Generation of the Early Cenozoic adakitic volcanism by partial melting of mafic lower crust,Eastern Turkey: Implications for crustal thickening to delamination |
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| Authors: | Orhan Karsli Abdurrahman Dokuz İbrahim Uysal Faruk Aydin Raif Kandemir Jan Wijbrans |
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| Affiliation: | 1. Department of Geological Engineering, Gümü?hane University, TR-29000, Gümü?hane, Turkey;2. Department of Geological Engineering, Karadeniz Technical University, TR-61080, Trabzon, Turkey;1. CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China;2. Guangzhou Institute of Geochemistry, Guangzhou 510640, China;1. Department of Geological Engineering, Karadeniz Technical University, 61080 Trabzon, Turkey;2. Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA;3. Department of Earth Sciences, University of Adelaide SA 5005, Australia;4. School of Earth Sciences and Resources, China University of Geosciences, 29 Xueyuan Road, Beijing 100083, China;5. CAS Key Laboratory of Crust–Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China;6. Korea Basic Science Institute, YangCheong 804-1, Ochang, Cheongwon, Chungbuk 363-883, South Korea;7. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;1. Department of Geological Engineering, Karadeniz Technical University, TR-61080 Trabzon, Turkey;2. Department of Geological Engineering, Dokuz Eylül University, TR-35160 Izmir, Turkey;3. Department of Geology & Environmental Earth Science, Miami University, Oxford, OH, 45056, USA;4. Universidad Nacional de Salta, Buenos Aires 117, 4400 Salta, Argentina;5. General Directorate of MTA, Geological Research Department, 06520 Ankara, Turkey;6. Department of Geology and Mineralogy, Graduate School of Science, Kyoto University, Kitashirakawa Oiwakecho, Sakyo-ku, Kyoto 606-8502, Japan |
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| Abstract: | Early Cenozoic (48–50 Ma) adakitic volcanic rocks from the Eastern Pontides, NE Turkey, consist of calc-alkaline and high-K calc-alkaline andesite and dacite, with SiO2 contents ranging from 56.01 to 65.44 wt.%. This is the first time that Early Eocene volcanism and adakites have been reported from the region. The rocks are composed of plagioclase, amphibole, quartz, and Mg-rich biotite. They have high and low-Mg# values ranging from 55 to 62 and 13 to 42, respectively. High-Mg# rocks have higher Ni and Co contents than low-Mg# samples. The rocks exhibit enrichments in large ion lithophile elements including the light rare earth elements, depletions in Nb, Ta and Ti and have high La/Yb and Sr/Y ratios. Their relative high ISr (0.70474–0.70640) and low εNd (50 Ma) values (? 2.3 to 0.8) are inconsistent with an origin as partial melts of a subducted oceanic slab. Combined major- and trace element and Sr–Nd isotope data suggest that the adakitic magmas are related to the unique tectonic setting of this region, where a transition from a collision to an extension stage has created thickening and delamination of the Pontide mafic lower crust at 50 Ma. The high-Mg adakitic magmas resulted from partial melting of the delaminated eclogitic mafic lower crust that sank into the relatively hot subcrustal mantle, and its subsequent interaction with the mantle peridotite during upward transport, leaving garnet as the residual phase, elevates the MgO content and Mg# of the magmas, whereas low-Mg# magmas formed by the melting of newly exposed lower crustal rocks caused by asthenospheric upwelling, which supplies heat flux to the lower crust. The data also suggest that the mafic lower continental crust beneath the region was thickened between the Late Cretaceous and the Late Paleocene and delaminated during Late Paleocene to Early Eocene time, which coincides with the initial stage of crustal thinning caused by crustal extensional events in the Eastern Pontides and rules out the possibility of an extensional regime before ~ 50 Ma in the region during the Late Mesozoic to Early Cenozoic. |
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