U-Pb zircon SHRIMP age, geochemical and petrographical characteristics of tuffs within calc-alkaline Eocene volcanics around Gumushane (NE Turkey), Eastern Pontides

摘要

Eocene units are widespread in the Eastern Pontides, unconformably overlying Upper Cretaceous volcanoclastic and sedimentary rocks. The Eocene units consist mainly of lava flows and lesser tuffs and sediments. In the study area, the Giimushane region, tuffs are interbedded with lava flows, over an area of -15 km~2. Petrographically, the tuffs can be classified into vitric tuff, crystal tuff, lapilli tuff, and locally silicified tuff. The tuffs, which are cut by dolerite dykes, contain plagioclase (An_(28-50)), amphibole, glass shards, and minor pyroxene, K-feldspar, opaque oxides, and zircon, as well as secondary clay, sericite, calcite, chlorite, and devitrified glass. The U-Pb zircon SHRIMP age from the tuff is 45.8 ±1.2 Ma, which is regarded as the crystallization age. U and Th concentrations within zircon grains vary from 343 to 744 ppm and from 203 to 605 ppm, respectively, and Th/U ratios range from 0.59 to 1.27, indicating precipitation from a melt. Geochemically, the tuffs have calc-alkaline volcanic-arc characteristics. The trace element compositions are indicative of subduction-zone volcanism. The tuffs have high large ion lithophile elements (LILEs) contents and low high field strength elements (HFSEs) contents compared to N-type Mid-Ocean Ridge Basalt (MORB). Chondrite-normalized rare earth element (REE) patterns are concave upwards indicating significant fractional crystallization during evolution of the melt, with (La/Lu)_(CN) = 1.28-13.35. Moderately negative (Eu/Eu*)_(CN) ratios are in the range 0.45-1.07, reflecting plagioclase fractionation. From the trace element signature it is concluded that the parental melt was derived from an enriched upper mantle and formed in an intra-arc basin.