Petrogenetic evolution of late Cenozoic, post-collision volcanism in western Anatolia, Turkey

Following an Eocene continent-are collision, the Western Anatolia region experienced a complete cycle of thickening and orogenic collapse. The early stage of collision-related volcanism, which was most evident during the Early Miocene (<21 Ma), produced a considerable volume of lavas and pyroclastic deposits of basaltic andesite to rhyolite composition. The volcanic activity continued into the Middle Miocene with a gradual change in eruptive style and magma composition. The Middle Miocene activity formed in relation to localised extensional basins and was dominated by lava flows and dykes of basalt to andesite composition. Both the Early and Middle Miocene rocks exhibit calc-alkaline and shoshonitic character. The Late Miocene volcanism (<11 Ma) was marked by alkali basalts and basanites erupted along the zones of localised extension. The Early-Middle Miocene volcanic rocks exhibit enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to the high field strength elements (HFSE) and have high Sr-87/Sr-86 (0.70757-0.70868) and low Nd-143/Nd-144 (0.51232-0.51246) ratios. Modelling of these characteristics indicates a mantle lithospheric source region carrying a subduction component inherited from a pre-collision subduction event, perturbation of this subduction-metasomatised lithosphere by either delamination of the thermal boundary layer or slab detachment is the likely mechanism for the initiation of the post-collision magmatism. Petrographic characteristics and trace element systematics (e.g. phenocryst assemblages and relative depletion in MREE and heavy rare earth elements (HREE)) suggest that the Early-Middle Miocene magmas underwent hydrous crystallisation (dominated by plagioclase + pyroxene + pargasitic amphibole) in deep crustal magma chambers. Subsequent crystallisation in shallower magma chambers follows two different trends: (1) anhydrous (pyroxene + plagioclase - dominated); and (2) hydrous (edenitic amphibole + plagioclase + pyroxene dominated). AFC modelling shows that the Early-Middle Miocene magmas evolved through assimilation combined with fractional crystallisation, and that the effects of assimilation decreased gradually from the Early Miocene into the Middle Miocene. This may indicate a progressive crustal thinning related to the extensional tectonics that prevailed from the latest Early Miocene onwards. In contrast, the Late Miocene alkaline rocks are characterised by low 87Sr/86Sr (0.70311-0.70325) and high Nd-143/Nd-144 (0.51293-0.51298) ratios and have GIB-type like trace element patterns characterised by enrichment in LILE. HFSE, LREE and MREE, and a slight depletion in HREE, relative to average N-MORB. REE modelling indicates that these rocks formed by partial melting of a garnet-bearing Iherzolite source. Trace element and isotope systematics are consistent with an origin by decompression melting of an enriched asthenospheric mantle source. (C) 2000 Elsevier Science B.V. All rights reserved.