Geology, petrography, geochemistry and petrogenesis of intermediate subvolcanic units in the south of Zahedan, Sistan and Baluchestan province, Iran

Document Type : Research Article

Authors

• Hamid Rahmani ¹
• Mansour Ghorbani ²

¹ Ph.D. Student, Faculty of Earth Sciences, Shahid Beheshti University, Tehran, Iran

² Associate Professor, Faculty of Earth Sciences, Shahid Beheshti University, Tehran, Iran

10.22067/econg.2022.77188.1048

Abstract

The study area is located 12 km south of Zahedan city, Sistan and Baluchistan province. Rock units in this area include a series of slightly altered flysch facies units with Eocene age, subvolcanic rocks and basic to intermediate dikes with probably Oligocene-Miocene age. Subvolcanic rocks in the form of stocks and domes intruded into the flysch unit and were cut by intermediate dykes in the later stages. The subvolcanic units of the region have two main compositions diorite-monzodiorite and quartzdiorite-quartzmonzodiorite with porphyry texture. The subvolcanic units of this area are in the range of metalalumin to slightly peralumin and I-type. This is consistent with the evidence such as the widespread presence of hornblende, apatite and sphene in the studied subvolcanic units, which indicates the oxidic conditions and high oxygen fugacity for these rocks at the time of formation. Spider diagrams drawn for trace and rare earth elements show the single origin and the role of fractional crystallization in the formation of these rocks. Based on the spider diagram of rare earth elements, the studied subvolcanic units of LREE elements show a enrichment toward to HREE elements, which is one of the prominent characteristics of calc-alkaline rocks in the subduction zones of the continental margin. These rocks are placed in the tectonomagmic environment diagrams in the adakitic range and in the category of shoshonite and high potassium calc-alkaline rocks, related to subduction environments.
 
Introduction
The study area is located 12 km south of Zahedan, Sistan and Baluchestan province. This area includes the height south part of Zahedan city. The heights in this area are mainly mountain ranges with northwest-southeast direction (Fig. 1). The Zahedan magmatic belt is one of the most important magmatic belts in eastern Iran, and it includes Eocene-Oligocene intrusive rocks from the southeast of Khash to Chehelkoureh with northwestern trend that have intruded the flysch-Eocene sequences (Rezaei-Kahkhaei et al., 2010). After the Laramide orogenic phase and the closure of the Sistan Ocean, the Alpine orogenic phases are the major geological events in this area. The main effect of Middle Alpine tectonic events is the intrusion of the Zahedan granitoid masses in the Sistan flysch, which has had extensive alteration and mineralization effects. The magmatic units of this area are generally known as Zahedan granitoid batholiths. This batholith includes an intermediate to acidic complex consisting of diorite to granodiorite (Sadeghian et al., 2005). The Zahedan batholite granodiorites are associated with scattered and small masses of diorite, quartz-diorite rocks as well as dikes and aplite, pegmatite and quartz veins. It seems that the subvolcanic units in the study area are also relation to the diorite to quartz-diorite complex of the Zahedan granitoid batholith. Unfortunately, the types of these subvolcanic rocks are not separated on the geological maps, and only the names of granite, granodiorite and diorite rocks are given together in the form of a geological unit.
 
Materials and methods
After collecting information, evaluating and validating previous data and information, field studies and sampling were conducted to prepare a geological map with a scale of 1:5,000 for an area of 5 square kilometers (Fig. 4). During these studies, 81 samples were collected for petrography and mineralogy studies and 32 samples were collected from the subvolcanic units of the region (fresh or with low alteration) for XRF and ICP-MS analysis (Table 1 and Table 2), as well as for checking the alteration in the area. A number of different samples were also collected for XRD studies. Chemical analyzes of all samples have been done in  Iran Mineral Processing Research Center laboratory (IMPRC).
 
Result and Discussion
The rock units exposed in this area include a set of metamorphic flysch facies units (phyllite, shales and meta-sandstone) with Eocene age, subvolcanic rocks and basic to intermediate dikes up to probably mid-Oligocene-Miocene age. Subvolcanic rocks in the form of stocks and domes intruded to the flysch unit and were later cut by intermediate dikes with a northeast-southwest trend. All subvolcanic rocks in the study area have porphyry texture. The dikes in the study area in the petrographic studies contain a set of major minerals plagioclase and hornblende. The presence of phenocrysts of these minerals in the microlithic pulp has caused porphyry texture. Also, the accumulation of plagioclase crystals in some parts has caused glomeroporphyritic texture. The oldest rock units that are in the study area, include sandstone, sandy tuff, shale, siltstone, phyllite and meta-siltstones attributed to the facies of eastern Iran flysch. The subvolcanic units of the area have two main compositions of diorite-monzodiorite and quartzdiorite-quartz monzodiorite. These rocks generally have porphyry and sometimes glomeroporphyritic textures with fine-grained background. The main minerals of this unit are plagioclase, hornblende, with less abundance of biotite, potassium feldspar and quartz.
Based on geochemical diagrams, samples taken from subvolcanic units in the study area are in the range of quartz monzodiorite-diorite to monzonite-diorite, and samples of porphyry andesite dikes are in the range of gabbrodiorite to diorite (Fig. 8).
These results are in good agreement with microscopic studies performed on rock units in the area. Moreover, the rocks of the study area show magmatic fractionation and decrease of ferromagnesian minerals along with increase of felsic minerals during the process of fractional crystallization based on the diagrams of changes of elements in relation to each other. Subvolcanic units in this area range from meta-aluminous to slightly per-aluminous and I-Type. This is consistent with evidence such as the widespread presence of hornblende, apatite and sphene in the studied subvolcanic units, and they indicate the oxidic conditions and high oxygen fugacity for these rocks at the time of formation. The spider diagrams drawn for trace and rare earth elements in the samples of this range have a close scattering pattern, which indicates the single origin and role of fractional crystallization in the formation of these rocks. Based on the spider diagram of rare earth elements, the studied subvolcanic units show an increase in LREE elements compared to HREE elements. This is a prominent feature of calc-alkaline rocks in the subduction zones of the continental margin. The presence of negative anomalies of Nb element as well as deficiency of other HFSE elements, the rod pattern and sharp spider diagrams which are an important feature of subduction-related magmas, can also be detected in geochemical studies of subvolcanic units in the area (Fig. 12). These
rocks are in the adakitic range in the diagrams of tectonomagmatic environments and fall in the category of high potassium shoshonitic and calc-alkaline calcareous rocks, related to subduction environments (Fig. 13 and Fig. 14).
 
Acknowledgements
This research has been carried out with the financial support of Iranian Mines & Mining Industries Development & Renovation Organization (IMIDRO), Exploration Empowerment Project. We hereby thank the managers of this collection for their efforts.

Keywords

• Zahedan
• Subvolcanic
• Adakitic
• Subduction

20.1001.1.20087306.1401.14.3.6.1

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