نشریه پترولوژی
پیاپی 52 (زمستان 1401)

The magmatic evolution of the Urumieh-Dokhtar arc is related to the Neo-Tethys subduction and the continental collision between the Arabian and Eurasian plates, which has led to the creation of diverse and different magmatism along different parts of this belt (Shahabpour, 2005; Agard et al., 2011; Richards, 2015; Karimpour et al., 2021).Volcanic rocks, with various lithological compositions located in the central part of the Urumieh-Dokhtar belt, are part of the extensive Eocene activity, Knowing the nature of basic-intermediate volcanic rocks northwest of Nain requires more detailed investigations. For this purpose, the exploration areas of Chakad, Safafoulad, and Mehrando, located northwest of Nain, were selected for further investigations. The main goal of the present paper is to understand the origin and the tectonic of the volcanic rocks of this part of the Urumieh-Dokhtar magmatic belt using their petrography and geochemical characteristics

Regional Geology:

Many outcrops belonging to Tertiary volcanic lavas and pyroclastic deposits are widespread 50 km northwest of Nain, within the central part of the Urumieh-Dokhtar magmatic arc (UDMA). The volcanic units are basic to acidic composition ranging in age from Eocene to Oligocene (Chiu et al., 2013) and pyroclastic rocks including tuff, breccia, and ignimbrite, low in altitude, overlain by a sequence of Quaternary alluvium. In the exploration areas of Chakad, Safafoulad, and Mehrando, the Eocene basic-intermediate volcanic rocks with the combination of basalt, basaltic-andesite and pyroclastic rocks exposed and play as the host rocks for copper mineralization. The youngest igneous units observed in the region are a set of parallel diabase and dolerite dikes as swarms, intruded the volcanic rocks. Based on field observations, silica and carbonate veins with different trends are observed along with copper mineralization in basaltic- andesite and basalt units. This ore mineralization occurs mainly as an oxide (malachite and azurite) accompanies with propylitic alteration and less argillic and silicification along and at the intersection of faults and fractures.

Following the preparation of thin sections and petrographic and mineralogical investigations of these units, 26 volcanic samples have been analyzed by the ICP-MS method for trace and rare earth elements. Petrography The basic-intermediate volcanic rocks of the region are mostly basalt and basaltic-andesite, as well as diabase and dolerite dikes. The studied basalts, mineralogically, dominated by Euhedral to subhedral phenocrysts of plagioclases (30 to 50 vol%) and clinopyroxene phenocrysts, with porphyritic texture. Small amounts of olivine occur as phenocryst and iddingsite basaltic- andesite consists of plagioclase (40 to 60 vol%), and clinopyroxene (15 to 20 vol%) phenocrysts set in a fine-grained to microlithic groundmass. Clinopyroxene and plagioclase with intergranular texture are the dominant minerals in diabase and dolerite. Chlorite, calcite, epidote, and quartz are the main altered minerals of the rocks under study.

Based on the geochemical data, the basic- intermediate volcanic rocks with the combination of basalt and basaltic- andesite have a calc-alkaline nature, consistent with the features of volcanic arcs in the subduction zone of the active continental margin. In the primitive mantle-normalized multi elements diagram, the patterns of basic- intermediate volcanic rocks show enrichment of LILE (e.g., Ba, K, Rb) and depletion of HFSE (e.g., Nb, Ti, Zr) one of the remarkable features of subduction zone- related magmas (Yang and Li, 2008; Kuscu and Geneli, 2010). In the chondrite-normalized REEs diagram, these rocks exhibit LREE enrichment relative to HREEs. Rare earth elements such as La and Sm are not significantly affected by the mineralogical changes of the source rock, so they can provide information on the chemical composition of the total source rock. The volcanic rocks of the region generated by partial melting of 5 to 10% (Aldanmaz et al., 2006) of the enriched garnet- lherzolite mantle at a depth of 90 to 100 km. It seems that the parent magma has been metasomatized under the influence of fluids and sediments derived from the oceanic lithosphere. As the tectonic setting identification diagrams display the studied samples plotted in the range of magmas related to the subduction zone of the active continental margin. The magmatism under discussion, is the result of the subduction of the Neo-Tethys oceanic lithospheric beneath the central Iranian plate which have given rise to great magmatism during the Eocene and following it.

