Geochemistry and U-Pb zircon age of low-grade metavolcanic rocks from the Biga Peninsula, Northwestern Turkey

Northwest Anatolia and especially the Biga Peninsula is the area having special important in the case of understanding of geology of Turkey and its surrounding. The Biga Peninsula has a Variscan basement affected by Alpine tectonics which is mainly composed of metavolcanic rocks. NE-SW-directed metavolcanic rocks occur in the basement of Çamlıca metamorphic association and made up of brown, green, yellowish green metalava, metatuff and small amount of metasedimentary rocks. The common mineral assemblages of the metavolcanic rocks are mainly composed of quartz + chlorite + epidote + albite + actinolite + calcite ± sphen ± zircon. This mineral assemblage indicates that these metavolcanic rocks underwent greenschist-facies metamorphism.
   Major, trace and rare earth element (REE) geochemistry for metavolcanic rocks from the Biga Peninsula has been determined to reveal their origin and tectonic setting. The metavolcanic rocks have compositions of andesites with calc-alkaline character. Calc-alkaline chemistry is represented by intermediate SiO₂ content, low MgO and low Cr. Chondritenormalized REE patterns are moderately fractionated (La_N/Yb_N ~ 2.2 to 8.9). Europium anomalies are variable (Eu/Eu* 0.6 to 1.9) and generally negative (average Eu/Eu* 0.83). The plagioclase fractionation is confirmed by a slight development of negative Eu anomaly. The metavolcanic rocks have a distinct negative Nb anomaly with negative Sr, Ba, Hf anomalies in extended element diagrams. The large negative Nb, Sr, Ba and Hf anomalies in the metavolcanic rocks exhibit a crustal involvement in their derivation. The crustal influence may be related to either partial melting at the base of continental crust or contamination of mafic magma with crustal material. On tectonic discrimination diagrams, all metavolcanic rocks cluster within the volcanic arc field away from either the within plate or ocean ridge fields. Those within the volcanic arc field indicate calc-alkaline magma type. Such a magma type is a characteristic of volcanic arc setting for the metavolcanic rocks. Moreover, negative Nb anomalies are also characteristic of the volcanic arc.
   Zircon grains from these metavolcanic rocks, which are euhedral with typical magmatic morphologies, were dated by LA-ICPMS. Zircon ages of two samples yielded 328.6 ± 3.5 Ma and 343.2 ± 2.6 Ma, respectively. These are interpreted as the time of protolith crystallization of metavolcanic rocks. This volcanic episode of the Biga Peninsula can be attributed the Variscan magmatic activity and also collisional event leading to the amalgamation of tectonic units took place during Variscan orogenic event.