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CENOZOIC VOLCANO-STRATIGRAPHY AND PETROLOGICAL EVOLUTION OF POST-COLLISIONAL VOLCANIC ROCKS IN EASTERN BIGA PENINSULA, NW ANATOLIA, TURKEY
Abstract
Post collisional volcanic rocks cover large areas in the east of the Biga peninsula, which is located in the NW Anatolian magmatic belt along the southern margin of Marmara Sea. These volcanic units span a compositional range from calc-akaline (i.e. andesite, dacite, and rhyodacite to rhyo lite), to alkaline (olivine basa lts) lavas. All rock units display coherent variation trends on the major and trace element Harker diagrams. Cenozoic volcanic sequences exposed in the east of Biga are divided into three volcano- stratigraphic formations in this study: (1) the I ЕџД±keli and (2) Arabaalan volcanics, and (3) the TaЕџtepe basalt. The first two units are calc-alkaline in character (these are named in this study as the calc-alkaline suite) in contrast to the TaЕџtepe basalt which is alkaline. The IЕџД±keli volcanics are composed of rhyodacitic and rhyolitic tuffs and lavas of Upper Oligocene age (new K-Ar age for this formation is 28.3В±1.3 Ma). The Arabaalan volcanics consist of andesitic and dacitic lavas and is Upper Oligocene to Lower Miocene in age (26.3В±1.0 - 18.4В±1.3 Ma). The Ta Еџtepe basalt is made up of alkali olivine basaltic dykes and lavas that were erupted along the zones of local extension during a period from Upper Miocene to Pliocene (12.7В±0.6 Ma). Our analytical results indicate that the SiO 2 content of the I ЕџД±keli volcanics varies between 59.88% and 67.26%, while that of the Arabaalan volcanics varies between 54.24% and 64.88%. Alkaline basaltic dyke from the Ta Еџtepe unit has the lowest SiO 2 content by 49.4%. Except for K 2O, all major elements gradually decrease with increasing SiO2 in e linear fashion. The calc-akaline suit (i.e. the I ЕџД±keli and the Arabaalan volcanics) is characterized by a distinct enrichment in the large ion lithophile elements (LILEs), and also in light rare earth elements (LREEs) and depletion in the high field strength elements (HFSEs). This can be interpreted as a reflection of either hydrous melting of mantle wedge in a subduction zone or enrichment of the magma sources with an inherited component from an ancient subduction plate. Our geochemical data also indicate that a number of magma chamber processes involving magma mi xing, fractional crystallization (FC) and assimilation with fractional crystallization (AFC) might ha ve been operational with together. The Ta Еџtepe alkali olivine basalt is characterized by the enrichment of LILE, HFSEs, LREEs and MREEs, and a slight depletion in HREEs, implying a metasomatized mantle source.
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References10
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