Petrographical, Geochemical and Petrological Characteristics of the Bayburt Tuffs: Eocene Calk-Alkaline Felsic Volcanism in the Southern Zone of Eastern Pontide

Mehmet ARSLAN, Zafer ASLAN, Abdurrahman DOKUZ

Abstract


In the Bayburt area, the tuffs named as “Bayburt tuffs” outcropping in Eocene basin are
made of two levels as bottom and upper units, interstratified with a claystone-marl interlayer. Lower
layer contains two whereas upper one unit, and every unit show gradating from coarse towards finegrained
tuffs. Tuffs contain dominantly glass shards, pumice and crystal fragments (plagioclase, quartz,
biotite, sanidine). Coarse and fine-grained levels are characterised by their crystal fragment/glass shard
ratio. Based on modal composition, coarse-grained level can be classified as vitric-crystal tuff and finegrained
level as vitric tuff. Geochemically, tuffs are rhyodacite to dacitic in composition and exhibit
medium-K calcalkaline and peraluminous characteristics. They show chemical variation trends,
probably reflecting pre-eruption magmatic processes, especially crystal fractionation by volatile effect.
Rare earth element patterns are spoon-like in shape, with (La/Lu)N=14-20, and pronounced Eu anomalies
indicating plagioclase fractionation. The investigated tuffs differ from the rest of Eocene volcanics in the
southern zone by their field characteristics and acidic composition. In the tuffs, presence of very fresh
and angular glass shards and common crystal fragments, but lack of lithic fragments may indicate
phreatomagmatic acidic volcanism in or very near to Eocene basin in the region. Furthermore,
geochemical data indicate that these rocks derived from an intermediate magma source (andesitic
parent).


Keywords


Tuff; Eocene Volcanics; geochemistry; petrology; Eastern Pontide; Bayburt

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