نشریه پترولوژی
پیاپی 60 (زمستان 1403)

Schist and gneiss metamorphic rocks, especially in orogenic belts, mainly host microcrystalline, amorphous, and flaky graphite deposits in the Earth’s crust. Due to its modern technological use as graphene source and commercial lithium-ion batteries, graphite has a growing economic value and known as critical minerals (International Energy Agency, 2021). Graphite can be formed through maturation and metamorphism of biogenic carbonaceous material; as precipitation from C-O-H fluids; mantle-derived; and through reduction of carbonates (Simandl et al., 2015). Important graphite deposits are found in carbonaceous sedimentary rocks subjected to regional or contact metamorphism and in veins precipitated from fluids.The Band-e-Cherk district located in the Kuh-e-Dom metallogenic area and consist of graphite-bearing schists that superposed contact metamorphism and are associated with Eocene volcanoplutonic suits. However, the microscopic characteristics, geochemistry, ore genesis, carbon source, and other features of graphite-bearing schists in the district have not been thoroughly explored. This study determined the geochemical features of graphite-bearing schists from the Band-e-Cherk district to reconstruct the metamorphic protoliths and Palaeo-sedimentary environment.

The study area lies in the Kuh-e-Dom metallogenic area at the central Iran zone. This region forms an important part of the Anarak metallogenic belt. Magmatic rocks in the Kuh-e-Dom metallogenic area are Eocene volcanic and granites, whose extension is controlled by west–east trending faults. The Kuh-e-Dom metamorphic complex consists of phyllite and various schist units in contact with magmatic rocks and with the same trending. The Kuh-e-Dom metallogenic area is characterized by the Fe–Cu–Au–Mn–Pb–Zn–Au–Ag mineralization.Lithologically, the Band-e-Cherk district are dominated by Permo-Triassic metamorphic units (including muscovite schist, epidote-hornblende-schist, muscovite-chlorite schist, biotite-graphite-schist), and Lower Cretaceous limestones as the oldest unit, as well as Eocene andesite, andesite-basalt volcanic rocks and equivalent tuff. Graphite-bearing schists are mostly in contact with marl, limestone and small amount of diorite intrusion as well.

For this research, sampling was carried out on graphite-bearing schists, and twenty-five thin-polished sections were prepared and studied with a Zeiss Axioplan 2 transmission-reflection optical microscope at the Kharazmi University. Twenty-six samples of graphite-bearing schists were analyzed for major elements by XRF and trace elements by ICP-MS method at Zarazma Mineral Processing Research Center and Iranian Mineral Processing Research Center (IMPRC). The analytical uncertainties were determined based on several internal certified reference samples. For the XRF these are <1% for SiO2 and Al2O3, and <5% for other major and minor elements. The detection limits were 0.1% for SiO2 and 0.01% for other major elements.

The results show that the SiO2 content of the metamorphic rocks is high (52.98% to 80.68%), while Na2O is 0.07% to 5.32%, K2O is 0.33% to 5.61%, K2O > Na2O, and K2O/Na2O + K2O > 0.5. SiO2 is significantly negatively correlated with Al2O3. P2O5 ranges from 0.01% to 4.86% (with average 0.54%, which is generally low, and MnO is between 0.001% and 0.61%, with a small variation range. On Harker diagrams, SiO2 is negatively correlated with Al2O3, CaO, K2O, MnO, Fe2O3, and TiO2, reveal that chemical differentiation of the rocks is constrained by sedimentary differentiation (Cheng et al., 2023). The fractionation degree of light rare earth elements (LREEs) is greater than that of heavy rare earth elements (HREEs), with LREE/HREE ratios of 3.33 to 34.2; LaN/YbN is 3.58 to 24.98, with a mean value of 10.44. The rocks have moderate negative Eu anomalies (δEu = 0.42 to 1.93, mean = 0.91). Ionic lithophile elements (e.g., Rb, and K) are relatively enriched, but Sr is fairly depleted.

