GEOCHEMICAL DISCRIMINATION AND PETROGENESIS OF ALKALIC BASALT SEQUENCES IN PART OF THE ANKARA MELANGE, CENTRAL TURKEY

East of Ankara, Turkey, within the major thrust-bound tectonic units o f the Ankara melange (Karakaya and Anatolian Nappes), are numerous blo cks of low-grade metamorphosed submarine basaltic lavas. Stable incomp atible element geochemistry indicates that the majority of these basal ts are alkaline and exhibit a range of characteristics typical of many within-plate oceanic islands (OIB). There is a broad chemical overlap between alkaline basalts from within the Triassic Karakaya Nappes (me tamorphic and limestone block melanges) and the Cretaceous Anatolian N appe (ophiolitic melange). A complete sequence of submarine alkaline b asalt lavas was sampled within the ophiolitic melange (the 500 m Kilic lar section) to illustrate the variation and petrogenesis of a small s eamount-related crustal segment, prior to tectonic dismemberment. The basalts can be divided into three chemical groups that relate to discr ete petrographic segments of the lava pile and probably reflect variab le degrees of partial melting of a garnet-bearing enriched source. The chemistry of alkaline basalts within the ophiolitic melange, together with their sedimentary associations, suggests an initial submarine en vironment adjacent to the volcaniclastite debris-filled moats of ocean ic islands or seamounts and their volcanically-active arches. Dismembe rment of the seamount-related basalts and pelagic sediments took place during subduction of the oceanic crust and their eventual accretion w ithin a forearc platform.