The Gorgan schist complex, composed of low-grade metamorphic rocks, are widespread in the Alborz highlands in the south of Mazandaran Sea and extended from Behshahr to Aliabad. The complex is characterized by alternating  of phyllite, sericite, chlorite schist, and quartzite along with ophiolite interlayers at  the base part of  the interval (Tietze, 1877; Stahl, 1911). The shale layers of the studied area are discontinuously covered by non-metamorphosed sediments, in the lower part, these sediments include a layer of conglomerate in which the  fragments of Gorgan schists occurred (Hubber, 1957).

25 thin sections were prepared from the collected samples. and were studied in the microscopic laboratory of Kharazmi University in Tehran with a Zeiss Axioplan 2 polarizing microscope. 10 samples with minor alterations were analyzed  by ICP-MS and XRF methods at the Zarazma laboratory in Tehran. The data obtained from the chemical analyses are given in Tables 1 and 2. Abbreviations used in the text, figures, and tables of the present  study are based on Whitney and Evans (Whitney and Evans, 2010). GCDKit, Excel, and Adobe Illustrator softwares were used.to analyze the geochemical data and to draw the required diagrams.

Regional Geology:

The studied area lies in the north of Iran, the east of Mazandaran province, and the middle-eastern Alborz zone. The northern part of this zone is Gorgan-Rasht zone, including the marginal areas of the Mazandaran Sea as well as  the north of Alborz fault (Darvishzadeh, 2013). Some workers believe that the protoliths of these metamorphic complexes belonging to  Paleozoic or Mesozoic rocks deformed by collisional tectonics in the Late Triassic (Khosrotehrani, 2009). The Behshahr and the Galogah regions are located in the geological divisions of Iran in the central Alborz zone and the west of  eastern Alborz zone.

Petrography:

According to the petrography, the investigated masses in the south of Behshahr include metagabbro, metabasalt and orthogneiss. Metagabbros in the hand sample are often seen as grayish-green, fine-grained to medium-grained and granular texture. In the microscopic studies, plagioclase and pyroxene phenocrysts are place in the microlithic to crypto-crystalline matrix, which shows porphyry texture. Also, metamorphic amphiboles such as tremolite-actinolite time with pseudomorph texture have grown in the pyroxene forms. Plagioclases are illustrated with polysynthetic twinning. Some plagioclases have been sericitized, and pyroxenes altered to serpentine. Pyroxene and plagioclase are the major rock-forming crystals and make the major granular texture. Mineral deforming such as amphiboles bending can show the syn-tectonic phases with the peak of metamorphism. The petrographic evidence demonstrates high-grade metamorphism, in the border of the upper amphibolite facies and granulite facies. The presence of gneisses containing high-grade metamorphic minerals such as garnet and clinopyroxene proves the high-grade metamorphic series in the central part of the Gorgan metamorphic complex. The identification of dynamo-thermal metamorphic fabrics such as Bookshelves and Micafish indicates the influence of special tectonic processes on the margin of the subduction zone with exhumed metamorphic complexes during the closing subduction systems and upwelling the metamorphic complex into the upper crust levels.

Geochemistry:

The chemical analysis show, the amount of SiO2 varies between 35.94 and 49.26 and they are in the range of basic rocks. Also, the Al2O3 content in these samples is between 13.80 and 20.32 which mines our samples are in the range of per aluminum. Also, the results of the chemical analysis represented our samples belonging to the calc-alkaline rock series. Low titanium content (1.59 to 2.72 wt%) and low potassium content (0.06 to 1.87 wt%) are special characteristics of these rocks. The low amount of P2O5 and Ti2O in the samples, which are less than 0.5 and 2.5, respectively, are similar to subduction-related rocks (Defant et al., 1992). Based on the TAS diagram (Cox et al., 1979), SiO2 versus Na2O+K2O, all the studied samples except the BMG125 sample are placed in the gabbro range. These study rocks are often included in the group of intrusive rocks resulting from melts created in subduction zones. The study of rare elements shows that Behshahr's intrusive complex shows features close to volcanic arcs on the edge of the subduction zone and close to oceanic islands. In the analyzed samples, LREE elements are more enriched than HREE elements, and LILE elements are also more enriched than HFSE elements, which indicates magma contamination with the crust and partial melting.

