Crust–mantle interaction in western Turkey: implications from Sr and Nd isotope geochemistry of Tertiary and Quaternary volcanics

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Abstract
The isotopic composition of Sr and Nd together with the abundance data for major and trace elements are reported for Tertiary to Quaternary volcanics from a variety of localities in western Turkey. These data are used to evaluate the role of crust–mantle interaction in the petrogenesis of the western Turkish volcanics. The major and trace element chemistry reveals a general change in the nature of volcanism from dominantly calc-alkaline in Tertiary to alkaline in Quaternary times. The calc-alkaline rocks are quartz-normative and comprise andesitic to rhyolitic compositions of Miocene–Pliocene age; the trace element patterns are typical of continental margin volcanics with high Ba/Nb ratios and negative Ti anomalies. The alkaline rocks are nepheline-normative and dominantly Quaternary in age; they are basic in composition, with a change from potassic nature in Miocene–Pliocene to sodic in Quaternary times. Most of the Tertiary alkaline volcanics display trace element patterns similar to those of the calc-alkaline ones, whereas the Quaternary alkaline volcanics have low Ba/Nb ratios without negative Ti anomalies; they resemble intraplate volcanics.The calc-alkaline rocks have high 87Sr/86Sr (from 0.705011 to 0.709529) and low 143Nd/144Nd ratios (from 0.512294 to 0.512691). With the exception of two Tertiary samples, all the alkaline volcanics plot within the so-called mantle array of the isotope correlation diagram, 87Sr/86Sr ratios ranging from 0.703128 to 0.703628 and 143Nd/144Nd ratios ranging from 0.512749 to 0.512998. The two Tertiary alkaline samples, with trace element patterns similar to those of the calc-alkaline ones, have considerably higher Sr (0.707741–0.707918) and lower Nd (0.512494–0.512514) isotope compositions. The combined isotope and chemical data suggest the derivation of the western Turkish volcanics from variable mixtures of melts generated in two different mantle regions. The calc-alkaline volcanics were essentially derived from the continental lithospheric or shallow asthenospheric mantle which was contaminated with upper crustal material during earlier subduction events. The generation of the alkaline volcanics was controlled by melts derived from relatively deep, isotopically depleted mantle regions. Most of the volcanics were subjected to contamination at crustal levels, through the operation of an assimilation–fractional crystallization (AFC) process. The nature of contaminant changed from upper crustal in the calc-alkaline to lower crustal in the alkaline volcanics, accompanying the overall decrease in the amount of contamination from about 50% down to about 10%, and broadly paralleling the transition from compressional to extensional tectonics in the region.