Provenance and tectonic setting of the Middle Eocene lower Akhoreh Formation, Nain area, Central Iran, assessed using petrography and geochemistry

Abstract

The Middle Eocene Akhoreh Formation is superbly exposed in the western corner of the Central-East Iranian Microcontinent (CEIM). This formation covered the northeastern flank of the Cretaceous Nain Ophiolite Mélange (NOM) and is adjacent to the Paleogene Urmieh–Dokhtar Magmatic Arc (UDMA) formed in the southwest of the CEIM. This terrigenous succession is composed of a thin basal conglomerate followed by mostly pink to purple sandstones alternating with shales. The clast composition and clast imbrication of the conglomerates show local source areas towards the north-north-east. Modal components of lower Akhoreh Formation sandstones reveals immature lithic arkose (Q₈F₄₈L₄₄) and feldspathic litharenite (Q₈F₄₄L₄₈) sandstones that are rich in mafic and ultramafic igneous and volcanic rock fragments. Mafic to ultramafic source rocks are also indicated by geochemical data (enrichment of Mg, Cr and Ni and Cr/V) in the sandstone and shale samples analyzed. However, geochemical data suggests an intermediate igneous rock origin for the shale samples studied, most likely from the nearby continental arc. Based on petrographic data, these sandstones have characteristics of a transitional to undissected arc tectonic setting. Geochemical discrimination diagrams using major and trace elements indicate an oceanic island arc tectonic setting for the lower Akhoreh Formation sandstones and shales, probably due to a predominance of ophiolitic source rocks. Furthermore, the chemical index of alteration and modal analysis indicate a weak to moderate degree of chemical weathering with arid climatic conditions in the source area. The exhumed NOM, together with the UDMA in the southwest, were dominant sources of sediment to the lower Akhoreh Formation, that lay to the northeast in a local retroarc basin of the Central Iranian Microplate, during the Middle Eocene.