~(40)Ar-~(39)Ar dating, whole-rock and Sr-Nd-Pb isotope geochemistry of post-collisional Eocene volcanic rocks in the southern part of the Eastern Pontides (NE Turkey): implications for magma evolution in extension-induced origin

摘要

The Eocene volcano-sedimentary units in the southern part of the Eastern Pontides (NE Turkey) are confined within a narrow zone of east-west trending, semi-isolated basins in Bayburt, Gumiishane, §iran and Alucra areas. The volcanic rocks in these areas are mainly basalt and andesite through dacite, with a dominant calc-alkaline to rare tholeiitic tendency. ~(40)Ar-~(39)Ar dating of these vol canic rocks places them between 37.7 ± 0.2 and 44.5 ± 0.2 Ma (Middle Eocene). Differences in the major and trace element variations can be explained by the fractionation of clinopyroxene ± magnetite in basaltic rocks and that of hornblende + plagioclase ± magne tite ± apatite in andesitic rocks. Primitive mantle-normal ized multi-element variations exhibit enrichment of large-ion lithophile elements and to a lesser extent, of light rare earth elements, as well as depletion of high field strength elements, thus revealing that volcanic rocks evolved from a parental magma derived from an enriched mantle source. Chondrite-normalized rare earth element patterns of the aforementioned volcanic rocks resemble each other and are spoon-shaped with low-to-medium enrichment (La_N/ Lu_N = 2-14), indicating similar spinel lherzolitic mantle source(s). Sr, Nd and Pb isotopic systematics imply that the volcanic rocks are derived from a subduction-modified subcontinental lithospheric mantle. Furthermore, post-col lisional thickened continental crust, lithospheric delami-nation and a subduction-imposed thermal structure are very important in generating Tertiary magma(s). The predominantly calc-alkaline nature of Eocene volcanic rocks is associated with increasing geodynamic regime-extension, whereas tholeiitic volcanism results from local variations in the stress regime of the ongoing extension and the thermal structure, as well as the thickness of the crust and the mantle-crust source regions. Based on volcanic variety and distribution, as well as on petrological data, Tertiary magmatic activity in Eastern Pontides is closely related to post-collisional thinning of the young lithosphere, which, in turn, is caused by extension and lithospheric delamina-tion after collisional events between the Tauride-Anatolide Platform and the Eurasian Plate.