=0.40-0.89), confirm a correlation to the CAO and are typical of boninitic magmas from forearc settings. Structural observations and measurements reveal high strain zones and concordant foliations at contacts between metagabbro and metasedimentary units, consistent with tectonic emplacement. In contrast to other exposures of the CAO, the Niğde metagabbro exhibits a much higher grade of metamorphism and degree of ductile deformation (upper amphibolite facies as compared to greenschist facies or lower). Thermometric results for synkinematic hornblende and plagioclase reveal that the gabbro was metamorphosed/deformed at 550-785°C.

Abundant gabbro clasts occur in Tertiary conglomerates deposited along a suture zone between the Central Anatolian Crystalline Complex (CACC) to the N and the Tauride terrane to the S. Previous studies have attributed ophiolite material in basin sediments to erosion of the Tauride ophiolites, however the presence of remnant ophiolite in the Niğde Massif, at the southernmost end of the CACC, indicates that catastrophic ophiolite erosion occurred in both terranes. Based on the distinct characteristics of the CAO (common metamorphic hornblende and boninitic compositions), it is possible to distinguish between the two ophiolite source areas. Paleogene conglomerates from the suture zone contain gabbro cobbles with boninitic and MORB compositions, whereas younger (Miocene) sediments contain gabbro that is consistently boninitic. Features of the gabbro cobbles suggest that the older sediment was at least partly derived from the Tauride ophiolites and that younger sediment was sourced in the CAO. These results provide insight into the evolving landscape of a tectonically-active region during the transition from subduction to continental collision.