According to the studies, all of the samples are in the calcalkaline range in the related diagrams. The calc-alkaline magmatic series is related to subduction setting. Therefore, it can be expected that the formation setting of the studied gabbroid mass is a subduction setting. Of course, due to the fact that some samples are in the intermediate range of calcalkaline and alkaline, the intraplate scenario is also possible to describe the tectonomagmatic setting of the region, and in this regard, we examine both subduction and intraplate theories. Considering that the age of the metamorphisms of Gorgan is related to the Paleozoic and Triassic periods (Zanchi et al., 2009), most probably the age of the studied intrusive masses and gabbros before the Paleozoic is related to the Cambrian. It is noteworthy that dating and providing a more accurate theory about the tectonomagmic setting of the studied area requires more sampling and geochronologic data. In this regard, it is suggested that in the continuation of this research, dating studies should be carried out to investigate and determine the tectonomagmic pattern and dating of intrusive masses in the Gorgan schists.

The Kahdelan area in the SW of Sarab is situated in the northern part of the Urumieh–Dokhtar geotectonic zone and Tarom-Hashtjin metallogenic zone. Some of the most important porphyry Cu ± Mo ± Au mineralizations in Iran occur along the NW–SE trending Urumieh–Dokhtar volcano-plutonic belt, lying between the Sanandaj–Sirjan zone and Central Iran. The Urumieh–Dokhtar magmatic arc, extends over a strike length of about 2000 km from northwest to southeast and is characterized by subduction-related calc-alkaline rocks. Due to extensive Tertiary magmatism and extensive alterations, this zone is one of the remarkable Cu-bearing regions in Iran. Therefore, many studies, regarding different aspects of the area, have been carried out by the Geological Survey of Iran as well as some private companies. Differentiating fertile and barren intrusive bodies could be important factors in reducing exploration expenses, and it is possible to identify susceptible areas by using geochemical data. Thus, the main purpose of the present study is to evaluate the granitoid rocks of the Kahdelan area as the possible potential for Cu-mineralization based on mineralogical and geochemical evidence.

Regional Geology:

The main intrusions of the Kahdelan area are Oligocene granitoid bodies composed of syenite, quartz syenite, and monzosyenite with light color and granular texture. Syenites are the most dominant plutonic rocks and quartz-syenites are the main host of Cu-mineralization. These intrusive bodies intruded the Upper Eocene pyroclastic and volcanic rocks which gave rise to alteration and mineralization occurrences in the area. Volcanic rocks are composed of basalt, basaltic-andesite, andesite, and trachyte mainly with porphyritic texture.

The present study evaluates the copper mineralization potential of the Kahdelan area for the first time. The represented information can be divided into three parts: 1) ore mineralogy and mineralogy of the Kahdelan’s rocks; 2) the investigating data related to I-type and magnetite series, and 3) the relationship between the obtained mineralogical and geochemical data with Cu-mineralization in the area to compare with porphyry copper deposits along the Urumieh–Dokhtar magmatic arc.