As the Zr/TiO2–Ni diagram displays (Winchester et al., 1980; Renmin et al., 1986), graphite-bearing schists samples were projected into the zone of sedimentary rocks (Figure 1A).Figure 1. A) Ni vs. Zr/TiO2 (Winchester et al., 1980; Renmin et al., 1986); B) Composition diagram of sedimentary-metamorphic rocks in different climatic zones of (Renmin et al., 1986) (1: Terrestrial facies clay compositions in humid and hot climatic zones; 2: Marine facies, lacustrine and lagoon facies clay compositions in dry climatic zones; 3: Terrestrial facies clay compositions in cold or moderately cold climatic zones); C) La/Th versus Hf diagram (Floyd and Leveridge, 1987), which shows that the studied samples are in the range of acidic-intermediate states.Based on Simonen’s diagram (Simonen, 1953) (Al+fm−C+alk−Si), all samples fall into the argillaceous sedimentary rock zone, confirming that the protoliths of the metamorphic rocks were sedimentary and that the metamorphic rocks are para-metamorphic. For determining of Palaeo-sedimentary environment, we used a ternary diagram of claystone composition in different climatic zones, indicating a shallow depth terrestrial facies zone of a cold or moderately cold climate for the samples under study (Figure 1B). Based on Ni–TiO2 discriminating diagram (Floyd et al., 1989), the protoliths of metamorphic samples plot in the sandstone zone and felsic rocks of the magmatic zone. All samples, on La/Th–Hf diagram (Figure 1C; Floyd and Leveridge, 1987), fall into within the mixed felsic–intermediate source zone, and on Th–Hf–Co ternary diagram (Taylor and McLennan, 1985), plot within the upper crust district.

The lithogeochemical analysis of graphite-bearing schists shows that the protolith are sedimentary rocks are of feldspathic sandstone and clay-rich materials, which are in a paleo-sedimentary environment of fresh to saline water. It was formed in a continental environment in a cold or relatively cold climate. Sediments originated from the upper crust and the main components of their origin were argillaceous rock and sandstone with an acidic-intermediate mixture composition. The tectonic discrimination diagrams show that the precursors of metamorphic rocks were probably deposited in an organic-rich river-flood facies environment in the continental margin. Feldspathic sandstone (possibly arkose), clay, and organic-rich graywacke were deposited over a long period and metamorphosed during regional metamorphism, and graphite formed by organic carbon metamorphism. However, the degree of metamorphism was not sufficient to obtain flake graphite, and the graphite is the microcrystalline amorphous type, which has undergone a weak degree of graphitization, and the temperatures have not exceeded 450 ºC.

The central Alborz, a tectono-stratigraphic terrane within the Alborz Mountains, underwent significant tectonic evolution linked to the Late Triassic Cimmerian orogeny and the subsequent collision between the Iranian microplate and the Eurasian plate. Previous geological investigations (e.g., Alavi, 1996; Stöcklin and Hassanzadeh, 2001) have delineated distinct tectono-stratigraphic units and proposed polyphase orogenic models for the northern Iranian Alborz. These studies highlight the tectonic stability of the central western Alborz from the Late Eocene to Late Miocene, correlating with Miocene sedimentation patterns in central Iran. Furthermore, studies of major intrusive bodies in the central Alborz (e.g., Ghorbani et al., 2014) have characterized their calc-alkaline to alkaline nature and syn-orogenic emplacement, with examples such as the Parachan intrusion exhibiting intermediate compositions (monzonite-monzodiorite) and shoshonitic to metaluminous geochemical signatures.

Regional Geology:

Given the significance of understanding magmatism and related processes in the context of Neogene tectonomagmatic events in Iran, a detailed petrological and geochemical investigation of the Aroud intrusive body, located north of Alam Kuh and previously unstudied, is warranted. This study utilizes field observations, petrography, and whole-rock geochemistry to constrain the petrogenesis and magmatic processes responsible for its formation. The Aroud pluton, exhibiting significant outcrop (Baharfirozi et al., 2002), is situated within a complex geological setting encompassing units ranging from the Late Precambrian to Recent (Axen et al., 2001; Asiabanha et al., 2012; Alavi, 1996; Ballato et al., 2015; Stöcklin and Hassanzadeh, 2001; Esmaeli et al., 2007; Valizadeh et al., 2008). These include Precambrian sedimentary sequences, Paleozoic conglomerates and shales (equivalent to the Lalun Formation), Paleozoic dolomites and limestones, Ordovician micaceous sandstones, and Carboniferous (Mubarakeh Formation) carbonate units. The Aroud pluton, with a north-south trend, is mapped as Neogene in age (Baharfirozi et al., 2002) and its emplacement is likely linked to the major thrust tectonics of the central Alborz, suggestive of a back-arc setting. Detailed petrographic and geochemical analyses will elucidate the petrogenesis of this pluton and its implications for the regional tectonomagmatic evolution.

