Interaction of Asthenospheric and Lithospheric Mantle: The Genesis of Calc-alkaline Volcanism at Erciyes Volcano, Central Anatolia, Turkey

摘要

The quaternary Erciyes stratovolcano and its associated monogenetic cones exhibit tholeiitic, alkaline, and calc-alkaline suites, with the calc-alkaline series dominating. Calc-alkaline products are andesitic, dacitic, and rhyodacitic, showing moderate to low MgO and high K_2O, plus LIL and HFS element contents. The andesites display trace-element patterns similar to those of the Central Mexican Volcanic belt, the Colville igneous complex (northeastern Washington, USA) and the Taos Plateau volcanic field (in north-central New Mexico, associated with northern Rio Grande Rift). Dacites from Erciyes also resemble these igneous complexes and give patterns similar to those of the upper crust. Furthermore, the high Ba/Nb ratio (>28) is typical of arc volcanism (Fitton et al., 1988) ranging from 27 to 47 for andesites, and 27 to 50 for dacites at Erciyes. Calc-alkaline suites also show high Rb/Nb ratios (4-7.5). Moreover, ~(87)Sr/~(86)Sr and ~(143)Nd/~(144)Nd ratios range from 0.703434 to 0.705143, and 0.51294 to 0.5126 for andesites and dacites, respectively. These incompatible element and isotopic ratios suggest two possible mechanisms for the genesis of calc-alkaline suites: subduction enrichment or crustal involvement. Isotopic similarities between tholeiitic and andesitic rocks, LIL and HFS element compositions, and spider plots all indicate that Various source components have been involved in the genesis of these products. The calc-alkaline products of Erciyes volcano were generated by the mixing of partial melts derived from depleted (tholeiitic) and enriched asthenospheric sources. In addition, the calc-alkaline suite was derived from an initially tholeiitic basalt end-member via a 25-30 percent assimilation of crustal material. Thus, post-collisional extension led to development of the Erciyes volcanism in central Anatolia. Calc-alkaline products were widely generated by the combined effects of mixing of depleted and enriched asthenospheric sources, together with a progressive AFC process that started from an initially tholeiitic component, without direct melting of the pre-subducting slab.