Sr-Nd Isotope Geochemistry and Tectonomagmatic Setting of Granitoid Intrusions of Balazard Prospecting Area, Southwest of Nehbandan

Document Type : Research Article

Authors

• Roohollah Miri Bydokhti ¹
• Mohammad Hassan Karimpour ²
• José Francisco Santos ³
• Mahdi Bemani ⁴

¹ Assistant professor, Department of Geology, Faculty of Science, University of Gonabad, Gonabad, Iran

² Professor, Department of Geology, Faculty of Science, Ferdowsi University of Mashhad, Mashhad, Iran

³ Associate Professor, Department of Geosciences, Geobiotec Research Unit, University of Aveiro, 3810-193 Aveiro, Portugal

⁴ Assistant professor, Mining engineering Department, Faculty of Engineering, University of Gonabad, Gonabad, Iran

10.22067/econg.2023.83438.1083

Abstract

Balazard prospecting area is located in eastern Iran, about 120 km south-west of Nehbandan in the Central part of Lut Block. This area consists of exposed Eocene volcanic rocks, intruded by several diorite and monzodiorite dykes and stocks. These intrusive rocks display porphyritic textures with mm-sized phenocrysts, most commonly of plagioclase, clinopyroxene and hornblende, embedded in a fine-grained groundmass with variable amounts of plagioclase, quartz, and opaque minerals. The assessment of geochemical properties of the major elements showed that these granitoids are metaluminous and of high-K calc-alkaline variety. The patterns of trace elements are identical, denoting enrichments in the light REE (LREE) to heavy REE (HREE). Moreover, LILE enrichment relative to HFSE and Nb, Ti, and P show negative anomalies. The Eu/Eu* ratios range from 0.96 to 0.76, which means that the plagioclase is a slag remnant in the origin of magma. The (87Sr/86Sr)i values of the assessed intrusive rocks range from 0.706 to 0.707, assuming an Oligocene age, while the εNdi values are between -1.9 to -3.2. These results manifest that the magmas were contaminated by continental crust. The contamination has probably occurred over the ascent of magma to crustal levels, and geochemical data verifies the preposition that the investigated intrusions were intruded in a volcanic belt resting on a subduction zone. Early magmas were created through the melting of mantle wedge peridotite, occasioning magma differentiation through crystal fractionation and crust contamination as they ascended to crustal levels.
 
Introduction
Balazard prospecting area is located 120 km of the south-west of Nehbandan in South Khorasan Province, Iran. The area is part of the volcanic-plutonic belt known as Lut Block, which owns an elongated shape stretched north-southwardly. Nehbandan Fault forms the eastern boundary of Lut Block. This is while it has been bordered by the Great Kavir Fault in the north, and by Nayband Fault in the west. The southern part of the block is likely defined by South Jazmourian Fault. Karimpour et al. (2012) argue that ~65% of the rocks cropped out in Lut Block are volcanic and plutonic. The magmatic activity of Lut Block initiated over the Middle Jurassic, particularly between 165-162 million years ago, which was associated with the intrusion of Shah-Kuh batholith (Esmaeily et al., 2005). This activity reached its highest level over the Tertiary period, particularly over the Middle Eocene (Arjmandzadeh & Santos, 2014). Over half of Lut Block is overlain by volcanic and subvolcanic rocks of Tertiary. These rocks show a thickness of up to 2000 m. They have been developed as a result of subduction before the collision between the Arabian and Asian plates (Berberian & King, 1981).
There is a perceptible potential for a variety of mineralization in Eastern Iran and particularly in Lut Block, which is due to its past tectonism as a subduction zone that led to extensive magmatic activity. The Middle Eocene to the Early Oligocene (30-39 million years ago) is perceptible, particularly, in respect of magmatism and mineralization (Karimpour et al., 2012). Karimpour et al. (2012) believe that a great deal of the magmatism and mineralization events in eastern Iran have occurred over the Tertiary. However, mineralization associated with Cretaceous magmatism has also been identified, such as Sn-Cu mineralization observed in Cretaceous monzonitic rocks in Kalateh Ahani (Karimpour et al., 2012). Here we present new geochemical data (elemental and isotopic) from shallow intrusive rocks, with the aim of providing a more detailed understanding of the petrogenetic processes and geodynamic evolution of Lut Block.
 
