Albin volcanism in Marivan region: Indication of extensionin the Sanandaj-Sirjan continental arc

The Marivan region located in the west of Kurdistan province, northwest of Iran and in the structural geological divisions of Iran (Stöcklin and Nabavi, 1973). The Mesozoic volcanic rocks are widespread in the Northern Sanandaj- Sirjan zone in comparison to the central and southern parts of this zone. The basic term of this volcanic belt with a calc-alkaline tendency is dominant where was formed in association with the Sanandaj-Sirjan arc magmatism in the Mesozoic which generated as a result of the subduction of the Neotethys oceanic crust under the active continental margin of Central Iran (Omrani et al., 2008; Azizi and Jahangiri, 2008; Moinevaziri et al., 2015).The studied area located in the Sanandaj Cretaceous volcanic belt (SVB) (Azizi and Moinevaziri, 2009), is characterized by the presence of basalts and andesite-basalts, often intruded shales, sandstones, and Cretaceous limestones. Rahimzadeh et al. (2021) investigated the North of Sanandaj-Sirjan magmatism in two basic and acidic phases and attributed their formation to a continental arc tectonic environment with extension. Also, the general age of the North Sanandaj-Sirjan zone magmatism varies from 110 to 130 million years (Barmian-Aptian). Ali et al. (2016) determined a bimodal model for the volcanic rocks of the Kata-rash region in Iraq Kurdistan (north of the study area), which originated 108 Ma (Albian) in an oceanic arc tectonic environment. The main purpose of the present paper is to study the petrology and geochemistry of the volcanic rocks of the area, in order to determine their tectonic setting.

Regional Geology:

The The studied area is located in the north and east of Marivan city and is a part of the 1:100,000 geological map of Marivan (Sabzehi et al., 2009). Structurally, it is located in the northwestern part of Sanandaj-Sirjan zone. The rock units with Albian age, are a sequence of low metamorphosed volcanic-sedimentary rocks and consist of basaltic lavas, andesite-basalt, rhyodacite, pyroclastic, shale, calcareous shale, metamorphosed limestone, sandstone and minor amounts of conglomerate. A great volume of basic rocks is found in the north of Marivan while the acidic domes are seen in the east of the Marivan. The field relationships of volcano-sedimentary sequence indicates that the volcanic rocks erupted from deep to shallow marine environment (Rahimzadeh et al., 2021).

Eleven samples selected from the Marivan volcanic rocks for chemical analyses. The major elements were measured by ICP-OES method and trace and rare earth elements (REE) were measured by the ICP-MS method in the Canadian MS-Analyses laboratory.

Petrography:

Cretaceous volcanic rocks with bimodal composition are often basic to slightly intermediate and acidic outcrops. The basic phase includes a large amount of basalt and a small amount of basaltic andesite and the acidic phase is composed of rhyolite and rhyodacite. Porphyritic and microlithic-glassy are the common textures in the rocks under study. The presence of zoned-plagioclases, quartz with embayed texture in rhyodacite, regrowth in crystals margin in both basic and acidic phases indicate the chemical imbalance of phenocrysts with melt in the magma forming these rocks.

Geochemistry:

Based on the results of chemical analyses, the Marivan volcanic rocks are classified as the basic and acidic where the bimodality of rocks can be seen. The basic rocks show calk-alkaline with tholeiitic tendency whereas the acidic term display the calc-alkaline nature. The geochemical results in the spider diagrams show that the basic rocks of the region are enriched in LREE and LILE (K, Cs and relatively Ba and Rb), and depleted in HFSE (Nb, Ti, P) except U and Th.The acidic rocks are enriched in LILE (K, Cs, Rb, Th) except Sr and depletion of HFSE (P, Ti, Nb) except U and Zr. Nb and Ti depletion is one of the characteristic features of magmatic arcs. Also, these rocks have an LREE-enriched pattern in both the basicand acidic phases with a high LREE/HREE ratio and a specific negative Eu anomaly are found only in the acidic phase. The acidic rocks were originated from the lherzolitic spinel mantle with a 5 to 8% partial melting degree whereas the basic phase is generated from the lherzolitic spinel-garnet mantle with 10 to 22% partial melting degree.

Tectonic setting:

This volcanic complex is a part of the Sanandaj-Sirjan magmatic arc, which shows both subduction and extensional components. Extension in the continental arc happened relation to roll-back or slab collapse (Wei et al., 2017; Rahimzadeh et al., 2021). Rollback occurs as a result of its pressure, the subducting crust turns back and breaks, and extension occurs for a short period. So, in this geodynamic environment, bimodal volcanism can be happened where basic term show the tholeiitic affinity. It’s happened from continental arc position. In this tectonic setting environments, most of the volcanic rocks are basaltic rocks while in a typical continental arc such as the Andes arc and the Urumieh-Dokhtar abundant andesite rocks are widespread (Gill, 2010).

Marivan volcanic rocks are part of the northern Sanandaj-Sirjan zone continental arc, which is roll-back happened in the Albian and as a result occurred the local extension in the regional compression setting. Its signs include the presence of shales associated with volcanic rocks, the abundance of basalts over andesites, the tendency of calc-alkaline to tholeiitic nature in most of the basic rocks and as the several geodynamic diagrams display. However, the obvious evidence of subduction in the region can be seen as the depletion of Nb and Ti elements and enrichment of LILE and LREE compared to HFS elements.