Petrography, Minerals:

Petrographic analysis reveals the Aroud pluton comprises granite, quartz monzonite, and syenite. The granite is characterized by a granular to intergranular texture with quartz (∼30 vol%), plagioclase (∼45 vol%), and alkali feldspar (∼25 vol%) as major constituents. Biotite and amphibole are the dominant mafic minerals. Quartz monzonite exhibits major mineral proportions of plagioclase (∼55 vol%), quartz (∼15 vol%), and alkali feldspar (∼30 vol%). Syenite, which is volumetrically minor, is predominantly composed of alkali feldspar (∼88 vol%), primarily orthoclase, with minor plagioclase (∼8 vol%) and quartz (∼4 vol%).

Whole rocks Geochemistry:

Whole-rock geochemical analyses (Table 1) indicate that the Aroud granites exhibit SiO₂ contents ranging from 69 to 72 wt%, and Al₂O₃ from 14 to 15 wt%. In contrast, the quartz monzonites and syenites display lower SiO₂ (quartz monzonite: 64-65 wt%; syenite: 60 wt%) and higher Al₂O₃ (quartz monzonite: 16-17 wt%; syenite: 17 wt%) amounts. Iron, magnesium, titanium, and phosphorus concentrations are also somewhat elevated in the quartz monzonites and syenite compared to the granites (Table 1). Total alkali versus silica (TAS) diagrams (Fig. 3A) and R1-R2 diagrams (Fig. 3B) classify all granitoid samples as calc-alkaline (Fig. 3C), magnesian, and alkali-calcic (Figs. 3D and 3E).Geochemically, the samples plot at the boundary between volcanic arc granite (VAG) and within-plate granite (WPG) fields, closer to the VAG field. This suggests an extensional setting within a magmatic arc (back-arc), transitioning toward an intracontinental environment akin to a back-arc basin (Fig. 7). Major oxides and derived tectonic discrimination diagrams place the samples within the WPG field, trending towards the VAG field, characteristic of predominantly post-collisional granites. The geochemical evidence points towards a post-collisional extensional environment primarily associated with a back-arc setting (Fig. 8).

The Aroud granitoid pluton, located north of Alam Kuh within the central Alborz structural zone, is interpreted to represent a post-collisional, extensional magmatic arc environment. The Aroud intrusive body exhibits diverse textures, including perthite and various intergrowths, reflecting a complex petrogenesis. Magmatic differentiation, water fugacity, decompression, fluid activity, and metasomatism are inferred as the primary factors influencing textural variability. Whole-rock and mineral chemistry data classify the Aroud samples within the shoshonitic series. Trace element and rare earth elements (REE) patterns, normalized to chondrite, reveal enrichment in LREEs and elements such as Pb, K, La, Rb, and Th, coupled with relative depletion in Ti, P, and Y. These geochemical characteristics indicate a complex magmatic history influenced by various petrogenetic processes. The geochemical signature is marked by enrichment in large ion lithophile elements (LILEs) and light rare earth elements (LREEs), coupled with relative depletion in high field strength elements (HFSEs) and a relatively steep HREE slope. These geochemical characteristics suggest a petrogenetic model involving delamination and partial melting of a mafic lower continental crust in the Aroud region, resulting in the generation of continental-type adakites (C-type adakites). Adakites are a specific type of subduction-related melt distinguished from typical granitoids of arc settings by their unique geochemical signature. Simplest models posit that adakites are predominantly generated through partial melting of the subducting oceanic crust.

Molybdenum-copper deposit of Shele boran is located in East-Azarbaijan province, northeast of Ahar. The area is dominated by the oldest rocks including Paleocene-Eocene andesitic-dacitic units into which Oligocene sub-volcanic intrusions have penetrated. Quartz-sulfide, quartz-oxide, and sulfide vein-veinlets were developed within the intrusion bodies that exhibit typical stockwork texture. Three types of hydrothermal hypogene alterations, potassic, phyllic and propylitic are developed in these bodies. The Oligocene intrusive bodies range, in composition, from granite, tonalite to porphyry microdiorite. The major constituent minerals including plagioclase, alkali-feldspar, quartz, biotite and hornblende accompanied by minor amounts of clinopyroxene, apatite, sphene, zircon. and common textures granular porphyry to porphyroid, and porphyritic textures. The parent magmas are high-K calc-alkaline to shoshonite showing LILE positive anomalies with high LREE/HREE ratio. These bodies were emplaced in a post-collision volcanic arc and an active continental margin setting.