The dominant alteration zones in the area under study are argillic, phyllic, carbonatization, silicification, and hematitization. The primary ore minerals are magnetite, chalcopyrite, and bornite which are generally replaced by secondary minerals including chalcocite, covellite, and malachite. The ore textures are predominantly disseminated, open space-filling, replacement, and brecciated. Gangues (carbonate and quartz minerals) textures are crystalline, open space-filling, cementation of brecciated zones, and colloform.The volcanic rocks of the Kahdelan area are calk-alkaline to shoshonitic and the granitoids are shoshonitic affinity and I-Type nature (magnetite series). In the area of study, sinking solutions washed away most of the pyrites and left behind empty pyrite molds and iron oxides and hydroxides along with different amounts of malachite and neotocite. As the chondrite-normalized REE diagrams display the studied granitoids are enriched in LREEs and fairly depleted in LREEs relative to HREEs The enrichment of LILE elements and depletion of HFSE elements are features similar to those of the fertile granitoids. The rate of decline of the slope in the diagram is similar to that of the copper porphyry deposits also slightly negative Eu anomalies of these granitoids are similar to fertile granitoids (Karimpour et al., 2021). The (La/Yb)n and Eu/Eu* are used in evaluating the oxidation state and investigating the depth changes of parent magma of granitoids. The (La/Yb)n anomalies vary from 4.05-23.17, and Eu/Eu* anomalies vary from 0.32-2.65 with an average of 0.8 that are different from copper porphyry deposits.Based on spider diagrams, the depletion of titanium could be related to the low oxygen fugacity in subduction zones. The Pb and U enrichment point to the role of the earth’s crust in the petrogenesis of these rocks. P depletion could be the consequence of apatite crystallization from the parent magma.Based on the relationship between geochemical data as well as mineralization, three diagrams have been used to distinguish fertile granitoids. The Eu/Eu* versus (La/Yb)n diagram (Karimpour et al., 2021) shows that Kahdelan’s granitoids have a reduced nature. The SiO2-K2O diagram (Peccerillo and Taylor, 1976) points out that the intrusive and the volcanic rocks of Kahdalan are characterized by the much less SiO2 amount compared to fertile granitoids in SNJMB (Saveh-Nain-Jiroft Magmatic Belt) (Karimpour et al., 2021). On A/NK versus A/CNK diagram (Meinert, 1995) the granitoids under study are close to fertile copper porphyry deposits.

On the basis of mineralogical and geochemical data, Kahdelan’s intrusive rocks are I-type and magnetite series. The tectonomagmatic setting of the rocks under study lies within Volcanic Arc Granites (VAG), active continental margins, and arc systems. On A/NK versus A/CNK diagram, these granitoids are close to fertile copper porphyry deposits. Quartz syenites are the main mineralization hosts. Based on geochemical data, the average Cu content is 3492.76 ppm in a total of 170 samples collected all over the area and up to 230534 ppm Cu in mineralized Quartz syenites veinlets. Mineralogical, geochemical, and alteration data in combination with fertile–barren discrimination diagrams indicate that the granitoids of the Kahdelan area can be evaluated as the possible potential for Cu mineralization.

The occurrence of tourmaline in the metamorphic complex of the north of Golpayegan is observed within the granitoid mass and schists in contact with granitoid and marbles. Speaking of abundance, tourmaline is less frequently a minor mineral found in granites and granitic pegmatites as well as in low- to high-grade metamorphic rocks (Krmícek et al., 2020). The minerals of the tourmaline group can adjust their composition to adapt to varied settings, and therefore show a remarkable stability range in terms of pressure, temperature, fluid composition, and host rock composition (Van Hinsberg et al., 2011). Since tourmaline displays a negligible intra-crystalline diffusion, it can record the physical and chemical conditions of its setting and preserve this information in geological chronicles. Therefore, tourmaline accurately presents the composition of fluids and the melts from which it crystallized (Marks et al., 2013). Although tourmaline is common in many rocks, it is not common in metamorphosed carbonates (Krmícek et al., 2020).A petrographical and geochemical investigation of tourmalines in the north of Golpayegan was undertaken to know the formation mechanism of this mineral and to determine the differences between tourmalines crystallized at the contact, in granitoid and tourmaline in marbles.

Regional Geology:

The Golpayegan Metamorphic Complex is located in the Sanandaj-Sirjan Zone and the occurrence of tourmaline in this complex is observed in two locations. The first location is on the west side of Ochestan farm where tourmaline is found in three forms:1) In the granitoid mass (Gt); 2) In the amphibole schists (At) in contact with the granitoid mass, and 3) In the mica schists (Mt) in contact with the granitoid mass. The second area is in the north of Esfajerd where tourmaline occurs inside the marbles (Ct).

With the completion of field reconnaissance and preparation of thin sections, a petrographic study was fulfilled to determine the texture and mineralogy of the minerals, and then, some samples were selected for electron microprobe analysis.

