COMPLEX COMPOSITIONAL ZONING IN CLINOPYROXENE IN LATE CRETACEOUS POTASSIC-ALKALINE VOLCANIC ROCKS FROM NORTHERN TURKEY: IMPLICATIONS FOR MAGMA MIXING

Textural signatures and compositional variation in major and trace elements within individual phenocrysts are strong indicators of changing conditions in a crystallizing magma chamber caused by open system processes. The ferromagnesian silicates, particularly clinopyroxenes, from a series of dikes and flows from north-central Turkey show exceptional examples of such textural and chemical zonation patterns. As such they have the potential to reveal the evolutionary history of the Pontide arc magmas. The lava flows and dikes are believed to have come from a continental magmatic arc on the southern edge of Laurasia, north of the subducting Neotethyan Ocean. This magmatic arc produced andesitic lavas and leucite-bearing alkaline ultrahigh potassium volcanic rocks during the late Cretaceous. This study is designed to explore the evolution of potassic magmas along this arc.

The samples given special attention were gathered from three different localities spanning 300 km along the Ankara-Erzincan suture belt, the Late Cretaceous – early Tertiary tectonic suture zone that represents the Pontide arc, as part of a larger study inquiring into the origin of leucite-bearing basaltic and potassic volcanism that took place along the arc. The Ankara-Erzincan suture belt in northern central Turkey marks the junction between the Kirsehir Massif and the Central Pontides; these two continental fragments collided during the early Tertiary during the closure of the northern branch of the Neotethyan Ocean.

Primary investigations were conducted using a JEOL JXA-6800 microprobe at the University of Georgia. Examination of textural and chemical features present in the clinopyroxenes from these potassic volcanic rocks sheds light on magmatic processes involved in the parent magmatism. Textural features of importance include fossil phenocryst resorption features and identifiable sets of chemically distinct epitaxial bands. The compositional and spatial data of these epitaxial band sets denote different families of phenocrysts with distinct genetic origins; the presence of multiple phenocrysts families in the same system is evidence for magma mixing events. To supplement this information data on temperature and pressure conditions prior to eruption, magma ascent rates, and inferred crystallization rates will be considered.