Major-element, trace-element, and Sr-Nd isotopic geochemistry and genesis of Varto (Mus) volcanic rocks, eastern Turkey

Major-element, trace-element and Sr-Nd isotopic data are presented for the Upper Miocene-Quaternary Varto volcanic rocks (Eastern Turkey) and their relationship to the regional tectonic regime were investigated in this study. The Varto volcanic rocks are located in the eastern part of the North Anatolian Fault Zone (NAFZ), in the East Anatolian Convergence Region (EACR). Volcanic rocks are subdivided into two groups: Lower Volcanic Units (LVU) and Upper Volcanic Units (UVU). These volcanic rocks are composed of volcaniclastics and lavas of both alkaline and calc-alkaline characters. LVU lavas are dacite, andesite, trachyte, basaltic andesite, benmoreite, and mugearite, whereas UVU I;lavas are basalt, basaltic andesite, mugearite, hawaite, benmoreite, and basanite in composition. Variation diagrams of SiO2, with major- and trace-elements are consistent with fractional crystallization process involving olivine, plagioclase, pyroxenes, and Fe-Ti oxides. Both units appear to be on the same fractionation trend, implying similar origins, but different degrees of evolution. Correlation between Sr-Nd isotopes with SiO2, MgO, Rb/Sr, 100/Sr, and Nd/Sm emphasize the effect of crustal contamination on these volcanic rocks. Sr-Nd isotopic data plot along the mantle array. However, rocks of Lower and Upper Volcanic Units are isotopically distinct. UVU have relatively low Sr-87/Sr-86 (0.70334-0.70466) and high Nd-143/Nd-144 ratios (0.51279-0.51288), which plot in the field of Oceanic Island basalts, Ararat lavas and the Eastern Anatolian alkaline province; in contrast, LVU exhibit higher Sr-87/Sr-86 (0.70438-0.70507) and lower Nd-143/Nd-144 (0.51265-0.51276) ratios. All the Varto volcanic rocks are products of mantle-derived magma and have experienced assimilation and fractional crystallization (AFC) during uprise through the continental crust, and formed in a within-plate environment along the Eastern part of the NAFZ that is closely related to deep lithospheric fractures. (C) 1998 Elsevier Science B.V. All rights reserved.