Ar-Ar Geochronology and Sr-Nd-Pb-O Isotopic Systematics of the Post-collisional Volcanic Rocks from the Karapınar-Karacadağ Area (Central Anatolia, Turkey): An Alternative Model for Orogenic Geochemical Signature in Sodic Alkali Basalts

[摘要] The Plio–Quaternary post-collisional volcanism in the Karapınar area is represented by two occurrences: (1) KaracadağVolcanic Complex (KCVC) and (2) Karapınar Volcanic Field (KPVF). The investigated volcanic units are the southwestern part of the Neogene to Quaternary Cappadocia Volcanic Province (CVP) in Central Anatolia. The CVP generallydisplays calc–alkaline affinity in the Late Miocene to Pliocene rocks, but both calc-alkaline and sodic alkaline affinity inthe Plio–Quaternary rocks, all of which have an orogenic geochemical signature. Such a volcanic activity contradicts theWestern and Eastern Anatolian volcanism characterized by anorogenic OIB-like sodic alkaline volcanic rocks postdatingearly orogenic calc–alkaline ones. We hypothesize that such temporal and geochemical variations in the investigated rocksresult from crustal contamination and present major and trace element chemistry and Sr–Nd–Pb–O isotope geochemistry,coupled with 40Ar/39Ar geochronology data to restrict the genesis and evolution of the rocks. The Neogene Karacadağvolcanic rocks are represented by lava flows, domes and their pyroclastic equivalents constituting a stratovolcano, anddated by new 40Ar/39Ar ages of 5.65 to 5.43 Ma. They are mainly composed of andesitic, rarely basaltic, dacitic andtrachytic rocks and have a calc–alkaline character. Constituting a monogenetic volcanic field, the Quaternary Karapınarvolcanic rocks are typically formed by cinder cones, maars and associated lavas, including xenoliths and xenocrystsplucked from the Karacadağ rocks. They comprise basaltic to andesitic rocks with a transitional affinity, from sodicalkaline to calc–alkaline. Both the Karacadağ and Karapınar volcanic rocks display incompatible trace element patternsrather characteristic for orogenic volcanic rocks. The Sr, Nd and Pb isotopic systematics of both units show a relativelynarrow range, but their δ18O values are markedly different. The Karacadag volcanic rocks have δ18O values rangingfrom 7.5 to 8.9 ‰, resembling those of subduction-related basalts, but the Karapınar volcanics have δ18O ratios between5.7 and 6.5 ‰ corresponding to OIB-like rocks. Additionally, δ18O values and 87Sr/86Sr ratios correlate positively withSiO2 in the rocks, indicating that contamination played an important role during differentiation processes. All the dataobtained suggest that the Karacadağ basaltic rocks stemmed from a subduction-modified lithospheric mantle source.On the other hand, the origin of the Karapınar basaltic rocks can be explained in terms of OIB-like melts contaminatedwith the Karacadağ volcanic rocks to gain orogenic geochemical signature, which may be an alternative model for theorigin of the CVP sodic alkali basalts.

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[关键词] contamination;geochronology;isotope geochemistry;post-collisional;OIB