Mineralogy, Geochemistry and Petrogenesis of Mantle Peridotites of Nehbandan Ophiolitic Complex, East of Iran

Document Type : Research Article

Authors

• Hamid Karimzadeh ¹
• Mohammad Rahgoshay ¹
• Iman Monsef ²

¹ Shahid Beheshti University

² Institute for Advanced Studies in Basic Sciences (IASBS)

10.22067/econg.v12i2.76889

Abstract

Introduction
Ophiolites are a set of oceanic rocks with different appearance and mineralogy in the world's largest orogenic belt, from the Alpine to the Himalayas. The ophiolites of Iran are also located in this belt. Among ophiolites in Iran, the Nehbandan Ophiolitic Complex in the east of the country is of great importance. The complete ophiolitic sequence consists of two sets. The first is the crust sequence, gabbro, diabase and basalt, and the second is a mantle sequence or peridotites, both of which are sequences in the Nehbandan Ophiolitic Complex. The main purpose of this research study is mantle section. There are three study areas, located near the city of Nehbandan: Kalateh-Shahpori, Qadam Gah peridotites that are about 30 km northwest of the city of Nehbandan near the Chahar Farsang village and the third area is located between the Khansharaf village and Nasfandeh Kuh area that is 10 km east of Nehbandan.
 
Materials and methods
In this lithological and mineralogical research study, thin and polished sections were prepared from samples. The thin sections were analyzed by polarizing OLYMPUS microscope BH-2 and the polished sections were analyzed by the OLYMPUS BX-60 reflecting microscope. A CAMECA SX100 electron probe microanalyzer was used to determine the chemical composition of the minerals in samples. The analytical condition include 15 kV and 20 nA rays with periods of 10 to 30 seconds at peaks for different minerals that are analyzed at the electron probe microanalysis center in the University Of Toulouse, France. The stoichiometry of minerals was used to calculate the amount of Fe3+ for access to the structural formula of minerals (Droop, 1987.(
 
Results and discussion
In terms of petrography, the Kalateh Shahpori peridotites are of the Harzburgite type and the Nasfandeh Kuh peridotites are of the Lherzolite type. The Qadam Gah peridotites are both geographically and petrographically indicative of the state of transition between the two other regions. The mineralogy of the Kalateh Shahpori peridotites is composed of olivine (Fo91), orthopyroxene (En90 Fs9), and the Cr-spinel is of the high Cr type. The Nasfandeh Kuh peridotites have olivine minerals that are Chrysolite (Fo89), orthopyroxene (En89 Fs9) and (En86 Fs10), clinopyroxene (En46 Fs5 Wo49) and, the Cr-spinel is of the high Al type. The Qadam Gah peridotites are composed of olivine (Fo90), orthopyroxene (En89 Fs9.5), clinopyroxene (En47 Fs3 Wo50) and, the Cr-spinel is of the medium Cr type.
According to geochemical data and petrogenesis, the Kalateh Shahpori harzburgites are of the supra-subduction zone type in the forearc basin. The Nasfandeh Kuh Lherzolites are of the middle-oceanic type. The Lherzolites of Qadam Gah have the same characteristics of both regions in terms of the formation environment. However, they are much more similar to the middle-oceanic peridotites. The degree of partial melting of the peridotite has a direct relationship with the Cr content and it has an inverse relationship with the Al2O3 content in the chromium-spinel of the peridotite (Hellebrand et al., 2001). Probably, these lherzolites formed due to the re-fertilization of harzburgites (Monsef et al., 2018). Accordingly, Kalateh-Shahpori harzburgites with 20% partial melting are of high-grade, and the Nasfandeh Kuh Lherzolites with 5% partial melting are of the low grade type. The herzolites of the Qadam Gah are approximately 11% partial melting and are located between the Kalateh Shahpori peridotites and the Nasfandeh Kuh peridotites. The high degree of melting in the Harzburgites may indicate their remelting in the fluid environment because the hydrosis condition increases the degree of partial melting of peridotite (Hirose and Kawamoto, 1995). The Cr# in Cr-spinel, and the Mg# in olivine of the peridotites indicate the presence of at least 3 types of peridotites in the Nehbandan Ophiolitic Complex. According to mineralogy, petrography, geochemistry, and petrogenesis studies of the peridotites in the Nehbandan ophiolitic complex, it is recommended to explore possible chromite deposits, high melting and supra-subduction harzburgite zones such as Kalateh Shahpori harzburgites which should be considered to be the first priority. Then the peridotites of transition regions such as Qadam Gah should be at second priority and finally the low melting middle-oceanic lherzolites such as the Nasfandeh Kuh shuld be considered to be the third priority.
 
References
Droop, G.‌T.‌R., 1987. A general equation for estimating Fe3+ concentrations in ferromagnesian silicatesand oxides from microprobe analyses, using stoichiometric criteria. Mineralogical Magazine, 51(361): 431–435.
Hellebrand, E., Snow, J.‌E., Dick, H.‌J.‌B. and Hofmann, A.‌W., 2001. Coupled major and trace elements as indicators of the extent of melting in mid-ocean-ridge peridotites. Nature, 410(1): 677–681.
Hirose, K. and Kawamoto, T., 1995. Hydrous partial melting of lherzolite at 1Gpa: the effect of H2O on the genesis of basaltic magmas. Earth and Planetary Science Letters, 133(3): 463– 473.
Monsef, I., Rahgoshay, M., Pirouz, M., Chiaradia, M., Michel Grégoire, M. and Ceuleneer, G., 2018. The Eastern Makran Ophiolite (SE Iran): evidence for a Late Cretaceous fore-arc oceanic crust. International Geology Review, 60(1):1–27

Keywords

• Mineralogy
• Geochemistry
• petrogenesis
• Mantle peridotite
• Nehbandan ophiolite
• East of Iran

20.1001.1.20087306.1399.12.2.2.1

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