Regional Geology:

The studied area, a part of Arasbaran exploration area, lies in the north-west of Iran and is one of the copper-molybdenum-gold metallurgical states, known as the Ahar-Arasbaran zone. The area, due to its special geological features and in terms of gold, copper mineralization, Molybdenum is of special importance. This zone is divided into two eastern and western parts by the Rasht-Takestan fault. The eastern part consists of basic to acidic tuffs belonging to shoshonite to alkaline magmatic series, and the western part contains andesitic to rhyodacite lavas and several granitoid masses with high-K calc-alkaline and shoshonite. Qaradagh, Shiverdagh batholiths and Haft-Cashmeh as well as Songun porphyry stocks are among the most important intrusions igneous masses related to mineralization in Arasbaran metallogenic zone, where skarn, porphyry, stockwork and epithermal mineralizations occurred. One of the prominent features of this zone is the extensive Tertiary magmatic activity, initiated in the Paleocene-Eocene and reached its peak along with the extensive folding of volcanic and pyroclastic units, and the emplacement of intrusive masses in the Oligocene (Pyrenean phase). The oldest rock units in this area are Paleocene-Eocene volcanic and pyroclastic deposits with andesite-dacite, andesite to basaltic andesite and associated tuffs, penetrated by sub volcanic Oligocene porphyroid masses.

Simultaneously with the preparation of the geological map of the region with the scale of, a number of samples were taken from the surface units and boreholes that were drilled for the exploration of molybdenum and copper. On the base of lithological diversity, 45 samples of intrusive rocks were collected. Following petrography study, 15 samples with the least amount of alteration were analyzed by XRF and ICP-MS methods and by combining the information obtained from field observations, microscopic studies as well as the main and rare elements analyses using the GCDkit software, petrogenesis and the formation of intrusive rocks of the region have been investigated. 

Petrography:

According to the petrographic studies, plagioclase, potassium feldspar, quartz, biotite and hornblende are the main rock-forming minerals. Clinopyroxene, apatite, zircon, and sphene as the minor and chlorite, sericite, calcite, and clay minerals as the secondary minerals. The presence of porphyric granular, porphyroid and porphyry textures microcrystalline in the background are notable.

Geochemistry:

As the Co/Th, Ce/Yb versus Ta/Yb and Th/Yb versus Ta/Yb diagrams demonstrate, the parent magma has shoshonite and high-K calc-alkaline nature. On Nb vs. Y, Rb vs. (Ta+Yb), Ta vs. Yb, and Rb vs. (Y+Nb) the diagrams, the bodies under study were emplaced in a post-collision volcanic arc and an active continental margin setting.On the spider diagram of intrusive masses normalized to chondrite, the studied samples show LREE enrichment, enrichment of LREE compared to HREE and negative anomaly of Nb and Zr indicate the dependence of these rocks on it shows calc-alkaline magmatic series. On the normalized diagram compared to the primary mantle, clear and distinct negative anomalies of Ti, Zr, P, Pr, Y, Nb and also positive anomalies of K, U, Pb, Cs, Nd are observed. The P, Ta, Ti and Nb negative anomalies on spider diagram with a specific enrichment of LILE (i.e.  K, Sr, Sm, Th and Cs) are of the important characteristics of magmatic rocks associated with volcanic arcs, caused by the action of fluids derived from subduction. The negative Nb anomaly indicates the magmas related to the active continental margin environments and can be caused by the contamination of the crust and fluids released from the subducting lithosphere. The change process of these samples is compatible with the characteristics of the geo-structural environment related to subduction.

The parent magma of the intrusion bodies has shoshonite and high-K calc-alkaline affinity.  It should be noted that shoshonite and high-K calc-alkaline has also been reported in the volcanic units of Sonajil area of Harris. These masses display positive and distinct anomalies of K, Th, Sr, Sm, Cs, Rb, Ba as well as Ta, Ti, Zr, Nb, P, Pr, Y, Yb negative anomalies, the important features of the magmatic rocks related with volcanic arcs. The P, Ta, Ti and Nb negative anomalies on spider diagram along with a specific enrichment of LILE including K, Sr, Sm, Th and Cs are the important characteristics of magmatic rocks associated with volcanic arcs. Thus, the order of changes in these samples is consistent with the features of the geological environment related to subduction. These bodies were emplaced in a post-collisional volcanic arc and an active continental margin setting. The obtained data are consistent with the previous studies carried out on Oligocene granitoid intrusive masses of Western Alborz Zone-Azerbaijan (i.e., Moayyed, 2001, Aghazadeh et al., 2010).

