The evolution of arc magmatism related with Palaeotethys in the west of Salmas, north of Sanandaj-Sirjan zone (SSZ), Iran

Abstract

The Mingol-Mamakan gabbroic-appinitic intrusions are located in northwestern part of Iran and belong to the Sanandaj–Sirjan zone (SSZ). These intrusions have had a significant impact on evolution of the northwestern part of the SSZ during Upper Carboniferous. The rocks typically include layered and massive gabbros-gabbrodiorites. The age of layered gabbros is 303-300 Ma and they mainly consist of leuco-gabbro, mezzo-gabbro, melano-gabbro, anorthosite, and hornblendite (appinite) with gradational or sharp bedding contacts alternatively. Layered gabbros with 314-322 Ma are mostly composed of leuco-gabbro, mezzo-gabbro, melano-gabbro, and hornblendite. Most of these samples are appinite in composition. The intrusions show no obvious deformation. Therefore, mineral composition changes in the rocks have been controlled by crystallization processes, such as fractionation in the magma chamber. Our investigations indicate that different rock types with tholeiitic magma series are probably derived from partial melting of spinel lherzolite upper mantle co-genetic source. Geochemical information and dating from the Mingol-Mamakan intrusive rocks reveal that the intrusions were formed of subduction-related immature or sub-mature island arc tholeiitic basalt which is enriched in Al2O3, FeO, Sr and depleted in K and Nb. Subsequently, primary tholeiitic arc basalt magma underwent fractional crystallization to form intrusive rocks at the lower crust, relatively in high pressures conditions. Geochemical modeling based on the partition coefficient of elements in minerals indicates that trace elements concentrations (large-ion lithophile elements, LILEs, high field strength elements, HFSE, and rare earth elements, REEs) in the Mingol-Mamakan intrusions throughout the crystallization were controlled by variable amounts of common minerals such as amphibole, clinopyroxene (for all trace elements) plagioclase (only for LILE) and probably spinel in the source rock (only for HFSE). Moreover, elements of first transition series of periodic table mainly controlled by orthopyroxene, olivine and possibly by clinopyroxene and amphibole in much smaller amounts.