marzieh ghadirpour

In central part of the Mesozoic Ashin ophiolite (Northwest of Anarak, Isfahan province, Iran), the Upper Eocene monzonitic stock cross cuts the Ashin ophiolite and Middle Eocene volcanic rocks. Amphibolite xenoliths are enclosed in the stock and associated Eocene volcanic rocks. Xenoliths are more abundant in the margin of the monzonitic stock. Rock-forming minerals of the stock are plagioclase with andesine to labradorite composition (An=34-60%), Alkali-feldspar with orthoclase composition (Or= 70.8 to 96.1%), diopsidic clinopyroxene with (Mg# =0.71-0.90), and phlogopite mica with (Fe#=0.3). Opaque minerals are magnetite and titanomagnetite (TiO2=1.6-4.4 wt.%). Main textures of samples from this intrusive body are granular, intergranular and poikilitic. Samples from the margin of this stock represent porphyritic texture. 
Geochemistry of minerals and whole rock samples of this stock indicate that they belong to the calc-alkaline magmatic series and are similar to the samples from the continental magmatic arcs.
These magmatic rocks possibly were formed by subduction of the CEIM (Central-East Iranian Microcontinent) confining oceanic crusts (Ashin and Nain oceanic crusts) during Mesozoic and Early Cenozoic eras.

Iran is a part of the Alpine-Himalayan orogenic system, including the Paleozoic to Cenozoic ophiolites, magmatic and metamorphic rocks (Takin, 1972; Berberian and King, 1981; Berberian et al., 1982; Dercourt et al., 1986; Alavi, 1994; Mohajjel et al., 2003; Shahabpour, 2007). The main pulse of the Paleogene and Neogene magmatic (volcanic and intrusive) activities of Iran can be attributed to the two Cenozoic subduction events, including the western Neo-Tethyan oceanic crust subduction beneath the Sanandaj-Sirjan block in the west and the eastern Neo-Tethyan oceanic crust subduction beneath the Central Iran (e.g., Shirdashtzadeh et al., 2022). The former subduction possibly caused to the formation of the Urumieh-Dokhtar Magmatic Arc, but the later subdution results is not well studied yet. 
In the this research, the target region is located in the west of the Yazd block (Central Iran), where the Eocene volcanic and plutonic rocks represent subduction-related characteristics (Jamshidzaei et al., 2021). The investigated subduction-related monzonitic stock that cross cuts the central part of the Ashin ophiolite in the Kuh-e-Kalut-e-Ghandehari region, in the northwest of Anarak (Isfahan Province, Iran). The main lithologies in the Kuh-e-Kalut-e-Ghandehari are Mesozoic lithologies of Ashin Ophiolite, Paleocene limestone, Eocene volcanic rocks, monzonitic stock, Lower Red Formation, and Akhoreh Formation. Ashin ophiolite was formed in the mesozoic (Shirdashtzadeh et al., 2022) and emplaced in the Late Paleocene (~60 Ma; Pirnia et al., 2020; Shirdashtzadeh et al., 2022), before than Eocene volcanism and plutonism. The studied monzonitic stock of the Kuh-e-Kalut-e-Ghandehari intrudes the Mesozoic Ashin ophiolite and Middle Eocene volcanic rocks.
The calc-alkaline affinity of the volcanic and plutonic rocks of the area, tectonic activity of the Great Kavir fault caused to the crushing and mylonitization of the surrounding rock units, as well as the alteration evidences in the field studies point to suitable conditions for the ore deposit exploration in the area (e.g., copper). In this research, the petrology, mineralogy, and whole rock geochemistry of the Upper Eocene monzonitic stock are considered. This research will expand our understanding of the geochemical nature of subduction-related Cenozoic magmatism in Central Iran.

After detailed field studies and sampling, the selected fresh samples were used for microscopic thin section and polished-thin section studies by the polarizing binocular microscope (Olympus BH-2). The microprobe analyses were performed at the School of Natural Systems, College of Science and Engineering, Kanazawa University (Kanazawa, Japan) using a wavelength dispersive electron probe microanalyzer (EPMA) (JEOL JXA-8800R). The mineral analysis was achieved under an accelerating voltage of 20 kV, a probe current of 20 nA, and a focused beam diameter of 3μm. 14 whole rock samples analyses were performed by Brucker S4 PIONEER XRF in the central laboratory of the University of Isfahan and 3 samples were analyzed in the Isfahan Nuclear Technology Center by neutron activation analysis (NAA).

