OVERVIEW OF THE TECTONIC EVOLUTION OF THE SOGUT METAMORPHICS, VARISCAN BASEMENT ASSEMBLAGE OF THE CENTRAL SAKARYA TERRANE (NW ESKISEHIR, TURKEY)

The Sakarya Composite Terrane (SCT) in northern Turkey consists of a pre-Jurassic basement composed of several tectonic units. Among these, the Sogut Metamorphics, preserved within the Central Sakarya Terrane (a part of the SCT), is a Variscan tectonic unit, consisting of para- and ortho-gneisses associated with bands, lenses, boudins, and tectonic slices of amphibolites. Altogether, this assemblage is extensively deformed and affected by partial melting in some places, as evidenced by the migmatite development within paragneisses and amphibolites. The peak metamorphic paragenesis is defined by sillimanite+biotite±cordierite in paragneisses for prograde amphibolite facies, and epidote+chlorite+muscovite in amphibolites for retrograde greenschist facies metamorphism. The gneisses and amphibolites were intruded by the late Carboniferous arc-type granitoids during the Variscan Orogeny.

Orthogneisses display geochemical signatures ranging from volcanic arc granites (dominantly) to within-plate granites (Th/Nb=0.67-1.43; Sm/Ti=0.0019-0.0056). Amphibolites are mostly basaltic in composition, including both alkaline and subalkaline varieties. The trace element systematics of the amphibolites reveals two chemical types for their protoliths: (1) incompatible element enriched signatures (Nb/Yb=4.02-10.29) with significant Zr-Hf depletion (Nd/Zr=0.14-0.65), and (2) subduction-modified geochemical signatures with prominent negative Nb anomalies (Th/Nb=0.16-0.65; La/Nb=1.27-3.67). Overall trace element systematics are suitable with an origin of continental within-plate magmatism for the prior and an intra-oceanic island arc magmatism for the latter type of the amphibolites.

Available geological and geochronological data indicate that the generation of the orthogneiss protoliths starts from Cambrian and interpreted to conform with the formation of the Cadomian arc due the subduction of the Iapetus Ocean beneath northern Gondwana. Furthermore, the protoliths of the amphibolites were generated during (1) Ordovician, interpreted to mark the opening of the Rheic Ocean with the slab-roll back of the Iapetus slab, and (2) mid-Carboniferous within the Rheic Ocean, located to the north of the SCT. Accreation and metamorphism of all this material was concluded prior and/or synchronous to the Variscan Orogeny.