Geochemical and isotopic features of the mafic rocks in the çamdere ophiyolitic melange (Demi̇rözü-Bayburt)

Çamdere (Bayburt) area in the eastern Sakarya Zone is one of the two locations where the Late Carboniferous-Early Permian sedimentary rocks are cropped out. The unit, named the 'Çamdere Ophiolitic Melange' contains blocks of various lithologies, such as marble, limestone, sandstone, basalt, dolerite, gabbro, meta-basalt, meta-gabbro, which characterize distinct sedimentary and tectonic settings. Basic igneous rocks in the unit, which are divided in the two subgroups: meta-basites and unmetamorphized mafites, are investigated in terms of their geochemical features and Sr, Nd, Pb isotope ratios. The rocks of both the groups have a basic-intermediate geochemical composition (SiO2 = 44.0-56.6 wt. %). The samples from the mafic blocks are tholeiite in character, whereas those from the meta-basites with their low, medium and high K2O contents VII display a compositional range from tholeiite to soshonite. The meta-basites show light rare earth elelement (LREE) patterns inclined slightly to the right and almost horizontal heavy rare earth element (HREE) patterns (LaN/YbN = 4.55 - 29.01). As to the mafic rocks, they display mid ocean ridge basalt (MORB)-parallel, flat and slightly LREE enriched patterns (LaN/YbN = 1.44 - 2.19). The samples from both the groups show large ion lithophile element (LILE) enrichments and negative Nb-Ta-Ti anomalies. However, the degrees of enrichments and negative anomalies are weaker in the samples of the unmetamorphosed mafic blocks. The meta-basites show positive and negative Pb and P anomalies, whereas samples from the mafic blocks commonly present negative Pb and P anomalies. In general, the samples have low MgO (<6.5 wt. %), Cr (<250 ppm) and Ni (<150 ppm) contents and low Mg-numbers (<60), indicating that the samples have not a geochemical composition equilibrium with their mantle source. Partly low radiogenic 87Sr/86Sr(t) ratios (0.704335−0.706227), medium positive ԐNd(t) values (+2.7 − +6.4) and Pb isotope ratios, which plot slightly above the Northern Hemisphere Referans Line (NHRL), point to a mantle source modified by crustal rocks or contribution from Enriched Mantle II (EM II) reservoir. Geochemical features of the rocks refer to the formation above a subduction zone or interaction with solutions from the subduction zone. However this subduction feature of the rocks are not exposed obviously in the tectonic setting discrimination diagrams. In these diagrams, the meta-basites are scattered on the fields of within plate basalt, calcalkali basalt and island arc tholeite, whereas unmetamorphosed mafic rocks plot in the MORB field, that all are away from the subduction zone. Key Words: Çamdere-Bayburt, Geochemistry, Meta-basite, Ophiolitic melange, Radiogenic isotope, Tectonic setting