Petrography and geochemistry of volcanic rocks of the Natour area, southwest of Ardabil province

Introduction
The Natour area is located approximately 33 km south of Kowsar city, Ardabil province, northwestern of Iran. Based on the division of structural zones of Iran (Alavi, 1991), this area is considered a part of the Alborz magmatic arc. The Alborz sedimentary-structural zone includes the northern highlands of the Iranian plate, which continues in a compound anticline with a general east-west trend from Azarbaidjan to Khorasan. Many of the stratigraphic units of Alborz and Central Iran are similar in terms of facies and formation conditions, so that Alborz can be considered as marginal folds of Central Iran, in whose formation the collision of the two Iranian and Turanian plates and its consequences played a fundamental role (Aghanbati, 2004). Alborz is more similar to Central Iran, especially in the southern slope, but there are some differences in the northern slope (Stocklin, 1968). The Alborz magmatic arc encompasses a wide range of tectonic environments, such as normal arc, back-arc, and post-collisional and extensional environments. The Alborz magmatic cycle during the Eocene-Oligocene has resulted in the formation of a wide range of intrusive igneous, subvolcanic, and volcanic-sedimentary rocks. These rocks show calc-alkaline and high-k calc-alkaline and shoshonitic magmatic series. These units include granitic, granodioritic intrusions, basaltic, andesitic, dacite, rhyolite lavas, and their associated tuffs. In the present study, an attempt is made to investigate the geological, petrographic, and geochemical characteristics of volcanic rocks in the Natour area. Also, in this research, based on the results of chemical analyses of major, trace, and rare earth elements, has determined the composition of the volcanic rocks of the area, the parent magma, and their tectonic setting.
Materials and Methods
In general, this research was conducted in two parts: field and laboratory. First, in the field part, a visit to the study area was carried out to investigate the condition of volcanic rocks, and in the next stage, samples of these rocks were taken for laboratory studies. In the laboratory part, a number of thin sections were prepared from the collected samples and then petrographic studies were performed on them. Also, in order to conduct geochemical studies, 12 fresh and less altered samples of these volcanic rocks were selected and analyzed at the Zanjan Zarazma Company. The major elements were analyzed by XRF method and the trace and rare earth elements were analyzed by ICP-MS method.
Results and Discussion
Based on geological location, the Natour area is located on the 1:100000 geological map of the Kivi sheet (Hajalilou and Rezaei, 2001). The most important rock units of the study area are related to the Eocene, Oligocene, and Quaternary.
Eocene rock units in the area include E^ab, E^clt, E^an, and E^tr. The E^ab unit is the oldest Eocene rock unit in the area and is often observed in the northeast of the Natour area. This unit consists of gray basaltic andesite. The E^clt unit consists of lithic crystalline tuff in gray to reddish gray color and is mostly spread in the central part of Natour area. In some places, this unit itself has interlayers of pyroclastic units such as tuff, lithic tuff, and lithic tuff andesite. The E^an unit includes porphyry andesite to porphyry basaltic andesite, ranging in color from purple to gray, and is often exposed in the southwest of the Natour area. The youngest Eocene unit in the study area includes E^tr with trachytic composition and it is often visible in the area in bright color. The trachyte unit is mainly visible in the southwestern part of the Natour area. The Oligocene rock units in the area consist of the O_lr^c unit, which contains heterogeneous conglomerate (in red) and is usually seen in the north and northeast of the area and discontinuously overlying Eocene volcanic units. According to petrographic studies, the volcanic rocks of the Natour area mainly show a composition of basaltic andesite, andesite, and trachyte. Basaltic andesite rocks have porphyritic and glomeroporphyritic textures, composed of the main minerals plagioclase and pyroxene in a fine-crystalline matrix. Andesite rocks contain the main minerals plagioclase, amphibole, and biotite and have a porphyritic texture in a fine-crystalline matrix. Trachyte rocks also have a porphyritic texture, consisting of alkali feldspar, plagioclase, and biotite minerals in a fine-crystalline matrix. To determine the composition of volcanic rocks in the Natour area, the total alkali (Na₂O+K₂O) versus silica (SiO₂) diagram (Middlemost, 1994) and the Nb/Yb versus Zr/Ti diagram (Pearce, 1996) were used. In the total alkali versus silica diagram, the samples from the study area are in the range of rocks with a composition of andesite, basaltic andesite, basaltic trachyandesite, trachyandesite, trachyte, and trachydacite. In the Nb/Yb versus Zr/Ti diagram, samples from the Natour area are located in the range of andesite, basaltic andesite, trachyandesite, and alkali basalt rocks. In the Al₂O₃-K₂O-Na₂O ternary diagram, samples from the Natour area are located in the metaluminous and peraluminous