Petrography:

The tourmaline in granitoid mass (Gt), appears as idiomorphic and coarse-grained without any inclusions. Micaschists (Mt) in contact with the granitoid mass are sieve-shaped or spongy. Inside the amphibole schists (At) in contact with the granitoid mass, it is idiomorphic without any inclusions.Tourmaline in marbles (Ct) is fine-grained and blue, which can be observed around biotites with corrosion marks, indicating its reaction with fluid.

Tourmaline chemistry:

The tourmalines in Ochestan granite-pegmatite (Gt) are of alkali tourmaline variety with schorl composition, enriched in aluminum, and points to the replacement of Al in the Y (R2) position. The substitution type of these tourmalines is Al(NaFe+2)-1 owing to the high amount of Fe versus Mg,The tourmalines in amphibole schist (At) are alkali tourmaline with schorl-dravite composition and the tourmalines in mica schist (Mt) are alkali tourmaline with dravite composition, and both types are characterized by insignificant amounts of aluminum. This demonstrates the replacement of Al in the position of Y (R2) has not occurred. Due to the change in Mg and Fe content, the At-type tourmalines benefit from both Al(NaFe+2)-1 and Al(NaMg)-1 substitution varieties. However, the substitutions of Mt-type tourmalines are mainly Al(NaMg)-1 due to the high content of Mg versus Fe. Indeed, Fe+3Al-1 substitution can be observed in both types of tourmaline.Tourmalines in marble (Ct) are of the alkaline type of dravite composition and rich in aluminum, which indicates the replacement of Al in the Y (R2) position. Their substitution is Al(NaMg)-1 due to the high content of Mg versus Fe.

The composition of tourmaline in Gt is of schorl type (Fe/Fe+Mg= 0.89-0.91) and has an aluminum replacement in the Y position. The composition of tourmaline in Mt type is of dravite type (Fe/Fe+Mg= 0.45-0.47) and the composition of tourmaline in At type is of schorl-dravite type (Fe/Fe+Mg= 0.49-0.51), which, replacement of aluminum in the Y position does not occur in both types of tourmaline in schists. Consequently, hydrothermal tourmalines have less aluminum (i.e. At and Mt-type tourmalines), and tourmalines in the granite-pegmatite mass have much more aluminum (i.e. Gt-type tourmalines). Based on the values of Fe# (FeO/FeO+MgO), it is possible to determine the formation site of tourmalines. If the amount of Fe# in tourmaline is >0.8, it indicates the closed magmatic system, lack of fluids interference, and their contamination with Al-rich sediments. Meanwhile, if the ratio is <0.6, it means that boron is metasomatic with sediments rich in Al and also is of an extrinsic origin.The Gt tourmaline samples in the range of Li-poor granitoids and related pegmatites and aplites related to them, the At and Mt tourmaline samples placed in the range of Ca-poor metapelites, metapsammites and quartz-tourmaline rocks not coexisting with an Al-saturating phase.The tourmalines in the marbles of the north Esfajard (Ct) are of dravite type with the ratio of Fe/Fe+Mg= 0.42-0.45. In these tourmalines, both aluminum replacement in the Y position and Al(NaMg)-1 substitution can be observed, which manifests the non-magmatic origin of these types of tourmalines. The Ct-type tourmalines in the range of metapelites and metapsammites coexist with an Al-saturating phase.The postmagmatic/residual-hydrothermal fluids related to alkali syenite magma of the north Esfajard along with the fluids from the progressive metamorphism in micaschists, have developed these tourmalines in marble.The discrepancy in the results of two types of thermometers (thermometry based on the amount of Ti in biotite and Mg-Fe exchange between tourmaline and biotite minerals) in meta-carbonates, highlights that at temperatures >566°C biotite, and lower temperatures, Ct-type tourmaline is composed of biotites.

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.

The Urumieh-Dokhtar magmatic arc (UDMA) of Iran with a length of about 1700 km and a width of approximately 150 km is mainly composed of Tertiary volcanic rocks and acidic to basic intrusions (Berberian, 1981; Emami et al., 1992; Darvishzadeh, 2003; Ghorbani, 2003). The UDMA has been attributed to the subduction of the Neotethys oceanic crust under the Iranian plate, which occurred from the Triassic to the Eocene (Asiabanha et al., 2012; Pang et al., 2013). The UDMA extending NW-SE includes a large volume of Cenozoic magmatism, especially in the Eocene (Chiu et al., 2013; Kananian et al., 2014). The purpose of this research is to identify the petrographic and geochemical characteristics of the volcanic and intrusive rocks of the Lak area in NW Iran and also to determine the tectonomagmatic setting of these rocks.

