Petrogenesis and ages of the amphibolites within the Söğüt metamorphics, Central Sakarya terrane (NW Eskişehir, Turkey)

Yaşar, İsmail Doğancan
The Sakarya Composite Terrane (SCT) in northern Turkey consists of a pre-Jurassic basement composed of several tectonic units. The Söğüt Metamorphics, preserved within the Central Sakarya Terrane (a part of the SCT), is one of these tectonic units, consisting of para- and ortho-gneisses associated with bands, lenses, boudins, and tectonic slices of amphibolites. Considering their field relations and petrographic features, amphibolites have fine- and coarse-grained varieties. The metamorphic conditions of amphibolites are defined by hornblende+plagioclase+biotite±sphene and epidote+chlorite+muscovite paragenesis for pro- and retro-grade phases, respectively. However, the peak metamorphic conditions (low-to-medium P/high T amphibolite facies) is defined by the sillimanite+biotite±cordierite paragenesis of the paragneisses. Geochemically, the trace element systematics of the amphibolites reveals two different chemical types for their protoliths: Type 1 amphibolites have somewhat EMORB characteristics, with negative Zr-Hf anomalies and variable enrichment in highly incompatible elements, whereas Type 2 amphibolites have subduction modified signatures with negative Nb anomalies. A continental within-plate rift setting with a metasomatized subcontinental lithospheric mantle (SCLM) source, and an intra-oceanic subduction zone setting with a mantle source metasomatized by slab-derived fluids/melts are proposed for the original crystallization setting of Type 1 and Type 2 amphibolites protoliths’, respectively. U-Pb zircon dating revealed Middle Ordovician, and Middle Carboniferous crystallization ages for the protoliths of Type 1 and 2 amphibolites, respectively. Altogether, the new data obtained confirms the opening of an oceanic basin due to within-plate rifting during the Middle Ordovician. The Middle Carboniferous magmatism, on the other hand, suggests the presence of an intra-oceanic subduction zone within this ocean.