Magmatic Response to Slab Tearing: Constraints from the Afyon Alkaline Volcanic Complex, Western Turkey

• Published in: Journal of petrology 2015-03, Vol.56 (3), p.527-562
• Main Authors: Prelevic, Dejan, Akal, Cueneyt, Romer, Rolf L, Mertz-Kraus, Regina, Helvaci, Cahit
• Format: Article
• Language: English
• Subjects: Geochemistry; Lava; Mantle; Melting; Melts; Signatures; Slabs; Volcanic activity
• ISSN: 0022-3530; 1460-2415
• DOI: 10.1093/petrology/egv008

The Middle Miocene Afyon alkaline volcanic complex (western Anatolia) erupted lavas of highly variable geochemistry, ranging from silica-undersaturated to silica-oversaturated and from ultrapotassic to Na-alkaline compositions. There are two major volcanic groups showing substantial differences in K-enrichment and different Sr, Nd and Pb isotopic compositions: plagioclase-amphibole-bearing lavas and sanidine- and/or leucite-bearing lavas. The most remarkable feature of Afyon volcanism is the close relationship in time and space of these two lava types. There is clear stratigraphic evidence for a switch from early Si-oversaturated sanidine- and/or leucite-bearing lavas, towards Si-undersaturated sanidine- and/or leucite-bearing lavas, which eventually change to slightly Si-undersaturated to -saturated plagioclase-amphibole-bearing lavas that make up the youngest formations. This change in composition is coupled with a decrease in super(87)Sr/ super(86)Sr (whole-rock and in situ apatite, perovskite, melilite and clinopyroxene), super(207)Pb/ super(204)Pb, Zr/Nb and Th/Nb, and an increase in super(143)Nd/ super(144)Nd, super(206)Pb/ super(204)Pb, super(208)Pb/ super(204)Pb and Ce/Pb, thus delineating a systematic change from orogenic (crust-like) to anorogenic (within-plate) signatures. Magma genesis in the Afyon volcanic complex has been controlled by roll-back of a subducted lithospheric slab since the Early Tertiary and post-collisional extensional events in Miocene times. It is associated with the upwelling of asthenospheric mantle through a gap in the subducted slab under western Anatolia. Magmatism is concurrent with the collapse of the orogenic belt and the development of extension-related horst and graben structures. We interpret the geochemical transition from orogenic to anorogenic affinity as being due to the increasing role of lithosphere-asthenosphere interaction that is most strongly reflected in the geochemistry of the Afyon lavas. Melting of peridotite in the convecting mantle (asthenosphere) may be a viable model for the origin of the plagioclase-amphibole-bearing lavas. Their ubiquitous high K sub(2)O contents, orogenic trace element signatures and isotopic compositions imply that the asthenosphere-derived primary melts were contaminated by melts derived from lithospheric mantle containing an orogenic chemical signature. Conversely, the ultrapotassic sanidine- and/or leucite-bearing lavas are derived from at least two types of metasomatized