Petrogenesis of Middle Miocene To Early Quaternary Basalts From the Karayaz?-G?ksu Plateau (Eastern Anatolia, Turkey): Implication for the Role of Pyroxenite and Lithospheric Thickness

Abstract

In Eastern Anatolia, the Karayazi-Go center dot ksu basaltic plateau is one of the basaltic eruption centers of post-collisional volcanism, located between the convergence zone of the Arabian and Anatolian plates. The geological and geochemical evolution of the plateau is documented using new 40Ar/39Ar age data, major-and trace element abundances, mineral geochemistry, and isotope compositions (Sr-Nd-Hf) for better understanding the petrogenesis and mantle source characteristics. Samples yield whole-rock 40Ar/39Ar ages of 12.53 +/- 0.05 and 12.68 +/- 0.05 Ma for transitional basalts (Middle Miocene), 5.88 +/- 0.20 Ma for tholeiitic lavas (Late Miocene), and 2.28 +/- 0.03 and 2.48 +/- 0.12 Ma for alkali members (Early Quaternary). The results suggest that the fractional crystallization of olivine and clinopyroxene in transitional, tholeiitic and lower alkaline lavas, and clinopyroxene in upper alkaline lavas, played a crucial role in their magmatic evolution without significant crustal contamination. Clinopyroxene-liquid equilibria indicate that the crystallization of transitional basalts occurred at the Moho and lower crust (temperature range 1157-1215 degrees C), tholeiitic basalts at the lower crust (temperature range 1154-1195 degrees C), alkaline basalts at lower to mid-crustal depths (temperature range 1083-1195 degrees C). The subduction-related metasomatism is in the form of melts derived from subducted sediments and decrease from Middle Miocene transitional to alkaline Early Quaternary basalts, as indicated by the Sr-Nd-Hf isotopes and Ba/ La and Th/Yb contents. The Early Quaternary alkaline basalts contain contributions from isotopically depleted but chemically enriched (enriched-MORB-like) asthenospheric mantle domain. Major elements-based parameters and olivine mineral chemistry yield that Karayazi-Go center dot ksu basalts could not be derived from a sole peridotite mantle source; the pyroxenite/eclogite-like mantle component would have contributed to the members of the plateau. Three-component mixing results using Sr-Nd isotopes indicate that the altered oceanic crust contributes to 0.5%-1% of the melt (crust vs. sediment about 65:35) for tholeiitic and alkaline basalts and reach up to 4% for transitional basalts (crust vs. sediment about 55:45). Melt equilibration depths of the basalts indicate that the lithospheric thickness of the region has not been changed on a large scale since the Middle Miocene to the Early Quaternary.