Sr-Nd-Ca isotopic variations of Cenozoic calc-alkaline and alkaline volcanic rocks above a slab tear in Western Anatolia, Turkey

The tectonic mechanisms that may trigger a transition from calc-alkaline to alkaline volcanic activity above a subduction zone are enigmatic. We report major/trace elemental and Sr-Nd-Ca isotopic compositions of a suite of magmatic samples from the Miocene Gordes calc-alkaline dacites and the Quaternary Kula alkaline basalts from Western Anatolia, Turkey. Zircon sensitive high-resolution ion microprobe dating shows that the Gordes dacites formed at ca. 18 Ma. They are characterized by of light rare earth elements and large-ion lithophile elements (Rb, Th, U), positive Pb anomalies and negative Eu and high field strength element (HFSE) (Nb, Ta, and Ti) anomalies with high 87Sr/86Sri (0.70977- 0.71010) and low epsilon Nd(t) (-7.3 to -7.0). They have small delta 44/40Ca (0.59%0-0.69%0) values lower than that of mid-ocean ridge basalt (0.83% +/- 0.11%). The Kula basalts exhibit alkaline affinities and are more enriched in Ta, Pb, and Sr) than those of typical oceanic island basalts. Their low 87Sr/86Sri (0.70307- 0.70343) and high epsilon Nd(t) (+4.2 to +6.5) are consistent with an asthenospheric mantle source. Their delta 44/40Ca values, ranging from 0.67%0 to 0.81%0, are higher than those of the dacites and lower than that of the bulk silicate earth (0.94%0 +/- 0.10%0). Geochemical modeling suggests that variable degrees of partial melting of an asthenospheric mantle involving recycled oceanic crust may have resulted in low delta 44/40Ca values of the Kula basalts. Combined with reported mantle tomographic images, we interpret that sub -slab asthenospheric mantle up welling through a slab tear during subduction roll -back may have played a key role in the Sr-Nd-Ca isotopic variability from Miocene calc-alkaline dacites to Quaternary alkaline basalts in Western Anatolia.