Based on the field relation ships, this gray to light gray pluton intrudes into the Middle Eocene volcanic rocks and belongs to the Upper Eocene. The Middle Eocene volcanic rocks and Upper Eocene monzonitic stock crosscut the Ashin Ophiolite. This Eocene stock and volcanic rocks contain amphibolite xenoliths with the same mineralogy and petrography. Xenoliths are more abundant in the margin of the monzonitic stock. Gradual decreasing of modal plagioclase content indicates that the xenoliths range from amphibolite (plagioclase + amphibole) to hornblendite (only amphibole) in composition.
Rock-forming minerals of the stock are plagioclase with andesine to labradorite composition (An = 34-60 %), alkali-feldspar with orthoclase composition (Or = 70.8 to 96.1%), diopside clinopyroxene with Mg# = 0.71-0.90, and phlogopite mica with Fe# = 0.3. Opaque minerals are magnetite and titanomagnetite with TiO2 = 1.6-4.4 wt%. The main textures of samples from this intrusive body are granular, intergranular and poikilitic. Samples from the margin of this stock represent porphyritic texture. The SiO2 value in the whole rock compositions ranges from 47.9 to 61.65 wt.% (basic to intermediate). The average content of alkalis is 9.75 wt.%). The Kuh-e-Kalut-e-Ghandehari rocks show sodic affinity by higher Na2O than K2O, based on the Na2O/K2O versus SiO2 and K2O/Na2O versus SiO2 diagrams (Jaques et al., 1985). The Eocene intrusive and volcanic rocks of this area are similar in terms of mineralogy and texture. Petrography and whole rocks chemical analyses indicate that the studied stock is geochemically composed of gabbro, monzodiorite to monzonite in composition with metaluminous affinity. Monzonite is the predominant rock.

Various tectonomagmatic discrimination diagrams are used to determine the tectonomagmatic setting of the Kuh-e-Kalut-e-Ghandehari stock. Mineral chemistry and whole rock geochemistry of the Kuh-e-Kalut-e-Ghandehari monzonitic stock indicate a calc-alkaline magmatic series similar to the subduction-related magmas in the normal continental magmatic arcs formed during the mantle metasomatism. According to the the temporal and geological situation, as well as the geochemical characteristics of the Kuh-e-Kalut-e-Ghandehari stock, it is considered as a part of an arc magmatism, related to the subduction of Neo-Tethyan oceanic crust beneath the CEIM (Central–East Iranian Microcontinent) during the Late Mesozoic and Early Cenozoic eras.

We are grateful to the University of Isfahan and the Department of Geology of Kanazawa University (Japan) for their supports. We are also grateful to anonymous reviewers for their useful comments and suggestions that improved the quality of this paper.

The Kuh-e-Kalut-e-Ghandehari is located about 40 Km northwest of the Anarak (northeast of Isfahan). This area is a part of the Central Iran that is situated in west of the Yazd block; near to the Ashin ophiolite and drastic directional changes of the Great Kavir Fault. In the northwest of Anarak, the Ashin ophiolite is covered by the Cretaceous limestones, and crossed by the Eocene volcanic rocks. The Upper Eocene gabbro-diorite stock cross cuts the Eocene volcanic rocks. Andesites are the predominant unit of the Eocene volcanic rocks. They are grey to dark- grey in colour and have amphibolite xenolith. The main textures of these volcanic rocks are porphyritic, poikilitic, trachytic, sieved texture and glomeroporphyritic. The main phenocrystic minerals of andesites are plagioclase, amphibole and clinopyroxene situated in a matrix of the same minerals and sanidine, magnetite and apatite. Secondary minerals are formed as a result of the hydrothermal alteration and comprise zeolite, chlorite, epidot, sericite, kaolinite and calcite. Mineral chemistry indicates that plagioclases are andesine to labradorite (An= 30-52%), K-feldspars are sanidine (Or= 68-71%), amphiboles are calcic with pargasite composition (Mg#= 0.58-0.71), and clinopyroxenes are diopside (Mg#= 0.91-0.93) in composition. Whole rocks chemical analyses indicate that these andesite and trachy-andesite rocks have calc-alkaline property. The chondrite and primitive mantle-normalized REE and multi-element patterns show LREE and LILE enrichment and HFSE (Ti, Ta, Nb) depletion, which are characteristics of the subduction-related magmas. These data indicate that the primary magmas were genarated by medium degrees of partial melting of the lithospheric mantle spinel lherzolite. Subduction of the Central-East-Iranian Microcontinent (CEIM)-confining oceanic crusts (Naein and Ashin oceanic crusts) are possibly the reason of this volcanism. Geochemistry of these volcanic rocks are similar to the arc-related ones.