The studied volcanic rocks as a dacitic tuff layer intercalated with the Upper Red Formation (URF, Late Miocene) and are located in the vicinity of the North Tabriz fault. During the Neogene, the red highlands north of Tabriz fault (Eynali) were a different basin from its southern part, namely the Sahand Volcanic complex. Based on the studies carried out on the volcanic rocks and pyroclastics of Sahand Volcano, the volcanic centers of Sahand have been active intermittently from the Late Miocene to the Late Pleistocene (Ghauori, 2002; Ghalamghash et al., 2019). The Upper Red Formation consists of red conglomerate alternated with sandstone, shale and marl and is associated with evaporite units (Asadian, 1993). These sediments have been deposited following uplift in a back-arc basin and within the Neotethys volcanic arc in Central Iran (Shahabpour, 2007). The aim of the present study is to investigate the relation between the tuff layer of URF and the first volcanic manifestations of Sahand volcano.

Subduction of the Neotethys under the central Iranian plate, followed by the collision of the Iranian and the Arabian plates (continental-continental collision), is responsible for the development of four structural zones in Iran. These structural zones with northwest-southeast trend include Zagros-Folded-Thrust belt, Sanandaj-Sirjan metamorphic and magmatic zone and Urmia-Dokhter magmatic arc (Alavi, 1994; Mohajjel et al., 2003). Omrani et al. (2008) have divided the volcanic rocks of Urmia-Dokhtar magmatic arc (including the studied area) into two categories: Eocene and Miocene to Plio-Quaternary. Eocene volcanic rocks consist of andesite, tuff and intermediate pyroclastics with small amounts of basalt, andesite and rhyolite. Miocene to Plio-Quaternary volcanic rocks are composed of andesite to dacititc rocks with Late Miocene to Pliocene age, which are followed by mafic volcanic rocks (Jahangiri, 2007; Omrani et al., 2008). Dacitic domes belonging to Late Miocene in the north of Tabriz fault, with adakitic composition, intruded the Upper Red Formation or Eocene volcanic units (Jahangiri, 2007).

Due to the lack of textural and mineralogical diversity of the studied rocks, four fresh samples were sent to the laboratory of the SGS Company located in Toronto, Canada, for analysis of major, trace and rare earth elements with ICP-MS. In order to determine the chemical composition of the rock-forming minerals, a sample of the studied rocks, after preparing a thin-polished section, was analyzed with an electron microprobe (CAMECA SX100) device at the Mineral Processing Research Center of Iran. The analytical conditions for voltage, beam current and beam diameter were set to 15 kV, 20 nA and 5μm, respectively.

Petrography The studied rocks are dominated by the presence of quartz, plagioclase, alkali feldspar and biotite as phenocrysts with a glassy groundmass (Hyaloporphyry) Apatite is rare and calcite and iron oxides form the secondary minerals. Quartz as anhedral to subhedral with embayed texture accounts for about 15% of phenocrysts. Plagioclase is subhedral and forms for about 25% of the rock phenocrysts.

Plagioclase and potassium feldspar are Ab70An25Or5 and Ab32An1Or67, in composition respectively. Thermometry of the feldspars based on the Ab-An-Or diagram (Fuhrman and Lindsley, 1988; Nekvasil, 1992) shows that they are of relatively low temperature type (~700 ºC). The composition of micas varies from biotite to phlogopite in diagram of Fe/Fe+Mg vs. total Al and are classified as primary and re-equilibrated primary biotites on [(Fe*+Mn)-10*TiO2-MgO] diagram. The studied biotites belong to calc-alkaline orogenic suites originated from a crust-mantle mixed source.

The studied rocks have a distinct enrichment of LILE (i.e. Rb, Ba, Th, U, K) and LREE compared to HFSE (i.e. Ta, Nb, Ti, Zr, Hf, Y) and HREE. The rocks have high amounts of Sr (400-540 ppm) and Ba (930-1130 ppm) as well. The studied tuff indicates the features of metaluminous and high-K calc-alkaline to shoshonite magmatic suites, and has the characteristics of rare elements indicative of arc type magmatism. The Nb/Ta ratio in the studied samples varies from 14.7 to 15.8, which is higher than the predicted values for the continental (Taylor and Mclennan, 1985), but it is similar to arc volcanic rocks (Stolz et al., 1996). The above features in combination with the negative anomaly of Nb, Ta and Ti and the high ratios of Ba/La, Ba/Zr and Ba/Nb >30 (Gill, 1981) point to their similarity with magmas related to subduction.

