Authors: AYKUT GÜÇTEKİN, NEZİHİ KÖPRÜBAŞI
Abstract: Hasandağ and Erciyes stratovolcanoes, which are the two important stratovolcanoes in Central Anatolia, erupted volcanic products with both calc-alkaline and alkaline compositions, although the calc-alkaline activity is more widespread. There are three stages of geochemical evolution in the history of the Hasandağ stratovolcanic complex: (1) Keçikalesi tholeiitic volcanism, (2) Hasandağ calc-alkaline volcanism, and (3) Hasandağ alkaline volcanism. The geochemical evolution of Erciyes volcanic complex also exhibits three distinct evolutionary stages: (1) Koçdağ alkaline volcanism, (2) Koçdağ calc-alkaline volcanism and (3) Erciyes calc-alkaline volcanism. The volcanic rocks from both suites show enrichments in LREE relative to HREE. The rocks as a whole show enrichments in large ion lithophile elements (LILE) relative to high field strength elements (HFSE) in N-MORB normalized multi-element diagrams, although the thoeliitic and alkaline rocks have less pronounced effects of HFSE/LILE fractionation comparing to the calc-alkaline rocks. Theoretical fractionation models obtained using the whole-rock trace element data indicate two distinct fractionation trends for the Hasandağ volcanism: amphibole and plagioclase fractionation for the tholeiitic and calc-alkaline rocks and plagioclase, pyroxene and amphibole fractionation for the alkaline rocks. The alkaline and calc-alkaline rocks of Erciyes volcanism, on the other hand, indicate similar fractionation trends that can be explained by plagioclase and amphibole fractionation. AFC modelling indicate that the effect of crustal contamination on parental melt compositions is less pronounced in the Erciyes volcanic rocks compared to the Hasandağ volcanic rocks. Theoretical melting trends obtained using the non-modal batch melting model indicate degree of melting between 8-9% for the Keçikalesi tholeiitic rocks, 4-5% for the Hasandağ alkaline rocks and 3-8% for the Koçdağ alkaline rocks. The modeling also shows that the Central Anatolian volcanic rocks were originated by variable degree melting of a mantle source with garnet+spinel lherzolite composition, although the effect of residual garnet in the source is more pronounced for the Hasandağ alkaline rocks. Geochemical modeling indicates that Central Anatolian volcanic rocks are likely to have originated by partial melting of a metasomatised lithospheric mantle. Delamination of TBL (termal boundary layer) may be considered as a potential mechanism that may cause thermal perturbation melting of continental lithospheric mantle. The distribution of volcanic centres along the region-scale strike-slip fault systems may also indicate additional effects of strike-slip faulting on melt production and eruption.
Keywords: Central Anatolia, collision volcanism, lithospheric delamination, Turkey
Full Text: PDF