科研之友
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摘要
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 Sr-87/Sr-86 (whole-rock and in situ apatite, perovskite, melilite and clinopyroxene), Pb-207/Pb-204, Zr/Nb and Th/Nb, and an increase in Nd-143/Nd-144, Pb-206/Pb-204, Pb-208/Pb-204 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 K2O 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 lithospheric mantle. The dominant source is a phlogopite-pyroxene-rich metasome, which was generated by recycling of continental sediments during previous subduction episodes. This is responsible for the orogenic geochemical signature dominantly seen in lamproites and shoshonites. On the other hand, melting of recently generated phlogopite-wehrlite metasomes resulted in the parental melts of melilite-leucitites, which should be of proto-kamafugitic composition. The wehrlitic metasomes were generated when convecting mantle-derived precursor melts reacted with lithospheric mantle peridotite along the solidus ledge in the system lherzolite + CO2 (<22 kbar).
- 出版日期2015-3
