Petrogenesis and time-progressive evolution of the Cenozoic continental volcanism in the Biga Peninsula, NW Anatolia (Turkey)

The post-collisional Cenozoic magmatic activity in the Biga Peninsula (NW Anatolia) started in the Middle Eocene (45.3 +/- 0.9 Ma) and lasted continuously until the Late Miocene (8.32 +/- 0.19 Ma) with minor punctuations, producing widespread volcanoplutonic complexes. These magmatic suites are characterized by calc-alkaline, high-K calc-alkaline, shoshonitic, mildly alkaline, and alkaline series. The Middle Eocene to Middle Miocene lavas have high LILE/LREE, LILE/HFSE and Sr-87/Sr-86(i) (0.7046-0.7087) and low epsilon(Nd)(i) (+ 1.2 to - 6.4) values. By contrast, the Late Miocene lavas have the lowest Sr-87/Sr-86(i) (0.7030-0.7033) and the highest epsilon(Nd)(i) (+2.6 to +6.7) values. Geochemical features of these rocks indicate two episodes of enrichment processes during the evolution of the Eocene-Early Miocene volcanism in the Biga Peninsula. The first stage, source enrichment metasomatism, occurred within the continental lithospheric mantle as a result of the previous subduction event(s), and produced a heterogeneously enriched source. The second stage, crustal contamination, resulted from interaction between the metasomatized mantle-derived melts and those magmas derived from the Sakarya crustal basement during their ascent to the surface. Slab breakoff-induced asthenospheric upwelling caused the melting of metasomatized mantle lithosphere beneath the Sakarya continent and produced the Middle Eocene calc-alkaline lavas. Geochemical, isotopic and age relationships suggest increasing amounts of crustal contamination and a decreasing subduction signature during the evolution of magmas from the Eocene through the Early Miocene. Crustal input was peaked during the Early Miocene volcanism, coinciding with the exhumation and rapid uplift of the Kazdag metamorphic massif. Asthenospheric upwelling beneath the Kazdag core complex contributed to the generation of hybrid magmas formed from mixing of mantle and crustal melts during this stage. The Middle Miocene (15.2 Ma) mildly alkaline rocks show less pronounced enrichment in LILE and LREE and display intermediate major and trace element compositions between the Early Miocene and Late Miocene lavas. The Cenozoic volcanism in Biga Peninsula waned briefly during 15-11 Ma, and was subsequently rejuvenated (Late Miocene, 11.0 +/- 0.4-8.32 +/- 0.19 Ma) producing mainly OIB-type alkaline lavas. Advanced extensional tectonics throughout NW Anatolia in the Late Miocene led to upwelling of the asthenospheric mantle and its decompressional melting that collectively resulted in the eruption of short-lived episodes of mainly OIB-type alkaline lavas. This tectonomagmatic evolution of the Cenozoic continental volcanism in western Anatolia presents a well-documented case study of an idealized post-collisional magmatism in most orogenic belts around the world. (c) 2007 Elsevier B.V. All rights reserved.