Geochemistry and paleotectonomagmatic setting of metabasites protolith of metamorphic complex from NW Asadabad (W Hamadan)

Document Type : علمی -پژوهشی

Authors

1 Geology Department, Faculty of Earth Sciences, Imam Khomeini International University, Qazvin, Iran

2 Geology Department, School of Geology, Faculty of Science, University of Tehran, Tehran, Iran

Abstract

IntroductionProtolith investigation of metabasitic rocks and identification of their geodynamic setting is one of the significant subjects in metamorphic petrology and several tectonic discrimination diagrams have been invented in order to attain this goal. Metamorphic complex of North Asadabad is regarded a part of Sanandaj-Sirjan zone that cropped out in western Hamadan. Sanandaj-Sirjan belt is the most dynamic block of Iran’s tectonic zones and has experienced important compressional-extensional and magmatic events during Neotethys subduction. Materials and methodsMetamorphic rocks of this complex are mainly metapelite and there are some restricted exposures of massive and foliated metabasites in this complex as well. Amphibole, plagioclase, chlorite and epidote are rock-forming minerals of these metabasites that are indicators of greenschist facieses metamorphic conditions. Plagioclase composition is albite and amphibole is from tremolite and edenite types. Chemical composition of amphibole and plagioclase are compatible with greenschist facieses’ metamorphic conditions, which are isofacial with respect to the adjoining metapelites.Results and DiscussionWhole rock chemistry of 10 metabasitic samples from north Asadabad metamorphic complex were evaluated via ICP-MS method. According to the whole rock geochemical data, metabasites of NW Asadabad are of orthoamphibolite type and contain mostly alkali basal compositions. The patterns of REE and multi-elements spider diagram exhibit subduction-related characteristics. Geochemical signatures of these metabasites are compatible with intra plate and continental plate margin tectonic settings.According to modern tectonic setting discrimination diagrams and those logarithmic diagrams that have been based on multi-elements, the protolith genesis of metabasite from north Asadabad metamorphic complex is mostly linked to the subduction setting. Metabasite with subduction geochemical signatures, have been reported from the rest localities of northern Sanandaj-Sirjan zone like Songhor-Kangavar, Saghez-Pyranshahr and Shahindezh as well.Results and DiscussionProbably, metabasites from NW Asadabad, are representing a mafic magmatism in the continental back arc setting, arising from Neotethys oceanic basin subduction beneath continental lithosphere of Sanandaj-Sirjan zone, which has subsequently undergone collision or continental active margin metamorphism.

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