Mantle source characteristics and melting models for the early-middle Miocene mafic volcanism in Western Anatolia: Implications for enrichment processes of mantle lithosphere and origin of K-rich volcanism in post-collisional settings

Abstract

In Western Anatolia, early-middle Miocene post-collisional volcanism is represented by K-rich volcanic rocks; including calc-alkaline, high-K calc-alkaline, shoshonitic and ultrapotassic products. Low-SiO 2 and relatively high-MgO mafic volcanic rocks in the region are represented by high-K volcanic rocks (HKVR) in the west, where strike–slip tectonics related to the İzmir–Balıkesir Transfer Zone dominates, while shoshonitic and ultrapotassic volcanic rocks (SHVR and UKVR) occur in the eastern part of the region, where mid-crustal units were exhumed along detachment faults during orogenic collapse. All three rock groups have nearly identical Sr–Nd isotope ratios, but the SHVR and UKVR are more enriched in incompatible trace elements relative to the HKVR. Comparison of the geochemical characteristics of the most primitive lavas (SiO 2 < 55 wt.%, Mg# > 65 and MgO > 6 wt.%) provides an important tool in discussing the origin and evolution of the K-rich magmatic activity in this post-orogenic setting. Geochemical features of the three rock groups require they were all derived from a primitive mantle-like source that, on the basis of Sr–Nd isotope mixing models, was supplemented by addition of 7–15% sediment components, including both sediment fluid and sediment melt. Low-degree partial melting (~ 5–10%) of this modified mantle source can account for the incompatible trace element budgets of the HKVR, but an additional trace element enrichment process is required to explain the compositions of the SHVR and UKVR. Numerical modeling suggests that this enrichment developed by multi-stage melting and melt percolation processes in a thicker metasomatized mantle lithosphere than that which produced the HKVR. This petrogenetic model accords with tectonic observations of lithospheric thickening from west to east in the area (where strike–slip deformation occurred to the west and regional uplift and exhumation occurred to the east). Hence, the first enrichment stage of the mantle source (HKVR plus SHVR and UKVR) is interpreted to be the result of subduction-related processes during southward retreat of the Aegean subduction system, with the second-stage of enrichment (SHVR and UKVR) developing in the mantle lithosphere during lithospheric extension.