Intracontinental quaternary volcanism in the Niksar pull-apart basin, North Anatolian Fault Zone, Turkey

The Niksar Basin is sited along the eastern segment of the North Anatolian Fault Zone in Turkey. It is a young sigmoidal pull- apart basin bordered by two non- parallel master faults associated with earthquakes in 1939 and 1942. The fault geometry along the irregular ENE margin of the basin is complex where young Plio - Quaternary volcanic rocks reach the surface along pairs of steep strike- slip faults which cut the basin sediments. The volcanic rocks around the Niksar Basin have been dated by high precision K- Ar dating and the ages range between 542 +/- 9 ka and 567 +/- 9 ka. The lavas are mainly alkaline ( sodium dominated) in nature and include basaltic trachandesite ( mugearite) and trachyandesite ( benmoreite) with minor sub- alkaline compositions of dacitic andesite, rhyodacite and rhyolite. Despite the large compositional gap between basaltic and felsic lavas, major and trace element distributions indicate that both the basaltic and felsic lavas are cogenetic. Abundances of major oxides and trace elements vary systematically through this compositional spectrum. Fractional crystallization of the observed phases accounts for the diversity of intermediate and evolved products. Amphibole fractionation in basalts at depth causes the trend towards silica saturation while alkali feldspar fractionation dominates the final stages of crystallization. Significant crustal contamination has occured in the evolved magmas but contamination is generally minimal or absent in their basaltic parents. Alkaline basaltic rocks have OIB ( ocean island basalt) like trace element patterns characterized by enrichment in LILE, HFSE, LREE and slight depletion in HREE relative to primitive mantle values. Overall geochemical variations indicate the combined effects of different degrees of partial melting, fractional crystallization and variable degrees of crustal contamination.