Petrogenesis of A-Type Khorasanlu Granitoid, Southeast of Zanjan: evidence for Eocene Extensional Magmatism, in Western Alborz, Iran

Document Type : Research Article

Author

• Mojgan Salavati

Assistant Professor, Department of Geology, Lahijan Branch, Islamic Azad University, Lahijan, Iran

10.22067/econg.2022.70403.1023

Abstract

Khorasanlu intrusive body with monzonite to granite composition is located 75 km south-east of Zanjan city in the Western Alborz. This intrusive body has intruded into the Eocene rocks of the region.
Based on petrographic and geochemical studies, the Khorasanlo intrusive complex have alkali-calcic to alkaline nature and are peraluminum and ferroan. Enrichment of light rare earth elements, relatively high amounts of HFSE compared to other granitoids, high Ga/Al ratio and placement of samples in the A-type granitoids field on the discrimination tectonic setting diagrams, indicate that the Khorasanlu intrusion mass is A₂-type granitoid and was generated in the post-collisional setting. It seems that the tension resulting from the rollback or increase of the subducted plate dip and asthenosphere uplifting, after the compressional tectonics regim, caused the melting of the crust and generation of the Khorasanlo young alkaline granitoids.
 
Introduction
Different types of magmatism are found in large igneous rock states (Ernst 2014; Jowitt and Ernst, 2013; Hari et al., 2018). In Iran have been reported many types of granitoid rocks I, S, and A, at different times and geological settings and recently, there have been many reports about the presence of A-type granite in different parts of Iran, which are generally the result of post-collision extensional activity (Alirezaei and Hassanzadeh, 2012, Shafaii Moghadam et al., 2015; Azizi et al. 2017; Honarmand et al., 2017). Recent studies on tertiary magmatism in Alborz show the effect of the subduction pattern in the region as continental magmatism (Valizadeh et al., 2008). In Zanjan province, in the western part of the Alborz mountain range, the Khorasanlu granitoid mass is exposed into the Eocene rocks. This paper has been attempted to study the geochemistry of the Khorasanlu granitoid mass as a part of the Alborz-Azerbaijan Tertiary magmatism from the viewpoint of genesis and tectono-magmatic setting.
 
Materials and methods    
For this study, 60 rock samples were collected from different types of intrusive. After microscopic studies, 16 samples were chosen and were analyzed in SGS Lab in Toronto, Canada. Major elements by the ICP-95A method and trace elements by the IMS-95A method were analyzed. than in the following, with IGPET and GCDKIT software were drawn diagrams and interpreted and analyzed.
 
Geological of the study area   
The studied body is located 75 km south-east of Zanjan city and north of Abhar and adjacent to the villages of Gawdara and Alvand. The studied area is located between the structural zones of the West-Alborz - Azerbaijan (Nabavi, 1976), Central Iran (Stöcklin, 1997), as the northeast of the study area is a part of the Soltanieh Mountains. The Paleozoic and Mesozoic Alborz formations are outcrop there; in general, almost all parts of the area are covered by Cenozoic sedimentary units and igneous rocks.
 
Result and Discussion 
Field and petrographic studies of the Khorasanlu mass, representing a variety of monzonite, quartz monzonite to granite in these rocks. The dominant texture of these rocks is granular to porphyry granular, and is less visible to perthitic, granophyre, graphic, poikilitic, and sometimes mirmecitic texture. The dominant volume of rocks is formed by alkali feldspars, plagioclase and quartz. Also, biotite, pyroxene, amphibole (green hornblende), and opaque minerals are visible in rocks. Based on the results of the geochemical analysis of the studied samples have, the SiO₂ 56.04 to 71.2 wt.% contents, relatively low TiO₂ contents (0.00 to 0.84 wt.%), relatively high Al₂O₃ (12.6 to 17.2 wt.%), Low CaO and MgO content (1.5-23.5 wt.% and 0.1-44.49 wt.% respectively), high Alkali contents (Na₂O 2.9 to 3.7 wt.% and K₂O 4.50 to 6.48 wt.%) and Fe₂O₃tot contents between 2.81 to 7.37 wt.%. 
The studied granitoid rocks in the chondrite-normalized spider diagram show subparallel, linear and homogeneous REE profiles with a moderate positive slope from HREE to LREE, the REE patterns have a low ratio of LREE/HREE, highly fractionated LREEs, enrichment of LREE (Ce/Yb)_N =2.29 to 4.66), and significant negative Eu anomaly. A high ratio of HREE to LREE is one of the characteristics in subduction regions rocks. In the ORG-normalized spider diagram (Pearce et al., 1984), the studied granites show Ba, Ta and Nb, negative anomalies while Rb, Th, Ce, and U have positive anomalies. Which is similar to the pattern of post-collision granites (Pearce et al., 1984). In the primitive mantle-normalized trace element diagram, most samples show the characteristic negative anomalies in Ba, Nb, Sr, P, and Eu.
 The samples of the study area were plotted in the alkali-calcic to alkalic fields on the Frost et al. (2001) diagram and indicate that they have peraluminous and mainly ferroan nature. Khorasanlu granitoid samples on discrimination diagrams (Pearce et al., 1984) and (Whalen et al., 1987) show within plate to arc setting granitoeids. Based on the separation diagrams of different types of granites, the studied samples with a high Ga/Al ratio indicate A-type nature also all samples show an A₂-type granitoid signature.
A₂-type group has many similarities to the elemental ratios of the average composition of crustal and island arc basalts. In this group, magma is formed from continental crusts or subducted crusts that have been displaced through a cycle of continental-continental collisions or island arc magmatism. This group is found in a wide range of tectonic settings and includes post-collision granitoids and post-orogenic granitoids (granitoids that are located at the end of a long period of heat flow and granitic magmatism) (Whalen et al., 1987; Eby 1992). Alborz mountain range in northern Iran is a region with active deformation, which is located in the deformation zone due to the collision of two Eurasian and Arabian plates (Allen et al., 2003; Zanchi et al. 2006). The study area is a part of the Alborz mountain range and is a part of active continental margin of neothetyan subduction zone.
And finally, based on all the evidence, it seems that Khorasanlu granitoid is formed in an extensional tectonic setting of the post collision in active continental margin during rising of asthenosphere and partial melting of lower crustal melts.

Keywords

• A type- granitoid
• Extensional magmatism
• Western Alborz
• Zanjan
• Iran

20.1001.1.20087306.1401.14.3.2.7

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