The south Natanz volcanic zone located in the central part of Urumieh-Dokhtar magmatic arc and the north of Isfahan. The area is dominated by the Eocene volcanic rocks ranging from acidic to intermediate and relatively basic affinity. Petrologically, they consist of rhyolite, dacite, andesite, basaltic trachyandesite and basaltic andesite. Mineralogically speaking, except for the acidic rocks, the other rocks are mainly made of plagioclase and clinopyroxene up. According to EPMA, the composition of clinopyroxene and plagioclase is augite and andesine to bytownite. Having features such as zoning and sieve texture in phenocrysts of plagioclase, roundness, and corrosion gulf in minerals belonging to the volcanic rocks reflecting the existence of unequilibrium during magma solidification. Considering geochemical features of these rocks, we can realize that they are calc-alkaline with potassium from intermediate to high LILE enrichment, and negative Nb-Ti anomalies. On the spider diagram, primitive mantle- normalized with Pb-enrichment can be seen which may be attributed to crustal assimilation. The negative anomalies of Nb and Ti on the spider diagram reflect subduction process, the crustal involvement in magmatism processes, or an indication of deficiency of these elements at the origin, the stability of Nb-Ti bearing phases during partial melting or separating these elements in the course of fractional crystallization process. The chondrite- normalized REE pattern points to LREE enrichment in comparison with HREE. The volcanic rocks studied have been originated in a magmatic arc.

Clinopyroxene is one of the most common of the rock forming minerals. Its long formation period (from the earliest crystallization of magma in the core of phenocrysts to the final microcrystalline crystallization in the rock background) can show the history of the host magma crystallization. The composition of clinopyroxene, especially those phenocrysts, in volcanic rocks could well establish the magmatic nature of the host lava.The clinopyroxene composition can point out the magmatic series, the tectonic environment and the source rock (Kushiro, 1960; Nisbet and Pearce, 1977, Leterrier et al., 1982). In addition, it is possible to estimate the temperature and pressure of rock formation by studying the chemistry of clinopyroxenes (Nimis and Taylor, 2000; Putirka, 2008).The study area is located in the middle part of the Urumieh- Dokhtar magmatic belt. It exactly lies in the area between Tarq and Mazdeh, its longitude is 51° 43' to 52° 00' E and its latitude is 33° 15' to 33° 30' N. The Eocene magmatic rocks vary from rather basic to acidic in composition, but they are mainly intermediate and they are rather basic rocks (Ghadirpour, 2017). Previously, various studies on using the chemical composition of the major elements of clinopyroxene were conducted to discover the conditions for the formation of igneous rocks in different parts of Iran (Sayari and Sharifi, 2016; Falahaty et. al., 2016; and Mohammadi et. al., 2017).So far, in all of these studies that have been conducted on volcanic rocks, the mineral chemistry of clinopyroxene has not been used to evaluate the magma's features such as temperature, pressure, and oxygen fugacity. In this article, we are going to study the mentioned features of magma by using chinopyroxene chemistry. To determine the geotectonic setting and the physicochemical conditions of volcanic rocks, thirty thin sections have been prepared. Their minerals and texture have been studied by using polarizing binocular microscope (Olympus BH-2). After detailed mineralogy and selection of suitable samples, microprobe analysis is done by EPMA (JEOL- JXA) in the Naruto University, Japan. The mineral analysis is performed at 15 nA intensity of current and accelerate voltage of 15 Kev. The study area is situated in the South of Natanz, between Tarq and Mazdeh villages. The volcanic rocks vary from acidic to rather basic (basaltic andesite to andesite and rarely rhyolite). Microlitic porphyric, glomeroporphyric and vesicular are some textures which are observed in the volcanics. Plagioclase and euhedral clinopyroxene occasionally with simple twinning are characteristic minerals of rocks.According to Wo-En-Fs diagram (Morimoto, 1989), clinopyroxene shows mainly the composition of augite.It is concluded that the magmatic series of rocks is calc-alkaline which is in relation to the subduction of the Neotethys oceanic crust under the central Iranian plate.There are several diagrams that are used for this purpose which are as follows.The Al2O3-Ti2O diagram (Le Bas, 1962): In this diagram, the studied clinopyroxene shows the nature of calc-alkaline. One of the diagrams used to determine the tectonic setting according to clinopyroxene composition is the F1- F2 diagram (Nisbet and Pearce, 1977). Based on this diagram, the Tarq- Mazdeh volcanic rocks belong to the magmatic arc environment.The Clinopyroxene temperatures are calculated by using a variety of methods which indicate that most of clinopyroxene in temperature range of 1150 to 1200°C has been crystallized (Soesoo, 1997). The temperature indicates changes in crystallization of clinopyroxene. The calculated temperatures of clinopyroxenes by using various methods show that they are crystalized in the temperature range of 1150 to 1200˚c. It mainly means that there is a change in temperature during clinopyroxene crystallization. By considering the barometric diagram, the pressure of clinopyroxene formation has been determined below 10 kb, in the depth range of 2 to 5 km.