Petrogenesis of collision-related plutonic rocks, central Anatolia(Turkey)

Abstract

Central Anatolia exhibits good examples of calc-alkaline and alkaline magmatism of similar age in a collision-related tectonic setting (continent-island arc collision). In the Central Anatolia region, post-collisional plutonic rocks (around 79.5 to 66.6 Ma) intrude Palaeozoic-Mesozoic metamorphic rocks overthrust by Upper Cretaceous ophiolitic units to make up the Central Anatolian Massif. In the Massif, three different intrusive rock types are recognised based on their geochemical characteristics: (i) calc-alkaline (Behrekdag, Cefalikdag, Celebi-BCC); (ii) transitional (Baranadag-B); and (ii) alkaline (Hamit-H). The BCC and B plutonic rocks are metaluminous, I-Type ranging from monzodiorite to granite. The H plutonic rocks are metaluminous to peralkaline, predominandy A-Type ranging from nepheline monzosyenite to quartz syenite. Two types of igneous enclave have been recognised in the BCC plutonic rocks; (i) fine-grained (Type-I); and (ii) medium-grained to porphyritic with feldspar megacrysts (Type-II). Field, petrographic and geochemical evidence suggest that the Type-I enclaves originated from injection of mafic magma into more silicic magma. Field, petrographic and mineralogical evidence together with major and trace element geochemistry suggest that the Type-II enclaves formed as cumulates of early-crystallised minerals (e.g. clinopyroxene, plagioclase, amphibole and biotite) from the granitoid magmas. All intrusive rocks show enrichment in LILE and LREE relative to HFSE and have high (^87)Sr/(^86)Sr and low (^143)Nd/(^144)Nd ratios. These characteristics indicate an enriched mantle source region(s) carrying a subduction component inherited from pre-collision subduction events. Rb, Nb, Y versus SiO(_2) diagrams and the tectonic discrimination diagram of Rb and the (Y+Nb) suggest that the BCC, B and H plutonic rocks have been affected by crustal assimilation combined with fractional crystallisation processes. Coexistence of calc-alkaline and alkaline magmatism in the Central Anatolian Massif has been attributed to mantle source heterogeneity before collision. Either thermal perturbation of the metasomatised lithosphere by delamination of the thermal boundary layer (TBL) or removal of a subducted plate (slab breakoff) are the likely mechanisms for the initiation of the post-collisional magmatism in the Massif