Geochemical characterization of mid-distal Nisyros tephra on Datca peninsula (southwestern Anatolia)
Özet
We present new distal records of tephra deposits that overly the Kos ignimbrite in seven locations of Datca peninsula. Tephra in one of these locations were previously associated with Nisyros Kyra sub-unit based only on the field characteristics. We use different proxies such as field observations, petrography, mineral, glass, and whole-rock chemistry in order to characterize and correlate the previously and recently identified pumice fall deposits on Datca. The total thickness of the fall deposit reaches to 3.5 m. The size of the pumice clasts is generally within the range of lapilli, and they have vitrophyric texture consisting mainly of plagioclase (andesine to labradorite) with scarce clinopyroxene (diopside to augite), olivine (Fo(48-50)), amphibole (magnesio-hastingsite), and biotite crystals. Amphibole is a ubiquitous phenocryst in all Datca tephra units and used as a criterion for the correlation. Glass major element analyses by EMPA reveal two different groups with andesitic and dacitic compositions. Difference in silica content (up to ca. 4 wt%) detected in the same specimen also designates the heterogeneity in pumice glass. This heterogeneity in glass composition is also supported by the frequent occurrence of banded pumice clasts in Datca tephra. Whole-rock composition of the pumice is mainly andesitic with calc-alkaline affinity. Multi-element patterns on primitive-mantle normalized diagram display typical arc-magmatism signature (i.e. depletion in Nb, Ta, Ti, and P). In order to check and eliminate the potential alternatives, we compared the distal deposits on Datca not only with Kyra, but also with other Nisyros tephra units. Yet, Kyra is the only unit that has comparable depositional characteristics, calcic amphibole crystals, andesitic-dacitic glass and whole-rock chemistry, and distal tephra deposits on neighboring islands (Tilos and Chalki). Therefore, we associate Datca tephra deposits with some proximal Kyra subunits of intermediate composition. However, if further geochemical (especially glass and mineral chemistry) data are provided for the Lakki and Melisserf tephra units, a more thorough tephra correlation will be possible. Finally, our results may have implications on Nisyros tephra dispersal and geochemical characterization. (C) 2017 Elsevier B.V. All rights reserved.
