Pyroxene chemistry in the mafic rocks of the Mahirud Complex: A confirmation on Cretaceous island-arcs magmatism in Eastern Iran

The volcano-plutonic complexes are important for understanding of tectono-magmatic settings in geological terrains (Wilson, 1989). The chemical composition of pyroxenes depends on the chemical composition and physical conditions of their host magmas, therefore, it can be used in determining of magmatic series nature. The Paleogene Eastern Iranian ranges is the product of the large-scale, Indian-Eurasian collisional event and their subsequent convergent involving amalgamation of the Afghan and Lut micro-continents as well as the Neo-Tethyan accretionary complexes and associated island arcs (Bagheri and Damani Gol, 2020). This paper aims to investigate new geochemical data on the Mahirud volcano-plutonic complex which represents a key area of the Sistan fold-and-thrust belt (SFB).

In order to study of the pyroxene chemistry of the MVPC, 10 samples of the fresh Cretaceous volcanic rocks were selected. Electronic micro-processing analyses (EPMA) were performed by the JEOL.JXA-8600M automatic super-probe with an accelerating voltage of 15 kW and a radiation current of 2 × 10-8 am in the Department of Earth and Environmental Sciences of Yamagata University in Japan.

The MVPC, previously known as Cheshmeh Ostad Group located in northeastern part of the SFB, has been interpreted as a rift-related assemblage which was preserved on the Afghan rifted margin (Tirrul et al, 1983; Angiboust et al, 2013). However, our observation indicates the Lower Cretaceous tonalitic stocks intruded the ophiolitic pillow lavas which interbedded with pelagic sediments and associated Upper Cretaceous neritic limestone (Guillou et al, 1981) in this region (figure 1). The volcanic rocks of this complex are diabase, basaltic andesite, andesite, dacite and tuff that were intruded by the Lower Cretaceous tonalitic stocks (Keshtgar et al, 2016; Keshtgar et al, 2019). The clinopyroxene in the diabasic rocks is mainly augite–diopside, while the clinopyroxene in basaltic andesite has augite composition. The low abundance of the HFSE elements, especially Ti, is a common characteristic of these clinopyroxenes. Moreover, the chemical composition of the studied pyroxenes are silica rich compared to the similar types of pyroxene in alkaline rock. This is in consistence with the geochemistry of the host volcanic rocks which are associated with ​​tholeiitic and calc-alkaline magmatic series (figure 2).

According to the tectonic setting geochemical diagrams (Le Base, 1962), the MVPC has properties similar to the magmas were originated in the supra-subduction zone (SSZ) and Island arcs (IAT) (figure 3). The presence of an island arc complex in the Sistan fold-and-thrust belt, which some of its parts are considered here as the MVPC, is comparable to the Pakistani Chagai-Raskoh and Kuhistan Cretaceous-Eocene island-arcs (Siddiqui, 2004; Siddiqui et al, 2012), in the east (figure 4). The subduction of the Neo-Tethys oceanic lithosphere under Eurasia, including the Lut and Afghan blocks, has been divided into two zones; the southern one which was an intra-oceanic subduction zone resulted in the island arcs development and on the other side, the northern one was a continental margin brought accretionary prisms and magmatic arc along the Cimmerian block.