Adakitic post-Miocene magmatism of Qaraie sub-volcanic dome, Mahneshan area (west of Zanjan)

زمین شناسی اقتصادی

Volume 16, Issue 1 - Serial Number 40, 2024, Pages 51-78   

Document Type: Research Article

DOI: 10.22067/econg.2024.84571.1086

Authors

Sajad Moseni Badalabadi¹; Mir Ali Asghar Mokhtari^{* 2}; Hossein Kouhestani²

¹M.Sc., Department of Geology, Faculty of Science, University of Zanjan, Zanjan, Iran

²Associate Professor, Department of Geology, Faculty of Science, University of Zanjan, Zanjan, Iran

Abstract

Qaraie sub-volcanic dome in the west of Zanjan is part of the Urumieh-Dokhtar magmatic arc in the Central Iran zone. Qaraie dome with columnar joints was intruded into the Upper Red Formation sequence and Kahrizbeik granitoid intrusion with Upper Proterozoic age. Based on petrographical studies, this dome is composed of dacite-rhyodacite and consists of plagioclase, biotite, quartz, as well as occasionally hornblende and sanidine phenocrysts within the fine-grained groundmass. These rocks have a porphyritic texture and present vesicular plus flow textures. On the petrological diagrams, rock units of the Qaraie dome have dacite, rhyodacite, and trachy-dacite composition and indicate high-K calc-alkaline to shoshonitic nature. Based on primitive mantle normalized spider diagrams, samples of the Qaraie dome indicate positive anomalies of LILEs (Rb, Ba, Th, U, K, and Cs) along with negative anomalies of HFSEs (Nb, P, and Ti) together with distinctive positive anomaly of Pb. Chondrite-normalized REE patterns demonstrate LREE enrichment and a high ratio of LREE/HREE. Samples from the Qaraie dome demonstrate geochemical similarity with adakites and are classified as high-silica adakites. These rocks resulted from 25% partial melting of the Lower continental crust with garnet-amphibolite composition in a post-collisional setting.
 
Introduction
Neogene to Quaternary magmatism in NW of Iran occurred as sub-volcanic and volcanic acidic domes with an adakitic nature. Recent studies on Miocene and post-Miocene magmatic rocks from different parts of Iran have demonstrated that most of the dacitic-rhyodacitic rocks have an adakitic nature (Jahangiri, 2008; Jamshidi et al., 2015; Saadat, 2023). There are some small dacitic domes in the west of Zanjan (from the Qaraie village in the south to the Moghanlou village in the north) that have not been reported in published maps and geological reports. The Qaraie dacitic dome, the largest dome in this area, was marked as Kahrizbeik granitic intrusion in the Mahneshan 1:100000 geological map (Lotfi, 2001). Considering the importance of the Miocene-Pliocene sub-volcanic domes in the evolution of Iran's tectonic-magmatic settings, and their role in the formation of some Au-As mineralizations (e.g., Arabshah, Zarshouran, and Aghdareh in the Takab area; Najafzadeh et al., 2017), studying the Qaraie sub-volcanic dome can provide valuable information for this part of Iran.
 
Regional Geology
Based on Iranian tectono-stratigraphic zones, the Qaraie area is located in the Urumieh-Dokhtar magmatic belt within the Central Iran zone. This area is a small part of the Mahneshan 1:100000 geological map (Lotfi, 2001). Based on the prepared 1:25000 geological map for this study, the Qaraie area is composed of Cretaceous sedimentary rocks along with other rock units including the Lower Red Formation, Qom Formation and Upper Red Formation, and Pliocene conglomerate. Kahrizbeik granitoid with Upper Proterozoic age (Lotfi, 2001) is located in the central part of the area. The Qaraie sub-volcanic dome is exposed in the north of the Qaraie village. This sub-volcanic dome intruded into rock units of the Lower Red Formation and Kahrizbeik granitoid intrusion and revealed prismatic structure in marginal parts. There are some outcrops of dacitic sub-volcanic domes in the south of the Moghanlou village intruding into the limestones of the Qom Formation which had an important role in the formation of the Moghanlou Sb deposit (Bavi et al., 2023)
 
Materials and methods
This research includes field and laboratory studies. During the fieldwork, different rock units were identified and a geological map with a scale of 1:25,000 was prepared. In this base, 32 samples were collected from the Qaraie dome. Among the mentioned samples, 15 thin sections were examined using a transmitted polarized light microscope in the laboratory of the University of Zanjan. The chemical composition of rock samples (n = 15) was analyzed at the Zarazma Analytical Laboratories, Tehran, Iran using XRF and ICP–MS methods.
 
Results
Considering petrographical studies, the Qaraie dome compositionally includes dacite and rhyodacite. These rocks have porphyry along with glomeroporphyritic, vesicular, and flow textures. Dacites consist of plagioclase, biotite, quartz, and sometimes hornblende phenocrysts in the fine-grained groundmass. Sanidine presents along with the mentioned phenocrysts in rhyodacites.
Based on geochemical diagrams, Qaraie samples were classified as dacite, rhyodacite, and trachydacite. These rocks have a high-K calc-alkaline to shoshonitic affinity. Based on primitive mantle normalized spider diagrams, these rocks have similar patterns. These diagrams indicate positive anomalies of LILEs along with negative anomalies of HFSEs. Chondrite-normalized REE patterns demonstrate a steep slope pattern with LREE enrichment and a high ratio of LREE/HREE, devoid of specified positive and negative Eu anomaly (Eu/Eu* between 0.94-1.07), (La/Yb)_N and (La/Sm)_N ratio between 26.72-32.83 and 10.7-11.6, respectively.
Dacite-rhyodacites of the Qaraie dome demonstrate geochemical similarity with adakites and are platted in adakite field on Y vs. Sr/Y, La/Yb vs. Sr/Y, and SiO₂ vs. MgO diagrams. Based on Sr vs. CaO+Na₂O and Sr vs. Na₂O+K₂O diagrams, the Qaraie samples are classified as high-silica adakites.
 
Discussion and conclusion
Geochemical data including LILEs and LREEs enrichment and negative anomalies of HFSEs along with strong positive Pb anomaly demonstrate subduction-related magmatism for the Qaraie sub-volcanic dome. Based on tectonic-magmatic setting discrimination diagrams (Th vs. Ta, Th/Hf vs. Ta/Hf, Th/Ta vs. Yb, and Th/Yb vs. Ta/Yb diagrams), formation of the Qaraie dome has been related to an active continental margin tectonic setting. Based on the (La/Yb)_N vs. Yb_N diagram, the Qaraie adakitic dome resulted from 25% partial melting of garnet-amphibolite. Other diagrams such as SiO₂ vs. Ni indicate that the source rocks for the Qaraie adakitic dome resulted from a thick lower continental crust. Considering the Th vs. Th/Ce diagram, the Qaraie adakitic dome was formed in a post-collisional setting.
 
Acknowledgment
This 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 Economic Geology reviewers and editor are also thanked for their constructive comments.

Keywords

Geochemistry; Petrology; Sub-volcanic dome; Adakite; Qaraie; Orumieh-Dokhtar; Zanjan

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