The lack of geological evidence in the region, indicating the existence of active subduction at the time of formation of the rocks under study; Thus, the observed geochemical features seem to be related to the origin rather than a tectonic origin. The enrichment of the studied rocks with some elements (i.e. Ba, Sr and Rb) requires extensive crystallization, crust contamination or very small partial melting. The studied rocks show non-adakitic characteristics, and therefore their genesis may be different from the types of adakitic rocks of Sahand Volcanic Complex. The bedrock of Sahand volcano is composed of Paleozoic-Mesozoic sedimentary deposits, Eocene volcanic rocks, lower Miocene deposits (Qom Formation) and Upper Red Formation (including marls, and red sandstones and gypsum belonging to the middle to late Miocene) (Abbassi et al., 2021). On the other hand, the oldest activity of Sahand volcano is attributed to the Late Miocene (Old Sahand in the division of Ghalamghash et al. (2019) with an age of ~8 Ma; and the thick pyroclastic sequence on the western slope of the volcano named Ghermeziqul Formation in the division of Moine Vaziri and Amine Sobhani (1977) with an age of 9-12 Ma). Therefore, considering the stratigraphic position of the tuff layer and its geochemical similarities with the non-adakitic eruptions of Sahand, it is likely the tuff layer was originated as the result of the first explosive activity of Sahand at the same time with the formation of Upper Red sediments (Late Miocene).

The Sanandaj-Sirjan zone, one of the most critical areas for studying metamorphic events, originated by subduction of the Neotethys oceanic lithosphere beneath the Central Iranian microcontinent during Early Jurassic to Late Cretaceous (Berberian and King, 1981; Alavi, 1994; Hassanzadeh and Wernicke, 2016). The Qori metamorphic complex located in the southeastern part of the Sanandaj-Sirjan zone (Figure 1). Fazlnia et al. (2009), suggests a regional metamorphic phase (c.a. 187 to 180 Ma) concurrent with the orogeny activities, and another phase (c.a. 147 Ma) associated with arc magmatism.The main purpose of the present paper is to investigate the metamorphic evolution of Qori complex metapelites using the phase diagram calculations. This research can enhance the accuracy of previous studies and provide researchers with a better understanding of thermodynamic changes during progressive orogenic metamorphism related to the tectonic evolution of the southern Sanandaj-Sirjan zone.

Geological setting:

The Qori metamorphic complex mainly comprises alternating actinolite schists, garnet amphibolites, and marbles interbedded with metapelites (garnet-kyanite-biotite schists) and metaultramafic rocks (olivine-orthopyroxene-spinel-hornblende schists) (Figure 2), subjected by Barrovian-type metamorphism (Fazlnia et al., 2009). The previous studies have reported peak metamorphic conditions of 9.2 ± 1.2 kbar and 705 ± 40°C attributed it to crustal thickening in the course of Early Cimmerian orogeny(180 - 187 Ma) (Fazlnia, 2007, 2017; Fazlnia et al., 2009). The rift propagating activity in Gondwana (Golonka, 2004; Sears et al., 2005) led to non-orogenic magmatism in northeastern Neyriz and the intrusion of the heterogeneous Talle-Pahlevani batholith into the semi-pelitic to pelitic metamorphic rocks of the Qori complex (Fazlnia et al., 2009). This caused intense contact metamorphism and migmatization at 700 to 750°C and P> 5 kbar (Fazlnia et al., 2023; Miri and Fazlnia, 2024). By the closure of the Neotethys, the study area underwent deformation, as the other parts of the Sanandaj-Sirjan zone.

About 50 metapelite samples we studied using polarizing microscope. The 6 selected samples were analyzed for their major oxides by a Philips PW1480 XRF instrument in University of Kiel, Germany.

Petrography:

The major minerals include biotite, quartz, garnet, muscovite, kyanite, plagioclase, chlorite, staurolite, along with minor amount of magnetite, rutile and porphyro-lepidoblastic texture. Two foliation fabrics, S1 and S2, are traceable in the area (Figure 3a), leading to the preferred orientation of biotite and muscovite (Figure 3b). The main stage of the garnet and staurolite growth occurred during the second metamorphism stage along with S2 foliation. They were replaced by biotite, muscovite and chlorite through the retrograde metamorphism (Figures 3e, f). In higher P-T, the staurolite became unstable, and kyanite replaces it (Figure 3g) indicating the middle amphibolite facies condition (Bucher and Grapes, 2011). In the final stage of the metamorphic process and cooling, magnetite crystals formed post-tectonically, cutting through the rock foliation (Figure 3f). The lack of pressure shadows in these crystals point to  post-tectonic growth.

