Petrogenesis and geochemistry of the Eocene volcanic sequence in the northeast of Zanjan: Implications for active continental margin magmatism in the Alborz- Azarbaijan Zone

Abstract

The Eocene volcanic sequence in NE of the Zanjan and in the Alborz-Azarbaijan zone is mostly intermediate in composition and is accompanied with pyroclastic tuff and breccia. Petrographic studies reveal that these volcanic rocks are andesite and trachy-andesite. The common textures are porphyritic and glomeroporphyritic and the phenocrysts are composed of plagioclase, pyroxene and amphibole minerals. Also, calcite, quartz, chlorite and epidote are the secondary phases. According to geochemical classification, Theses rocks are plotted on the fields of andesite and trachy-andesite. Geochemically, the study rocks are enriched in LREEs and LILEs relative to HFSEs. Petrographical observations along with geochemistry of rare earth and trace elements suggest calc-alkaline affinity of the rocks under discussion as well as crustal assimilation, fractional crystallization and derivation from a subducted-related environment. Accordingly, the studied rocks are analogous with tectonic features of active continental margin arc settings.

Keywords

• Eocene volcanic sequence
• calc-alkaline magmatism
• active continental margin
• Zanjan
• Alborz-Azarbaijan zone