Material and methods
A total of six samples from unaltered intrusive rocks at Balazard prospecting area and representing the main lithologies were grabbed to chemically analyze major and trace elements through petrographic assessment. Major and trace elements were determined using fused disks and a Philips PW 1410 spectrometer through wavelength-dispersive X-ray fluorescence (XRF) spectrometry. The chemical analysis was undertaken at Amethyst Laboratory in Mashhad, Iran. Inductively Coupled Plasma-mass Spectrometry (ICP-MS) was applied to analyze four of the samples for trace elements at Acme Laboratories in Vancouver (Canada). The samples were tested by a lithium metaborate/tetraborate fusion and total digestion with nitric acid prior to analysis. Four whole-rock samples of Balazard granitoid rocks were analyzed for their Sr and Nd isotopic compositions at the University of Aveiro's Laboratory of Isotope Geology in Portugal.
The ground samples were treated with a HF/HNO3 solution in Teflon Parr acid digestion bombs, which were heated at 200°C for three days. The ultimate solution was evaporated and the samples were then dissolved in HCl (6.2 N) in acid digestion bombs and dried again. The elements were purified before being analyzed by means of a two-stage conventional ion chromatography technique: a) Sr and REE elements were separated in an ion exchange column by AG8 50W Bio-Rad cation exchange resin; b) Nd was purified from other lanthanides by means of columns with Ln Resin (ElChrom Technologies) cation exchange resin.
All reagents that were applied to prepare the samples were sub-boiling distilled, and the water was produced using a Milli-Q Element (Millipore) apparatus. Sr was loaded onto a single Ta filament with H₃PO₄, while Nd was loaded onto a Ta outer side filament with HCl in a triple filament arrangement. The ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd isotopic ratios were determined using a Multi-Collector Thermal Ionization Mass Spectrometer (TIMS) VG Sector 54, with data acquired in dynamic mode and peak measurements at 1-2 V for ⁸⁸Sr and 0.5-1.0 V for ¹⁴⁴Nd. The Sr and Nd isotopic ratios were corrected for mass fractionation relative to ⁸⁸Sr/⁸⁶Sr = 0.1194 and ¹⁴⁶Nd/¹⁴⁴Nd = 0.7219. While the investigation was in progress, the SRM-987 standard yielded an average ⁸⁷Sr/⁸⁶Sr value of 0.710266 ± 14 (conf. lim 95%, N = 13) and a ¹⁴³Nd/¹⁴⁴Nd value of 0.5121019 ± 75 (conf. lim 95%, N = 12) in comparison to the JNdi-1 standard.
 
 
Result and discussion
Balazard intrusive rocks consist of diorite, quartz diorite, and quartz monzodiorite, and exhibit characteristics typical of high-K calc-alkaline rocks from a volcanic arc setting. The primitive mantle-normalized trace element spider diagrams display significant enrichment in LILE, including Rb, Sr, Ba, Zr, Cs, and Th, and depletion in some HFSE, such as P, Nb, and Y. Chondrite-normalized plots exhibit LREE enrichment and a significant La/Yb fractionation.
Balazard granitoid rocks have (⁸⁷Sr/⁸⁶Sr)i values that vary between 0.7064 and 0.7066. In terms of isotopic compositions, Balazard granitoid rocks have εNdi between -1.9 and -3.2. The geochemical data are commensurate with the settlement of the investigated intrusions in a magmatic belt above a subduction zone suggesting contamination through being exposed to the continental crust during magma ascent to crustal levels. Balazard granitoid rocks display a range of (⁸⁷Sr/⁸⁶Sr)i values between 0.7064 and 0.7066. Additionally, the isotopic compositions of the rocks show εNd_i values ranging from -1.9 to -3.2. These geochemical characteristics suggest that the investigated intrusions were emplaced in a magmatic belt above a subduction zone and were subsequently contaminated during magma ascent to continental crust.
 
Acknowledgements
The authors wish to thank Mrs. Sara Ribeiro (Laboratório de Geologia Isotópica da Universidade de Aveiro) for the TIMS analysis. This research was financially supported by the Geobiotec Research Unit (funded by the Portuguese Foundation for Science and Technology, through project PEst-OE/CTE/UI4035/2014, University of Aveiro, Portugal).

Keywords

• Geochemistry
• Subduction Zone
• Balazard
• Nehbandan
• Lut Block

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