Contrasting complexities in the evolution of calc-alkaline and alkaline melts of the Nigde volcanic rocks, Turkey: textural, mineral chemical and geochemical evidence

Abstract

The Nigde volcanic province in central Turkey comprises four separate stratovolcanoes (Tepekoy, Cinarli, Melendiz and Keciboyduran) of Mio-Pliocene age (MPv) and several monogenetic cones of Pleistocene age (Pv). The MPv rocks are composed of large volume of andesitic lavas and pyroclastics with medium to high-K calc-alkaline character. They are strongly porphyritic with phenocryst assemblages of plag + opx + cpx + Fe-Ti oxides + bio ± amph ± quartz, and commonly exhibit disequilibrium textures such as complexly zoned plagioclases, reversely zoned pyroxenes, and reacted amphiboles and biotites. Geothermobaro-metric studies based on mineral chemistry data suggest that the MPv calc-alkaline magmas underwent a complex evolutionary history of cooling, crystallisation and mixing within two distinct magma chambers sited in the lower and in the upper crust, respectively. The rocks of Pv include small volumes of alkaline basalts showing poorly porphyritic textures and equilibrated phenocryst assemblages of ol +cpx +lesser plag and Fe-Ti oxides. Textural features, and strong compositional variations in single phenocrysts from andesites clearly reflect magma mixing-mingling and open-system modifications. Major and trace element evidence also argues against dominant fractional crystallisation processes and supports magma mixing for andesitic magmas. In contrast, textural, mineral chemical and geochemical evidence for Pv alkaline rocks indicate that magmas had a relatively simple evolutionary history dominated by small degrees of fractional crystallisation of mafic minerals along volcanic conduits. MPv show typical island-arc trace element signatures, such as negative anomalies in high-field strength element (HFSE) and high ratios between HFSE and large ion lithophile elements (LILE). Slight HFSE negative anomalies are also observed in Pliocene alkaline basalt, suggesting contamination by arc component, either in the source or during alkaline magma ascent.