Geochemical Constraints on the Petrogenesis of Cenozoic I-Type Granitoids in Northwest Anatolia, Turkey: Evidence for Magma Generation by Lithospheric Delamination in a Post-Collisional Setting

Abstract

Extensive magmatic activity followed major plate convergence and Eocene continent-arc collision in northwest Anatolia, Turkey. This produced a considerable volume of eruptive products, as well as a large proportion of plutonic bodies along the suture zone. The main plutonic bodies in northwest Anatolia are exposed in the Armutlu, Kapidag, and Lapseki peninsulas where they form an E-W trending magmatic belt that comprises three individual intrusive suites. The plutonic bodies intrude Paleozoic—Mesozoic metamorphic basement rocks overthrust by Upper Cretaceous—Tertiary ophiolite fragments. Petrographic and geochemical characteristics of these three plutons are remarkably similar, indicating that the magmas that formed each of them were generated from the same source and experienced similar petrogenetic processes. The plutons are generally calc-alkaline, metaluminus, and I-type, and range in composition from hornblende-monzogranite and granite to granodiorite. The rocks are characterized by enrichment in LILE and LREE and depletion in HFSE relative to N-MORB values (e.g., negative Nb and Ta anomalies). They follow assimilation and fractional crystallization (AFC) trends indicative of extensive crustal contamination of magma derived from a mantle source that had been modified by earlier metasomatic events. The chemical characteristics of the plutonic rocks are remarkably similar to those of subduction-related or active continental margin granites. Subduction-related dehydration metasomatism may therefore be inferred as the likely process that modified the mantle wedge source region from which the initial melts were generated. Late orogenic delamination of either the subducting slab or the lowermost part of the mantle lithosphere (e.g., the thermal boundary layer) and concomitant rise of hot mantle asthenosphere appears to be a suitable explanation of the heat that triggered partial melting within the metasomatized part of the mantle lithosphere after subduction waned