Geology:

The Lak area is situated 36 km southwest of Buin Zahra, in the northern part of the UDMA and the western part of the Central Iran zone (Aghanabati, 2006). The UDMA is an Andean magmatic arc with a NW-SE trend, which formed by the oblique subduction of the Neotethys oceanic plate under the central Iranian plate (Shearman et al., 1976; Berberian and King, 1981; Agard et al., 2011; Gohari et al., 2022). One of the remarkable features of the UDMA is the emplacement of intrusive masses due to late Eocene and early Oligocene pressure phase (Pyrenean tectonic phase) in volcanic-sedimentary sequences of Eocene age (Delavari et al., 2017). The main outcrops of the study area include Eocene volcanic and volcano-sedimentary rocks consisting of andesite, andesitic basalt, basalt, dacite, and rhyodacite. The volcano-sedimentary rocks of the Lak area comprise alternating lava flows (basalt to andesite) and pyroclastic materials (various types of tuff and agglomerate).

The studies carried out in the Lak area include field and laboratory parts. During the field studies, a geological map with a scale of 1:5000 in an area of 20 square kilometers was prepared. Forty-five samples of volcanic rocks and intrusive masses for preparation of thin sections and petrographic studies (20 samples), measurement of main oxides by XRF method (14 samples), and analyzing minor and rare earth element contents by ICP-MS method (17 samples) were collected and sent to the relevant laboratories for analysis.

Petrography:

In the Lak area and its surroundings, volcanic and intrusive rocks are exposed. These rocks mainly include andesite lavas, basaltic andesite, basalt and tuff, and pyroclastic deposits of lower Eocene age and intrusive masses and dykes with the composition of dacite, rhyodacite, microdiorite, and gabbro-diorite of upper Eocene-Oligocene age, which were injected into the Eocene volcanic rocks (Firouzbakht et al., 2018). Geochemistry and tectonomagmatic setting of the volcanic and intrusive rocks The amount of SiO2 in the rocks under study varies from 42 to 71% and on K2O versus SiO2 diagram, those are in the range of andesite, basaltic andesite, basalt, dacite, and rhyodacite. The range of K2O changes in these rocks as well as intrusive rocks is relatively wide so the investigated samples of volcanic and intrusive units are located in different groups of low- and medium-potassium rocks. According to the geochemical characteristics and also taking into account the temporal and spatial location of the volcanic rocks of the Lak area, it seems that these rocks are related to the magmatism caused by the subduction of the Neo-Tethys oceanic crust under the central Iranian plate and have been originated in a (magmatic arc) environment.

Possible origin of magma:

The widespread distribution of plutonic rocks in the study area, mineralogical similarity between these plutons and the volcanics as well as very similar chemical compositions of these two rock types in different geochemical and tectono-magmatic discrimination diagrams suggest that the plutonic and the volcanic rocks may have originated from the same source.The volcanic and plutonic rocks of the area are very similar to calc-alkaline lavas based on major and trace element geochemical data. The Al2O3 content of these rocks is high, but they are low in Mg#, their Zr/Y ratio is greater than 3, similar to the volcanic rocks of continental arcs (Pearce and Norry, 1979). In the diagram of normalized trace elements relative to the enriched mid-ocean ridge basalts (E-MORB), Ti (except basalts), Nb, P, and Rb have negative anomalies, but Pb and K show positive anomalies, pointing to magmatic rocks from a subduction zone (Morata and Aguirre, 2003).Magmas forming volcanic rocks in subduction zones usually originate from mantle wedges, fluids, and hydrous melts derived from subducting oceanic crust. The samples from the Lak area have high K2O contents and the downward trend in MgO values of the volcanic rocks of the area indicating that the magma originated from the mantle wedge (Gourgaud and Vincent, 2003).