KESKİN M., USTAÖMER T.
4th International Turkish Geology Symposium in Adana, Çukurova University, Turkey, 1 - 04 September 2001, pp.117, (Summary Text)
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Publication Type:
Conference Paper / Summary Text
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Country:
Turkey
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Page Numbers:
pp.117
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Istanbul University Affiliated:
Yes
Abstract
The Upper Cretaceous volcano-sedimentary belt, extending along the southern cost of the Black Sea from Srednogoria in Bulgaria in the west through the Central Pontides in northern Turkey, is one of the main tectonic elements of the Mediterranean Tethysides. This belt was interpreted as a magmatic arc sequence developed above a northward-dipping subduction zone of Tethys or a rift-associated volcanic margin sequence, developed during the opening of the W Black Sea Basin. In order to test these models, we have mapped out the belt in the north of Istanbul and we have studied geochemistry of the volcanics by XRF, ICP-MS and microprobe.
The Upper Cretaceous volcano-sedimentary sequence, termed the Kavaklar Group, is divided into three formations, deposited in tectonically active marine environment. From base to top these are: (1) Bozhane fm., consisting of terrigenous turbidites intercalated with thick and coarse-grained volcaniclastic beds, (2) Garipçe fm. composed of coarse-grained volcaniclastic sediments containing volcaniclastic boulderstones, volcanic breccias, hyaloclastites and sandstones with minor lava flows, (3) Kısırkaya fm. which is made up of basic lavas interlayered with associated volcaniclastics and occasional limestone lenses.
Our geochemical database indicates that there are two distinct magmatic series in this sequence: (1) calc-alkaline series (CAS) displaying a distinct subduction signature with a lithospheric mantle origin and (2) alkaline series (AS) exhibiting a within-plate geochemical signature, implying an asthenospheric origin.
Volcanic products of the Bozhane and Kavaklar formations are represented by CAS, whereas those of Kısırkaya fm. correspond to AS. Lavas of the CAS span a compositional range from basaltic andesite to dacite. They may be divided into three petrographic types: (a) aphyric, (b) Plg+Px-phyric (An<sub>40-85</sub>; ferroan diopside and enstatite, Mg-rich augite) and Plg+Amp-phyric (An<sub>46-60</sub>; ferrohornblende and pargasite) lavas. In contrast, lavas of the AS are represented only by olivine-basalts.
Variation diagrams involving highly incompatible and compatible element pairs indicate that mixing of magmas derived from lithospheric and asthenospheric sources was a major process in magma chamber evolution of CAS.
Neither stratigraphy nor tectonics is consistent with an arc setting during the Upper Cretaceous in the region. Furthermore, an arc-massif is missing all over the Western Pontides. Therefore, we argue that this sequence represents an extensional volcanism, related to the opening of the Black Sea by rifting during the Mid- to Late Cretaceous period. We believe that the arc signature in the CAS may not be related to an active subduction. A plausible explanation for this may be inheritance from an earlier subduction event that imprinted its signature on sub-continental lithosphere.
We propose a model where magma generation is associated with lithospheric thinning which caused derivation of magma from progressively deeper zones in the mantle during the opening of Western Black Sea.