Adakitic magmatism, a window to evolution on tectonic and mineralization in eastern Iran

In this research, geochemical data from 314 samples of volcanic and intrusive rocks with adakitic or adakite-like affinity reported from eastern Iran have been studied, these rocks are often known with Sr>400 g/ton and Y<18 g/ton. By comparing and analyzing the characteristics of these adakites, it is concluded that: (1) The adakite rocks of eastern Iran are mainly high silica adakites; (2) High-silica adakitic rocks in northeastern Iran have lower MgO, Th, and Th/Ce and relatively higher Cr, Ni, and SiO2 contents than other adakites in eastern Iran, indicating their connection with the subduction zone and melting of the oceanic crust, while the adakitic rocks of southeastern Iran with lower SiO2 content and higher MgO, Sr, and Th/Ce are categorized as post-collision adakites mainly formed from melting of the thickened lower crust; (3) Most of the adakites in the central parts (eastern Iran) were formed from the melting of the thickened lower crust after the collision, from an amphibolite garnet source, and the ratios of Sr/Nb, Ba/Nb and La/Nb of adakites decreases towards the northeast in this section; 4) The analyzed data and the results presented in this research show that the adakites of eastern Iran have characteristics associated with mineralization and often have the necessary potential to play a role in the formation of valuable reserves.

The term adakite refers to volcanic and intrusive rocks that have more than 56% SiO2, more than 15% Al2O3, and usually less than 3% MgO by weight, high Na2O content (3.5-7.5 %), and low ratio of Drummond et al., 1996K2O/Na2O (<0.5) (Defant and Drummond, 1990;; Martin, 1999; Martin et al., 2005; Condie, 2005; Castillo, 2012). Adakites are divided into (1) a high- SiO2 adakite (HSA) and (2) a low- SiO2 adakite (LSA). The HAS have >60 wt.% SiO2, low MgO (0.5–4 wt.%), CaO + Na2O contents <11 wt.% and Sr abundances <1100 ppm. In contrast, the LSA have <60 wt.% SiO2, higher MgO (4–9 wt.%), CaO + Na2O contents >10 wt.% and Sr contents 1000–3000 ppm (Martin et al., 2005).
The studied area in the eastern part of Iran (Figure 1-A) includes a large part of the structural zones in the south and east, including the Lut block, Makran arc, the Sistan suture and Binaloud. This region is a part of the extensive magmatism that has spread from Turkey to Pakistan and had numerous magmatic activities over time, especially from the Cretaceous to the Quaternary.
Adakitic series have received special attention in recent years in Iran and some articles have been published by various researchers. The main goal of this research is to review the published articles and documents related to the geochemical and isotopic characteristics of adakites in eastern Iran, in order to open a window for a better understanding of the relationship between adakite magmatism and magmatic-tectonic evolution and porphyry copper ± gold mineralization in the east of Iran.

Geochemical data of 314 samples of volcanic and intrusive rocks with adakitic nature were collected from eastern Iran. The location of the studied areas and a summary of data and references is presented in Figure 1 and Table 1. In this study, acidic and intermediate rocks (volcanic and intrusive) with adakitic characteristics were studied and mafic rocks such as basalt and samples with high LOI (above 3) were not considered in the database (Figure 2-A, B).

Volcanic and sub-volcanic adakitic rocks have been reported from Sabzevar, Neishabur and Quchan regions (Table 1, Figure 1-B). These rocks are mainly dacite to trachyandesite and andesite with calc-alkaline to high K- calc-alkaline affinity. Rocks with adakitic nature in central part of eastern Iran are reported from the areas of Garjagan, Khosuf, Shurab, Fadeshk, Pironj, Gurung, Shah Suleiman Ali, Sang-Rahuzag, Shadan, and Tighnab (Figure 1-C and Table 1). According to the chemical classification diagram (Middlemost, 1994), the composition of volcanic rocks is mainly dacite, rhyodacite, andesite and trachyandesite and intrusive rocks are mainly diorite, granodiorite and granite (Figure 2-A, B). They are mostly calc-alkaline to high-K calc-alkaline, sometimes shoshonite (Figure 5-A) and meta-aluminous affinity.
Southern part include adakitic rocks from Lar, Malek Siah Kuh, Lakhshak, Chah Serbi, Shaheswaran, Taftan and Karvander areas (Table 1 and Figure 1-D). The volcanic rocks of the southern part are mainly dacite to andesite (Figure 2-A) and the intrusive rocks are mainly diorite and gabbrodiorite (Figure 2-B). These rocks have the characteristics of calc-alkaline with high-K to and meta-aluminous affinity.

Based on the data presented in this study, it is clear that the adakite rocks of eastern Iran are mainly high silica adakites. These rocks in northeastern Iran have lower MgO, Th, Th/Ce and relatively higher Cr, Ni, and SiO2 contents than other adakites in eastern Iran, which indicate their connection with the subduction zone and melting of the oceanic crust.
The adakitic rocks of southeastern Iran with lower SiO2 content and more MgO, Sr, and Th/Ce are in the range of post-collision adakites, which are mainly formed from melting of the thickened lower crust.
Most of the adakites in the central parts (eastern Iran) were formed from the melting of the thickened lower crust after the collision, from an amphibolite garnet source.
Temporal-spatial relationship between adakites and porphyry copper deposits and/or epithermal gold-copper deposits is studied in many researches (e.g., Thiéblemont et al., 1997; Sajona and Maury, 1998; Li et al., 2011; Richards et al., 2012; Zhang et al., 2021). Porphyry mineralization in Iran mainly took place during the evolution of the branches of the Neo-Tethys Ocean and its final closure. The results presented in this research illustrate the adakites of eastern Iran have characteristics associated with mineralization and often have the necessary potential to play a role in the formation of valuable reserves. Changes in La/Sm and Dy/Yb ratios in adakites are considered as a geochemical sign for mineralization potential. The ratios of (LaN/SmN) and (DyN/YbN) help to determine the fertile magmatism (with the participation of amphibole) from the barren (without amphibole) (Richards et al., 2012). Amphibole-dominated adakites are clearly associated with economic porphyry copper mineralization (e.g., Kheirkhah et al., 2020). In the studied adakites, the changes of LaN/SmN and DyN/YbN ratios are 1.7 to 10.7 (average 4.5) and 0.7 to 2.5 (average 1.2), respectively. Based on these ratios, most of the studied adakites, except for some adakites that show changes in LaN/SmN ratios of less than 4 and DyN/YbN less than 1.1, have mineralization potential.
The analyzed data and the results presented in this research yields that the adakites of eastern Iran have characteristics associated with mineralization and often have the necessary potential to play a role in the formation of valuable deposits. The distribution of copper-gold indices, and the outcrops of adakites along with magnetic anomalies (Figure 10-A), and crust thickness variations in eastern Iran (Figure 10-B), emphasize the importance of focusing on future prospecting, drilling and isotopic studies in this area.