GEOCHEMISTRY AND TECTONIC ENVIRONMENT OF BASALTIC ROCKS FROM THE MISIS OPHIOLITIC MELANGE, SOUTH TURKEY

The Miocene Misis tectonosedimentary melange was formed during the closing stages of the Neotethys in what is now the Adana basin of southern Turkey. A major part is characterized by large disorientated limestone blocks set in a predominantly volcanogenic matrix of bedded and chaotic units comprising basaltic pillow lavas, associated hyaloclastites, serpentinites and various acidic tuffs. All the basaltic rocks are midly metamorphosed (zeolite facies), variably vesicular and differentially altered to smectite, chlorite, Fe-oxides, zeolites and carbonate. Petrographic and chemical data on the least altered pillow lavas indicate they were originally sparsely olivine-plagioclase-clinopyroxene-phyric tholeiites and minor amphibole-phyric basaltic andesites. The basaltic clasts in the melange matrix are genetically related to the pillow lava sequences, with the observed variation throughout the suite being governed by low-pressure fractional crystallization. As a group, the basalts are characterized by low Zr/Y (2-4), relatively high Th/Yb (0.5-0.7) and La/Nb (1.5-2.5) ratios, and moderate, chondrite-normalized, light REE enrichment [(La/Yb)N = 3] patterns. The basalts have a mildly enriched. subduction-related chemistry and were possibly formed initially in a back-arc basin setting. The supra-subduction zone characteristics of the Misis basaltic units are similar to other Tethyan basalts in eastern Mediterranean ophiolites.