Abstract

Late Cretaceous lithologies around Amasya region are represented by Pontide fore-arc basin units which corresponds a volcanoclastic sequence. This sequence has the products of alkaline ultrapotassic magmatism accompanying calcalkaline lavas which are abundant along Pontide arc. The ultrapotassic rocks which are classified as leucitite, minette and trachyte based on their mineralogical composition, occur as dikes, stocks and rarely lava flows as to be comprised by the Late Cretaceous Volcanoclastic Succession (LCVS). Fractional crytallization accompanied by assimilation (AFC) is a low pressure processes able to differentiate ultrapotassic parental melts to various compositions in a continental margin tectonic setting.The trachytes are the youngest and the most evolved members of LCVS. Therefore we performed AFC modelling using the most primitive minette sample as starting composition and calculated the fractionation trends based on the theoretical mineralogical compositions. We also used the Triassic metapelitic basement rocks of Central Pontides as assimilant. The AFC modelling results imply that it is possible to produce trachytes by adding Central Pontide basement rocks up to 5 %, begining from the most primitive phonolitic sample of Amasya. However the differentiation of leucitite and minette is able to be explained by neither fractional nor assimilation processes.

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