Geochemistry:

The chemical data of the whole-rock are represented in Table 1. The samples plot between the pelitic and mafic rock fields on a discrimination diagram (Figure 4a), although, the presence of kyanite and staurolite reveals their metapelitic natures (Bucher and Grapes, 2011). The FeO/K2O versus SiO2/Al2O3 diagram (Figure 4b) suggests a Fe-rich sandstone for their protolith.

Phase diagram modeling The sample Af-220, containing peak P-T mineral assemblage and sufficient Al2O3, FeO, and MgO contents to form the desired minerals, was selected for calculations. The calculations were performed using Theriak-Domino software (de Capitani and Petrakakis, 2010), version 10.0.19044.1526, released in 2018, with the tcdb55c2d database in a K2O-FeO-MgO-Al2O3-SiO2-H2O (KFMASH) chemical system (Figures 5 to 7). The fluid was considered as pure water and in-excess. The solid solution models used in calculations include GARNET (White et al., 2007) for garnet, CHLORITE (Holland et al., 1998) for chlorite, PHNG (Coggon and Holland, 2002) for muscovite, BIO (White et al., 2007) for biotite, CORD (Holland and Powell, 1998) for cordierite, and LIQtc (White et al., 2007) for melt.

Metamorphism conditions:

The stability fields of garnet + biotite + kyanite + sillimanite indicating peak metamorphic condition occur at T 650 to 780°C and P >7 kbar (Figures 5, the blue dash-line). At higher T, melt appears, suggesting that the sample did not experienced T>780°C. It should be noted that there is no field containing the kyanite and the sillimanite as well, but their coexistence in the samples display the polymorphic transformation P-T condition. However, the occurrence of sillimanite in the samples under study point to the higher T than that of kyanite, the field 1 is considered as the peak metamorphic condition prior to anataxis. This thermal shock occurred due to intrusion of the Talle-Pahlevani pluton into Qori metapelites (e.g. Fazlnia et al., 2023). Cordierite appears at P<7 kbar, thus, its absence in the samples indicates a minimum P of 7 kbar.

Influence of protolith composition:

A MgO/(MgO+FeO) vs. T phase diagram at 8 kbar P (Figure 6) shows that the peak assemblages occur at MgO/(MgO+FeO) ratios of 0.2 to 0.5. Also, an Al2O3 vs. T phase diagram (Figure 7) suggesting that the assemblage requires at least 10 wt% Al2O3 to form kyanite and sillimanite at peak condition.

The metapelites of Qori complex originated from a Fe-rich sandstone protolith;The parageneses include (1) chlorite + muscovite + biotite (greenschist facies) → (2) biotite + garnet + staurolite (lower amphibolite facies) → (3) biotite + garnet + staurolite + kyanite + sillimanite (medium to upper amphibolite facies);Thermodynamic phase diagram calculations indicate that the peak assemblages formed at 650 to 780 °C and 7 kbar;The MgO/FeO and Al2O3 contents of the protolith affected the peak mineral assemblage.

The Makran volcanic arc in southern Iran is one of the few active ocean-continent subduction systems in the Alp-Himalaya orogen (Priestley et al., 2021). It includes four volcanic fields: Shahsavaran (SVF), Bazman (BVF), west of Khash (KVF), and Taftan (TVF) (Biabangard and Moradian, 2008; Pang et al., 2014; Firouzkouhi et al., 2017a; Ghalamghash et al., 2022; Delavari et al., 2022) (Figure 1).In this work, we investigate the influence of crustal assimilation in the genesis of fractionated rocks of SVF, BVF and TVF, focusing on insights gained from Pb isotope ratios. We have also used Pb isotope contents of 5 basaltic samples represented by Saadat and Stern (2011) to distinguish the enrichment of the parental magmas in the source from crustal contamination during magma ascend and in the magma chamber.

