PETROCHEMISTRY OF THE CUMAOVASI VOLCANICS; IMPLICATIONS FOR EXTENSION RELATED SILICIC VOLCANISM IN THE WEST-CENTRAL ANATOLIA, TURKEY

The Lower Miocene aged Cumaovasý volcanics in the NE-SW trending Çubukludag graben is well preserved example of a large silicic volcanism in West-Central Turkey. This volcanic unit form an upper part of the graben sequence. It consists of a cluster of rhyolite domes and associated pyroclastic deposits, aligned along a NE-SW trend, reflecting a crustal fissure-fracture zone within the graben basin.

The first products of the volcanism were deposited by the phretomagmatic explosive eruptions. This phase is mainly represented by fine-grained pumice and ash-rich tephra deposits, which are partly deposited in a lacustrine environment. Following magmatic activities, pyroclastic density flows, produced widespread blocky tephra deposits.

The lava phase is mainly dacitic and rhyolitic lavas which extruded from domes and fissures which are aligned along NE-SW direction within the graben. Main lithologies are rhyolite, rhyodacite, dacite, obsidian, perlite and autobrecciated flows. The rhyolitic lavas displaying the flow-foliations and the foliated stony rhyolites are the most common lava types. Petrochemically, the Cumaovasý volcanics consist of dacitic and high-silica rhyolitic lavas, which are calc alkaline, peralumious and enriched in LILE. Extremely low Sr, Ba contents, strong Eu depletions and very low La_N/Yb_N ratios are typical for the rhyolitic lavas. The isotopic signatures of the lavas are ¹⁴³Nd/¹⁴⁴Nd= 0.51234-0.51242, δ¹⁸O = +9.7-+12.2 %o (VSMOW) and ²⁰⁷Pb/²⁰⁴Pb=~15.7, ²⁰⁸Pb/²⁰⁴Pb=~39.08. ⁸⁷Sr/⁸⁶Sr ratios of the dacitic lavas are 0.708-0.706, whereas the rhyolitic series represents very high and variable (0.724-0.786) ratios. These geochemical features are distinctly different from the neighboring same aged rhyolitic lavas and show close similarities to the "Topaz rhyolites".

Lower Miocene aged silicic volcanism in the Cumaovasý region is a typical for the volcano-stratigraphic and morphological features of the subaerial silicic domes and lava flows together with the pyroclastic deposits. Considering the Neogene tectonic evolution of the Western Anatolia and geochemical nature of the silicic volcanics, we may propose that the Cumaovasý acidic volcanism was formed from crustal melting controlled the extension-related tectonism.