| Summary: | Introduction
The Tighanab area is located in the Southern Khorasan province and 104km south-east of Sarbisheh, in the eastern part of Sistan suture zone (Tirrul et al., 1983). The Sistan suture zone has formed as a result of collision between the Lut and Afghan blokcs and its closure time is related to upper Cretaceous era (Bröcker et al., 2013). Eocene-Oligocene magmatism in eastern Iran (Lut-Sistan) crop out as volcanic rocks, pyroclastic and subvolcanic rocks (Pang et al., 2013) which have caused skarn mineralization in some parts. The relationship between skarn mineralization and adakites has been discussed by various researchers (Lei et al., 2018). Skarn deposits and their associated Cenozoic plutonic rocks in Iran, have outcrops in northwest, central and southeast of the Urumiyeh-Dokhtar magmatic belt, Sabzevar-Dorouneh magmatic belt and the eastern Iran magmatic belt (Sepidbar et al., 2017). The Tighanab subvolcanic bodies play an important role in skarn mineralization.
This research study is carried out for studying petrography, geochemistry and tectonic setting of subvolcanic bodies and their role in skarn mineralization since geochemistry and petrology of the mentioned masses have not been studied.
Material and methods
This research is based on field observations, thin sections, polished thin section studies and chemical analysis of samples. In this regard, 90 thin sections were prepared and studied by microscope. Then, 11 samples of subvolcanic rocks with the least alteration were selected. Then they were crushed and powdered. Next, they were analyzed by the ICP-ES method for major elements and the ICP-MS method for trace and rare earth elements. The magnetic susceptibility of the samples was measured by SM20 magnetic sensitivity device at university of Birjand.
Results
The study area is located in the eastern part of the Sistan suture zone and the Mahirud geological map (1:100000). Quartzdioritic subvolcanic rocks intruded the Paleocene-Eocene limestone and sandstone and formed iron skarn mineralization. The main textures in quartz diorite porphyry are porphyry with microgranular groundmass and poikilitic. Plagioclase, hornblende and quartz are the main constitutes of these rocks. Plagioclase phenocrysts have polysynthetic twinnig, zoning and resorption rim and are andesine and rarely oligoclase based on extinction angle. Different geochemical diagrams show correlation between the Tighanab igneous rocks and intrusions associated with iron skarns. Geochemical features as mean of SiO2(64.48%), Al2O3(16.68%), Sr(470ppm), Y(8.9ppm), Sr/Y(55.58), Yb(0.89ppm) and poor negative anomaly of Eu are representative of high silica adakitic features for these rocks. The amount of Mg#(55.48-68.1), Sr/Y(mean55.58), Th/La(mean0.32), La/YbN(4.2) and Th(mean1.8ppm) indicate oceanic crust melting with garnet-amphibolite composition to generation of adakitic magma.
Discussion
Field evidence, mineralogy, and magnetic susceptibility measurements show that granitoids of the Tighanab area belong to the magnetite series. Based on tectonic discrimination diagrams, the intermediate samples of the Tighanab area are located in the range of VAG and VAG + Syn-COLG. The studied rocks show depletion of HFSE such as Ti, P, Nb, Yb, Y and enrichment in LILE that indicates their association with the subduction environment. Negative anomaly of HFSE may be a result of contamination of magma by crustal materials during ascent and emplacement in subduction zones. Comparison of some major and trace elements of Tighanab samples with adakites indicated that these rocks have high silica adakitic nature.
Geochemical evidence shows that the studied rocks are similar to the rocks associated with iron skarns. Some geochemical characteristics such as HREE and HFSE depletion, high Sr, Sr/Y and (Gd/Yb)N>1 and poor negative anomaly of Eu in the studied samples, indicate that the adakitic magma has been formed at pressures above the plagioclase stability. The geochemical characteristics of the studied samples, such as low Y and high Sr/Y ratio, indicate the presence of garnet in the origin of these rocks (Mao et al., 2018). Trace and rare element diagrams show that adikatic magma of the Tighanab area subvolcanic rocks have been produced by melting of the oceanic slab. Adakitic rocks of the Tighanab area have been formed from a source with 10 to 25% garnet amphibolites composition.
References
Tirrul, R., Bell, L.R., Griffis, R.J. and Camp, V.E., 1983. The Sistan suture zone of eastern Iran. Geological Society of America Bulletin, 94(1): 134–150.
Bröcker, M., Rad, G.F., Burgess, R., Theunissen, S., Paderin, I., Rodionov, N. and Salimi, Z., 2013. New age constraints for the geodynamic evolution of the Sistan Suture Zone, eastern Iran. Lithos, 170–171: 17–34.
Pang, K.N., Chung, S.L., Zarrinkoub, M.H., Khatib, M.M., Mohammadi, S.S., Chiu, H.Y., Chu, H.Ch., Lee, H.Y. and Lo, C.H., 2013. Eocene–Oligocene post-collisional magmatism in the Lut–Sistan region, eastern Iran: magma genesis and tectonic implications. Lithos, 180–181: 234–251.
Mao, Q., Yu, M., Xiao, W., Windley, B.F., Li, Y., Wei, X., Zho, J. and Lü, X., 2018. Skarn–mineralized porphyry adakites in the Harlik arc at Kalatage, E. Tianshan (NW China): Slab melting in the Devonian–early Carboniferous in the southern Central Asian Orogenic Belt. Journal of Asian Earth Sciences, 153: 365–378.
Lei, X.F., Duan, D.F., Jiang, S.Y. and Xiong, S.F., 2018. Ore–forming fluids and isotopic (HOCS–Pb) characteristics of the Fujiashan–Longjiaoshan skarn W–Cu–(Mo) deposit in the Edong District of Hubei Province, China. Ore Geology Reviews, 102: 386–405.
Sepidbar, F., Mirnejad, H., Li, J.W., Wei, C., George, L.L. and Burlinson, K., 2017. Mineral geochemistry of the Sangan skarn deposit, NE Iran: Implication for the evolution of hydrothermal fluid. Chemie der Erde-Geochemistry, 77(3): 399–419.
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