THE ROLE OF LOW PRESSURE FRACTIONATION IN THE DIFFERENTIATION OF CALC ALKALINE ANKARA VOLCANICS

In Central Ankara, Turkey, Middle Eocene (38.8-42.0±1.6 Ma) volcanism has produced calc alkaline dacites and rhyolites. Previous work on these volcanics has failed to reach a consensus on the nature of the differentiation process. Our hypothesis is that the calc alkaline dacites and rhyolites of the Ankara Volcanics are the products of fractional crystallization of calc alkaline parental dacitic composition of D-16.

Linear trends in major oxide variation diagrams suggest that fractional crystallization of a dacitic parental magma may be responsible for the chemical spectrum shown by the Ankara Volcanics. In order to test this hypothesis, we chose a dacite composition (D-16 with Mg number of 48.64) as the parental magma and using the MELTs algorithm we allowed it to differentiate under fractional crystallization conditions at fO₂≈NNO.

Results of the MELTs calculations show that the best fit between the bulk composition of the Ankara Volcanics and calculated melt compositions is in the pressure range of 1.5-2.0 kbars with water content of 3.0-4.0 %.

These results suggest that low pressure fractionation of a dacitic magma located at about 6-8 km depth in the continental crust may be the principal process for the calc alkaline series of Ankara Volcanics.