realms. Also, to determine the alumina saturation index of volcanic rocks in the study area, the diagram presented by Shand (1943) was used. In this diagram, the metaluminous, peraluminous, and peralkaline ranges are distinguished. According to this diagram, samples from the Natour area fall into two ranges: metaluminous and peraluminous. The magmatic series of volcanic rocks in the Natour area was initially determined using the diagram of total alkalis (Na₂O+K₂O) versus silica (SiO₂) (Irvine and Baragar, 1971). In this diagram, two ranges of alkaline and subalkaline are separated. Based on this diagram, all samples in the study area belong to the alkaline range. Then, Th/Yb versus Ta/Yb (Pearce, 1983), Co versus Th (Hastie et al, 2007), and SiO₂ versus K₂O (Peccerillo and Taylor, 1976) diagrams were used to determine the magmatic series of the igneous rocks of the area. These diagrams are divided into the ranges of tholeiitic, calc-alkaline, high-k calc-alkaline, and shoshonitic magmatic series. Based on the Th/Yb versus Ta/Yb diagram, most of the samples in the study area are located in the calc-alkaline magmatic series. According to the Co versus Th diagram, the samples from the Natour area are within the range of calc-alkaline and high-k calc-alkaline and shoshonitic magmatic series. According to the SiO₂ versus K₂O diagram, the samples from the study area are mainly located in the range of shoshonitic magmatic series. The trend of changes in trace and rare earth elements in samples of volcanic rocks from the Natour area was determined using spider diagrams. In this regard, samples from the study area were normalized to the primitive mantle (Sun and McDonough, 1989) and chondrite (Nakamura, 1974). The spider diagram of the samples normalized to the primitive mantle shows a positive anomaly in large ion lithophile elements (LILE) such as Cs, K, and Pb, and a negative anomaly in high field strength stability elements (HFSE) such as Nb, Zr, and Ti. Positive anomalies of LILE elements and negative anomalies of HFSE elements are characteristics of arc-related regions, whose formation can be associated with subduction zones and contamination of magma with continental crust (Wilson, 1989; Rollinson, 1993; Thuy et al, 2004; Kuscu and Geneli, 2010; Yu et al, 2017). In the spider diagram of samples normalized to chondrite, enrichment of LREE relative to HREE can be identified. Also, a relatively weak depletion of the Ce element is observed in this diagram. The enrichment of LREE relative to HREE can indicate the formation of volcanic rocks in subduction zones or the contamination of magma by crustal materials (Kuster and Harms, 1998; Ulmer, 2001; Srivastava and Singh, 2004; 
Peccerillo et al, 2004; Goss and Kay, 2009). The relatively weak depletion of Ce element can most likely be due to the high mobility of this element during the subduction process (Hoyle et al, 1984). Using TiO₂-Al₂O₃ and Y-Zr diagrams (Muller et al, 1992), the tectonic setting of igneous rocks can be interpreted. In these diagrams, the within plate setting and the arc-related setting are separated. Based on these diagrams, all samples of volcanic rocks in the Natour area are placed in the arc-related tectonic setting. To determine the tectonic setting of the volcanic rocks of the Natour area, the Nb/Yb versus Th/Yb diagram (Pearce, 2008) was also used, in which the samples from the study area are located in the tectonic setting associated with volcanic arcs. To distinguish oceanic arc rocks from continental arc and post-collisional arc rocks, the ternary diagram TiO₂/100-La-Hf×10 (Muller et al, 1992) can be used. According to this diagram, samples of volcanic rocks from the Natour area are often located in the settings of continental and post-collisional arcs. Also, the Zr×3-Nb×50-Ce/P₂O₅ ternary diagram (Muller et al, 1992) was used to separate continental arc rocks from post-collisional arc rocks. According to this diagram, the samples related to volcanic rocks of the study area are mainly located in the post-collisional arcs.
Conclusion
Petrographically, the volcanic rocks of the Natour area represent a composition of basaltic andesite, andesite, and trachyte. These rocks are located in the chemical classification diagrams in the range of andesite, basaltic andesite, trachyandesite, trachyte, trachydacite, and alkali basalt. Volcanic rocks of the Natour area are located in the metaluminous and peraluminous ranges on alumina saturation index determination diagrams. In terms of magmatic series, these rocks represent high-k calc-alkaline and shoshonitic magmatic series. Spider diagrams related to volcanic rocks of the Natour area indicate a positive anomaly in large ion lithophile elements (LILE) and a negative anomaly in high field strength elements (HFSE) and enrichment of LREE relative to HREE, which can indicate the formation of volcanic rocks of the study area in subduction zones or contamination of magma by crustal materials. According to tectonic setting diagrams, the volcanic rocks of the study area were developed in arc-related tectonic settings. Also, in the diagrams of the separation of oceanic arc rocks from continental arc and post-collisional arc rocks, they are placed in the position of post-collisional arcs.