Geochemistry, mineralogy and petrogenesis of the northeast Nigde volcanics, central Anatolia, Turkey

The volcanics exposed in the northeast Nigde area are characterized by pumiceous pyroclastic rocks present as ash flows and fall deposits and by compositions ranging from dacite to rhyolite. Xenoliths found in the volcanics are basaltic andesite, andesite and dacite in composition. These rocks exhibit linear chemical variations between end-member compositions and a continuity of trace element behaviour exists through the basaltic andesite-andesite-dacite-rhyolite compositional range. This is consistent with the fractionation of ferromagnesian minerals and plagioclase from a basaltic andesite or andesite parent. These rocks are peraluminous and show typical high-K calc-alkaline differentiation trends with total iron content decreasing progressively with increasing silica content. Bulk rock and mineral compositional trends and petrographic data suggest that crustal material was added to the magmas by subducted oceanic crust and is a likely contaminant of the source zone of the Nigde magmas. The chemical variations in these volcanics indicate that crystal liquid fractionation has been a dominant process in controlling the chemistry of the northeast Nigde volcanics. It is also clear, from the petrographic and chemical features, that magma mixing with disequilibrium played a significant role in the evolution of the Nigde volcanic rocks. This is shown by normal and reverse zoning in plagioclase and resorption of most of the observed minerals. The xenoliths found in the Nigde volcanics represent the deeper part of the magma reservoir which equilibrated at the higher pressures. Copyright © 2002 John Wiley and Sons, Ltd