Eocene calc-alkaline volcanic rocks related to Cenozoic episodic magmatism during Neo-Tethyan subduction, Tarom–Hashtjin Belt, NW Iran

Abstract

Abstract The Eocene Rasht-Abad volcanic rocks are located in the Alborz-Azerbaijan magmatic belt (including the Tarom-Hashtjin province) of NW Iran. These are mainly mafic to intermediate volcanic rocks with calc-alkaline affinities, comprising andesite, andesite-basalt, trachy-andesite, and dacite. Clinopyroxene ranging in composition from diopside to augite is the most significant mafic mineral of the basic rocks. Aluminum partitioning between tetrahedral and octahedral sites shows that the mineral crystalized at low pressure. Ferric iron of clinopyroxene also indicates high oxygen fugacity of the host magma crystallization environment. Geothermobarometry using clinopyroxene constrains the crystallization temperature and pressure of this mineral as 1100–1200°C and 3-7.5 kbar. Ce/Pb values of the mafic lavas are lower than values expected for mantle-derived melts but do not support crustal contamination. Co-existing basalt to trachyandesite lavas display parallel and tight REE patterns that suggest these rocks originated from a common mantle source or parental magma. Tectonomagmatic discrimination diagrams and mineral compositions are consistent with the host volcanic rocks having the characteristics of continental margin arcs. Geochemical data are consistent with the parental magma of mafic-intermediate rocks of Rasht-Abad area being derived from a typical subcontinental lithospheric mantle which was enriched by subducted slab-derived fluids and melts during tectonic events in the active continental margin. The data support a model of Eocene flare-up magmatism associated with rollback of a flattened slab.