Regional Geology:

The SVF and KVF are mainly composed of basaltic andesite and basaltic lava flows, with minor andesitic rocks in SVF. In contrast, the BVF and TVF consist primarily of dacitic and andesitic rocks. SVF volcanic centers evolved from shields of thin basaltic lavas to composite cones of andesite and dacite with significant pyroclastic material (Figure 2).  BVF features a stratovolcano of andesitic and dacitic lava flows, pyroclastic rocks, and scattered monogenic cinder cones around Bazman volcano (Figure 2). KVF is characterized by small-volume cinder cones, some with multiple volcanic phases. TVF includes the Taftan stratovolcano, with alternating andesitic and dacitic lava flows, pyroclastics, and minor basaltic flows (Moinevaziri, 1985; Biabangard and Moradian, 2008; Saadat and Stern, 2011).

Geochemistry:

The major element concentrations of Makran arc volcanic rocks are shown in Table 1. Using the IUGS TAS classification (Figure 4A), the rocks are classified as andesite and dacite, falling in the sub-alkaline field. High Al2O3 and CaO levels classify them as calc-alkaline, supported by the AFM diagram (Figure 4B). K2O contents align with medium-high K calc-alkaline magmas (Figure 4C).Chondrite-normalized REE patterns and NMORB-normalized spidergrams (Figure 5) for andesitic and dacitic samples from SVF, BVF, and TVF show enrichment in LILEs relative to HFSEs and LREEs, with LREEs enriched over HREEs, typical of island arc and continental-margin magmatism. TVF andesites are more enriched thanBVF dacites and SVF andesites. Eu/Eu* values range from 0.73 to 1.15, with averages of 0.82 (SVF), 1.01 (BVF), and 0.89 (TVF), suggesting plagioclase fractionation, particularly in SVF and TVF.The average 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb ratios in the studied rocks range from 38.51421 to 39.09866, 15.58849 to 15.68117, and 18.45061 to 18.82136, respectively. For TVF andesites, the averages are 39.03451, 15.67155, and 18.77488; for BVF dacites, 38.70995, 15.62541, and 18.56748; and for SVF andesites, 38.83091, 15.64839, and 18.63444. While SVF and BVF samples show no systematic isotopic variation with fractionation, TVF rocks exhibit a positive correlation between Pb isotopes and SiO2, K2O, and Eu/Eu* (Figure 6). TVF rocks have the highest Pb isotopic ratios among all samples.

Elevated major elements variations with increasing SiO2 are consistent with crystallization of pyroxene, plagioclase, Hornblende, and somehow magnetite and ilmenite (Figure 7). Variation of Sr, Zr, and Th with fractionation trends suggest Pl fractionation, magma mixing and crustal contamination, respectively (Figure 8). Nb/Ta ratio is also negatively correlated with markers of fractional crystallization such as elevated SiO2 and K2O (Figure 8D). Decreasing Nb/Ta with increasing fractionation reflect amphibole and biotite (Muntener et al., 2018) fractionation and also, could be a clue to the role of lower crust in the magma evolution (Tan et al., 2022).AFC (DePaolo, 1981) and mixing models using Th/La and K/Rb (Figure 9) show the contribution of Makran granitic rocks, lower crust, and upper crust to magma evolution. Trace element AFC models and Pb isotope ratios, which correlate positively with SiO2 and K2O (Figure 6B), suggest crustal assimilation or mantle wedge fluid interactions. Pb isotopic modeling indicates up to 15% crustal contribution in andesitic and dacitic rocks, best fitting a mix of upper and minor lower crust. Taftan volcano samples show the highest crustal assimilation, while Bazman and Shahsavaran samples display less (2-8%) assimilation.The Pb isotope signatures of the andesites and dacites likely reflect a mix of crustal assimilation and an enriched mantle wedge. In addition to subduction-related fluids, Pb isotopic features may originate from the lower crust through subduction erosion. The 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios in MVA basaltic samples (Saadat and Stern, 2011) suggest an enriched EMII-type mantle source. If parental basaltic magmas were unaffected by crustal assimilation during ascent, excess Pb in fractionated rocks likely results from crustal assimilation. Pb isotope ratios of 5 basaltic samples from SVF and KVF (Saadat and Stern, 2011) (Figure 1) fit best with 1-3% crustal contribution in mixing models (Figure 10). These basaltic samples are considered parental magmas for SVF and KVF andesites. Thus, crustal assimilation in fractionated rocks (andesite and dacite) is estimated at up to 12% in TVF, 7% in SVF, and 2% (±1%) in BVF. Geochemical and isotopic data indicate the contaminant is a mix of upper crust (possibly Tethyan flysch) and minor lower crust.