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Petrology and geochemistry of eclogites from the Biga Peninsula, Northwest Turkey

The Biga Peninsula in northwest Turkey contains high-pressure metabasic eclogite that occur in two localities; as lenses within a 2 km long, 500 m thick quartz-phengite schist slice that is in turn found in the greenschist-facies metasedimentary rocks of the Çamlıca metamorphics, in Çamlıca area, and in two north-south elongated eclogites occurring as a tectonic slice between Kazdağ Massif and Çetmi Group, in Çetmi area. The P-T conditions under which these two exposures of eclogites formed are important to quantify the tectonic processes of subduction, exhumation and emplacement that high-pressure rocks of the Biga Peninsula record. New geochemical data suggest that both protoliths were N-type MORB basalt with high TiO2 and K2O–Na2O content and Nb/Y ratios. Most eclogite samples have tholeiitic signatures volcanic arc settings. ∑REE abundances range from 47.55 to 107.4 ppm. Europium anomalies are slightly variable (Eu/Eu* = 0.9–1.1) and trace element contents are similar to typical MORB based on tectonic discrimination diagrams. All eclogite protoliths were probably derived from depleted mantle source, modified by fluids within the subduction zone. The high-P mineral assemblage in eclogites from both regions is omphacite + garnet + glaucophane + phengite + epidote + zoisite + quartz. The inclusions in garnet are glaucophane, quartz, phengite, Ca-amphibole and rutile. P-T conditions are similar to each other and constrained at 550–700 °C and 16–24 kbar. Geochemical data and mineral chemistry indicate that the eclogites in the Biga Peninsula represent oceanic crust processed at significant depths (50–80 km) within the subduction channel and were juxtaposed with greenschist facies as a tectonic slice in the accretionary complex at higher structural levels.