Chemo-probe into the mantle origin of the NW Anatolia Eocene to Miocene volcanic rocks: implications for the role of, crustal accretion, subduction, slab roll-back and slab break-off processes in genesis of post-collisional magmatism

Abstract

Post-collisional Cenozoic magmatic activity in NW Anatolia produced widespread volcanism across the region. In the Biga Peninsula, in the west, medium-K calc-alkaline to ultra-K rocks with orogenic geochemical signature were emplaced at ~ 43–15 Ma (Biga orogenic volcanic rocks; BOVR). Volcanic activity in the Central Sakarya region, to the east, is mainly restricted to ~ 53–38 Ma, but also continued during the Early Miocene with small basaltic extrusives (Sakarya orogenic volcanic rocks; SOVR). This study presents a new set of geochemical data (whole rock major and trace elements and Sr–Nd–Pb isotopic compositions), obtained from the Cenozoic calc-alkaline volcanic rocks from these two regions. While there is considerable overlap in the emplacement time of volcanism in the two areas, the post-collisional volcanic rocks of these two regions differ in terms of their geochemical compositions: (1) the BOVR show an age-dependent increase in K and other large-ion lithophile elements (LILE), coupled with an increase in radiogenic Sr and Pb compositions from the Eocene to Miocene; whereas (2) the SOVR are characterized by more sodic compositions with lower K and less radiogenic Sr contents with respect to the BOVR, which were unchanged in Eocene and Miocene. We conclude that these geochemical features were principally related to the distinct modes of subduction-related mantle enrichment processes. We suggest that the Eocene to Miocene progressive enrichment in the BOVR mantle was related to successive subduction of oceanic and crustal materials in the western Aegean, while the SOVR mantle was dominantly enriched during the pre-collisional events. Magma generation in the western region was related to subduction roll-back processes associated with post-collisional extension. In the east, thermal perturbation of the mantle in response to asthenospheric upwelling due to slab break-off process was responsible for the magma generation. The time-dependent increase of K (and other LILE and radiogenic Sr) in the Cenozoic orogenic lavas from the Rhodope to Biga region emphasizes the importance of crustal imbrication and subduction in the genesis of orogenic K-rich lavas of the Alpine–Himalayan orogenic belt.

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