Petrology of Oligocene Ghaleh Yaghmesh granitoids in the west of Yazd province

• Published in: Journal of Economic Geology Vol. 8; no. 2; pp. 473 - 491
• Main Authors: Bahareh Fazeli, Mahmoud Khalili, Roy Beavers, Mahin Mansouri Esfahani, Zahra Loghmani Dastjerdi
• Format: Journal Article
• Language: English; Persian
• Published: Ferdowsi University of Mashhad 01.02.2017
• Subjects: Eocene-Oligocene; Ghaleh Yaghmesh; I-type granitoid; Urumieh-Dokhtar; Volcanic arc
• ISSN: 2008-7306
• DOI: 10.22067/econg.v8i2.43200

Introduction The generation and evolution of granitic magmas has been a hot debated subject among petrologists. The diversity of their origin has led different authors to propose that these rocks are not simple in their origin and might be generated in more ways than one. In the past several decades, many petrologists used a variety characteristics to subdivide the granitoid rocks. Such proposals have of course been forward by Chappell and White (1974) for the granitoids of Eastern Australia. They divided these granitoids into two distinct types (I-and S-type granitic rocks), which they interpreted as being derived from igneous and sedimentary source rocks, respectively. The Ghaleh Yaghmesh plutonic massif is located in the most western part of Yazd and it forms a part of the Urumieh-Dokhtar magmatic belt. The belt is response to subduction of Neo-Tethyan oceanic crust beneath central Iran (Alavi, 1994). During Cretaceous-Late Tertiary, numerous granitoid bodies were exposed in this belt, many of which have been studied by a number of workers (e.g. Sepahi and Malvandi, 2008; Honarmand et al., 2013; Kananian et al., 2014). The massif composed of diorite, quartzdiorite, tonalite, granodiorite and granite (Oligocene) intruded into the Eocene volcanic and pyroclastic rocks including rhyolite, rhyodacite, andesitic, rhyodacitic and rhyolitic tuff. The main purpose of the present paper is to describe the petrography, and whole rock geochemistry of the Ghaleh Yaghmish granitoids as well as discussing their petrogenetic and tectonic significance in the light of the regional geological framework of the study area. Materials and methods After field studies and sampling, fifty thin sections were prepared for petrographic study. Twenty-one fresh samples were selected for XRF chemical analysis performed at the Southern Methodist University (Dallas - USA). Thin polished sections of granodiorite rocks were prepared for composition determining and structure formula calculation of amphibole minerals by Cameca SX50 microprobe device at the Oklahomacity University (Norman - USA). Results The studied plutonic rocks are dominated by plagioclase, orthoclase, quartz, amphibole (magnesio hornblende and actinolite hornblende), biotite, and pyroxene. Zircon, apatite, sphene, tourmaline and opaque minerals as the common accessory and chlorite, epidote and calcite are the secondary minerals. On the base of petrographic observation as well as mineral-chemistry and geochemical data, the granitoid massif is classified as I-type (magnetite series), calc – alkaline and metaluminous composition. The rocks under discussion are characterized by the high level of LILE (Ba, Sr, K and Cs) and the negative anomaly of HFS elements (Ti, Nb, Zr and Y) indicating the subduction related magmatism. The Ghaleh Yaghmesh granitoids are cogenetic and possibly developed in subduction zone related to active continental margin calc – alkaline volcanic arcs. Mixing process of acidic and basic magmas is likely involved in generation of the rocks being studied. Discussion The parent magma probably formed by partial melting of amphibolites with some sedimentary materials. Fractional crystallization of melt in the higher levels of crust gave rise to various rock types. Mantle – derived basaltic magmas emplaced into the lower crust most likely provide heat for partial melting )Clemens et al., 2011(. Field evidences such as the presence of mafic microgranular enclaves having sharp boundaries with the host rocks (Zorpi et al., 1989; Didier, 1991), petrographic observations (similar mineralogy of MME and the host rock (Didier, 1991; Didier and Barbarin, 1991), the occurrence of accicular apatite (Zorpi et al., 1989; Didier, 1991), the corroded margin of amphibole and plagioclase (Zorpi et al., 1989; Shelley, 1993) and the abundance of biotite and hornblende in MME compared to the host rock (Ellis and Thompson, 1986)) and geochemical criteria (range of silica from 51.35 to 70.78) indicate that magma mixing process was likely responsible for the formation of the rocks being studied. Acknowledgements The authors would like to thank the University of Isfahan for the financial support. We also thank the Southern Methodist University (SMU) (Dallas - USA) for the XRF chemical analysis undertaken for this project. References Alavi, M., 1994. 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