پترولوژی (Apr 2023)
Lithological sequence, geochemistry and Sr, Nd and Pb isotopic data of Marshoun volcanic rocks, North Abhar (Tarom-Hashtjin subzone)
Abstract
Marshoun area located 120Km Southeast of Zanjan, is a part of the Tarom-Hashtjin metallogenic-magmatic subzone within the Alborz-Azarbaijan zone. Similar to most parts of the Alborz-Azarbaijan zone, the Eocene-Oligocene volcanic and the intrusive rocks of this subzone were formed as a result of the Alpine orogenic phase, which has a close spatial and temporal relationship with metallic mineralization (Kouhestani et al., 2019). Several studies have been conducted on metallic mineralizations in different parts of the Tarom-Hashtjin subzone. The petrological studies carried out in this subzone are mainly focused on intrusive rocks (e.g., Seyed Qaraeini et al., 2020) and volcanic rocks' geochemical and petrological characteristics have been less considered. Marshoun area is composed of volcanic-sedimentary sequences which are hosts for Pb-Zn-Cu mineralization (Kouhestani et al., 2019). A detailed scientific study has not been done on the lithological sequence and their geochemical and petrological characteristics in the Marshoun area so far. In the present study, the lithological and geochemical characteristics including Sr, Nd, and Pb isotopic data, as well as the tectonomagmatic environment of the volcanic rocks of the area have been investigated.Materials and methodsDuring fieldwork, a 1:25000 geological map prepared from different lithological units of the area and over 30 samples were taken. Also, 17 thin sections for petrographical studies, 10 samples for chemical and 4 samples (2 andesites and 2 dacites) for iaoopic analyses. Chemical analyses (XRF and ICP–MS methods) were carried out at Zarazma Laboratory, Tehran, Iran., and isotopic studies (i.e. Nd, Sr, and Pb isotope studies at Institute of Geology and Geophysics, Chinese Academy of Geosciences, Beijing, China.ResultsThe predominant rock units in the Marshoun area are Eocene acidic tuffs, dacitic-rhyodacitic lava, and occasionally ignimbrite at the base and alternation of intermediate tuff with minor andesite and basaltic andesite intercalation in the top, along with some intrusive rocks with (Zajkan intrusion), and some gabbroic dykes.Zajkan intrusion including pyroxene quartz monzodiorite, quartz monzodiorite, and granodiorite composition intruded acidic volcano-sedimentary rocks with a total thickness of 930 meters can be divided into 9 parts.Volcanic rocks of the Marshoun area are classified as rhyolite, rhyodacite, dacite, andesite, basaltic andesite, and trachy-andesite with high-K calc-alkaline affinity. Dacitic-rhyodacitic rocks have porphyritic, flow, and spherolitic textures, composed of plagioclase, quartz, alkali feldspar, and mafic minerals (amphibole and biotite) set in a quartz-felspathic groundmass whereas, andesitic rocks show porphyritic, glomeroporphyritic, and amygdaloidal textures, composed of plagioclase and mafic minerals (amphibole and some pyroxene) set in a fine-grained and occasionally microlithic groundmass.All samples under study on primitive mantle normalized spider diagrams, have similar patterns indicative of their genetic relations. LILEs and HFSEs. negative anomalies are remarkable features of these rocks. Chondrite-normalized REE patterns demonstrate a relatively steep to low slope pattern with LREE enrichment and a high ratio of LREE/HREE, (La/Yb)N, and (La/Sm)N ratio between 3.8-30.1 and 1.2-8.25, respectively. On tectonomagmatic setting discrimination diagrams, volcanic rocks of the Marshoun area have been formed in an active continental margin tectonic setting. Isotopic data of Sr (0.70485-0.70622), Nd (0.512695-0.712733), and Pb (206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb between 18.743-18.803, 15.5938-15.6112 and 38.8138-18.0721, respectively) point to dominant role of mantle in the formation of the investigated rocks. According to the Pb isotopes, the area's acidic rocks originated either from a more enriched mantle or were contaminated by crustal materials during ascending magma.Discussion and ConclusionAs the geochemical data indicate the primary magma of Marshoun volcanic rocks is generated by the partial melting of subcontinental metasomatized mantle lithosphere as a result of the subduction process within the continental margin environment. According to data obtained from the present study as well as the previous research, it can be concluded that the result of the subduction of the active continental margin and the shortening of the crust in Alborz during the Eocene gave rise to the thickening of continental crust and further led to the separation and subsidence of the lower part of the subcontinental lithospheric mantle (delamination).As a result of this event, the ascending of asthenosphere currents has led to an increase in the thermal gradient and partial melting of the subcontinental lithosphere and generation of basic magma which during ascending contaminated by crustal materials. Finally, the differentiation process led to the formation of intermediate and acidic rocks.AcknowledgmentThis research study was made possible by a grant from the office of the vice-chancellor of research and technology, University of Zanjan. We hereby acknowledge their generous support. The Journal of Petrology reviewers and editor are also thanked for their constructive comments
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