Geochemistry of volcanic rocks from the south of Gazik (east of Birjand): implications for the evolution of Sistan Ocean (eastern Iran)

Abstract

The study area is located the south of Gazik (east of Birjand) and along the eastern margin of the Sistan suture zone. The south of Gazic volcanics crop out as a north- south trending unit. They are predominantly intermediate extrusives including andesite and basaltic- andesite. Based on field relationships and the age of adjacent sedimentary rocks, the volcanics are late- Cretaceous (Maastrichtian) in age. Petrographically, plagioclase is the predominant phase and clinopyroxene appears as the main ferromagnesian mineral. The chondrite- normalized rare earth element (REE) patterns are characterized by moderate light REE (LREE) enrichment relative to heavy REE (HREE) and the ratio of (La/Yb)N in the ranges of 2.71 to 8.68. Furthermore, the chemical characteristics of the volcanics including enrichment of large ion lithophile elements (LILE) like K, Rb and Ba and depletion of high field strength elements (HFSE) like Nb, Ta and Ti are similar to subduction zone magmatism. Based on trace element concentrations, it seems that the melts were derived from a mantle wedge intensely modified by subduction zone fluids. Considering minor and trace element contents, the studied rocks are more similar to island arc volcanics. Therefore, the Sistan Ocean has probably recorded an active subduction zone in the late Cretaceous, which is in conflict with the pre- late Cretaceous Lut and Afghan continental collision.

Keywords

• volcanic rocks
• late- Cretaceous
• Sistan Ocean
• eastern Iran