Some remarks on the nature of mantle metasomatism beneath western Anatolian–Aegean region: Contrasting isotopic signatures recorded in the Miocene lavas from the Söke Basin

Abstract

The presence of a subduction signature with variable isotopic patterns in the melt products of mantle origin within the Aegean–Anatolian domain reflects the differential effects of slab‐derived material on the chemical composition of the supraslab mantle region. Here, we use Sr–Nd–Pb isotopic compositions and major and trace element abundances to evaluate the characteristics of the source components and possible roles of fluid/melt metasomatism in the genesis of the Middle to Late Miocene mafic–intermediate lavas from the Söke Basin in western Anatolia. This volcanic suite comprises a number of individual lava flows in which the dominant rock types are potassium‐rich basaltic andesites and andesites with trace element relative abundances indicative of melt derivation from subduction‐modified sources. These samples have significantly heterogeneous Sr and Nd isotopic ratios, with 87Sr/86Sr and 143Nd/144Nd values ranging from 0.70475 to 0.70927 and from 0.51218 to 0.51270, respectively; they also yield a range of Pb isotope ratios, with 206Pb/204Pb = 18.963–19.344, 207Pb/204Pb = 15.672–15.733, and 208Pb/204Pb = 39.096–39.264. Remarkably large variations in isotopic ratios with relatively limited changes in major elements suggest the involvement of heterogeneously enriched sources with diverse isotopic signatures. The modelling of trace element and isotopic data indicates that parts of the lithospheric mantle in this region are isotopically enriched, with geochemical signatures that reflect the interaction of variably depleted mantle peridotites with melts from subducted sediments of continental crustal origin, while other parts host phlogopite‐rich metasomatic components with signatures of devolatilization reactions from ancient subduction events that led to the preferential loss of Pb and Rb to create metasomatic domains with high U/Pb and low Rb/Sr values that ultimately generated highly radiogenic 206Pb/204Pb and relatively unradiogenic 87Sr/86Sr values. The observed isotopic diversity in relatively short‐time‐ and length‐scales may be interpreted to have resulted largely from the variable extent of melt contributions from a number of discrete lithological domains with contrasting long‐term isotopic evolution and is consistent with the view that the continental lithosphere in this region comprises a number of disparate lithospheric fragments with different petrogenetic histories.