Turkish Journal of Earth Sciences

Geochemical Characteristics of the Eastern Pontide Volcanics, Turkey: An Example of Multiple Volcanic Cycles in the Arc Evolution




The Eastern Pontides is located along the Alpine metallogenetic belt in the Eastern Black Sea region of Turkey and is composed predominantly of volcanic rocks, the products of the major orogenic activities that took place during Lias, Late Cretaceous, and Eocene. On the bases of field observations and geochemical data, four volcanic cycles were determined in the province: 1) Jurassic Cycle, 2) Late Cretaceous Lower Cycle, 3) Late Cretaceous Upper Cycle, and 4) Tertiary Cycle. The geochemical data indicates that the products of the Jurassic Cycle bear the characteristics of back arc lavas. The presence of picritic and nepheline-normative alkaline lavas and the absence of both a shoshonitic series and continental crust involvement indicate a rift-type extensional tectonic environment. The presence of tholeiitic, calc-alkaline, and shoshonitic lavas in the volcanic cycles of Late Cretaceous and Tertiary demonstrates that each cycle represents the beginning of a new geochemical and hence, geodynamical activity in the region. Subduction related major-, trace-, and rare earth-element characteristics require more than one subduction related activity during formation of these volcanics. The data indicates an embrionic back arc or an inter arc-type environment for the Late Cretaceous Upper Cycle volcanics, probably geochemically marking the opening of the present Black Sea basin during this cycle. Subduction related Tertiary cycle volcanics suggest that back arc or inter arc-type basin initiation during the Late Cretaceous Upper Cycle was abruptly or prematurely disrupted or volcanic activities ceased and new subduction commenced. Another possible explanation that Tertiary Cycle rocks represent collision related volcanics rather than new subduction has yet to be investigated. The petrogenetic relationships and their nature in and among the cycles of the subalkaline and alkaline lavas of the Eastern Pontide volcanics were investigated using major, trace, and REE data, and it is concluded that these volcanics, in general, were produced from a MORB-like mantle that was subjected to mobile element enriched dehydration fluids originating from crustal-derived sediment which metasomatized and initiated melting of the mantle wedge.

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