| چکیده انگلیسی مقاله |
Introduction
The central Alborz, a tectono-stratigraphic terrane within the Alborz Mountains, underwent significant tectonic evolution linked to the Late Triassic Cimmerian orogeny and the subsequent collision between the Iranian microplate and the Eurasian plate. Previous geological investigations (e.g., Alavi, 1996; Stöcklin and Hassanzadeh, 2001) have delineated distinct tectono-stratigraphic units and proposed polyphase orogenic models for the northern Iranian Alborz. These studies highlight the tectonic stability of the central western Alborz from the Late Eocene to Late Miocene, correlating with Miocene sedimentation patterns in central Iran. Furthermore, studies of major intrusive bodies in the central Alborz (e.g., Ghorbani et al., 2014) have characterized their calc-alkaline to alkaline nature and syn-orogenic emplacement, with examples such as the Parachan intrusion exhibiting intermediate compositions (monzonite-monzodiorite) and shoshonitic to metaluminous geochemical signatures.
Regional Geology
Given the significance of understanding magmatism and related processes in the context of Neogene tectonomagmatic events in Iran, a detailed petrological and geochemical investigation of the Aroud intrusive body, located north of Alam Kuh and previously unstudied, is warranted. This study utilizes field observations, petrography, and whole-rock geochemistry to constrain the petrogenesis and magmatic processes responsible for its formation. The Aroud pluton, exhibiting significant outcrop (Baharfirozi et al., 2002), is situated within a complex geological setting encompassing units ranging from the Late Precambrian to Recent (Axen et al., 2001; Asiabanha et al., 2012; Alavi, 1996; Ballato et al., 2015; Stöcklin and Hassanzadeh, 2001; Esmaeli et al., 2007; Valizadeh et al., 2008). These include Precambrian sedimentary sequences, Paleozoic conglomerates and shales (equivalent to the Lalun Formation), Paleozoic dolomites and limestones, Ordovician micaceous sandstones, and Carboniferous (Mubarakeh Formation) carbonate units. The Aroud pluton, with a north-south trend, is mapped as Neogene in age (Baharfirozi et al., 2002) and its emplacement is likely linked to the major thrust tectonics of the central Alborz, suggestive of a back-arc setting. Detailed petrographic and geochemical analyses will elucidate the petrogenesis of this pluton and its implications for the regional tectonomagmatic evolution.
Petrography, Minerals
Petrographic analysis reveals the Aroud pluton comprises granite, quartz monzonite, and syenite. The granite is characterized by a granular to intergranular texture with quartz (∼30 vol%), plagioclase (∼45 vol%), and alkali feldspar (∼25 vol%) as major constituents. Biotite and amphibole are the dominant mafic minerals. Quartz monzonite exhibits major mineral proportions of plagioclase (∼55 vol%), quartz (∼15 vol%), and alkali feldspar (∼30 vol%). Syenite, which is volumetrically minor, is predominantly composed of alkali feldspar (∼88 vol%), primarily orthoclase, with minor plagioclase (∼8 vol%) and quartz (∼4 vol%).
Whole rocks Geochemistry
Whole-rock geochemical analyses (Table 1) indicate that the Aroud granites exhibit SiO₂ contents ranging from 69 to 72 wt%, and Al₂O₃ from 14 to 15 wt%. In contrast, the quartz monzonites and syenites display lower SiO₂ (quartz monzonite: 64-65 wt%; syenite: 60 wt%) and higher Al₂O₃ (quartz monzonite: 16-17 wt%; syenite: 17 wt%) amounts. Iron, magnesium, titanium, and phosphorus concentrations are also somewhat elevated in the quartz monzonites and syenite compared to the granites (Table 1). Total alkali versus silica (TAS) diagrams (Fig. 3A) and R1-R2 diagrams (Fig. 3B) classify all granitoid samples as calc-alkaline (Fig. 3C), magnesian, and alkali-calcic (Figs. 3D and 3E).
Geochemically, the samples plot at the boundary between volcanic arc granite (VAG) and within-plate granite (WPG) fields, closer to the VAG field. This suggests an extensional setting within a magmatic arc (back-arc), transitioning toward an intracontinental environment akin to a back-arc basin (Fig. 7). Major oxides and derived tectonic discrimination diagrams place the samples within the WPG field, trending towards the VAG field, characteristic of predominantly post-collisional granites. The geochemical evidence points towards a post-collisional extensional environment primarily associated with a back-arc setting (Fig. 8).
Discussion and Conclusion
The Aroud granitoid pluton, located north of Alam Kuh within the central Alborz structural zone, is interpreted to represent a post-collisional, extensional magmatic arc environment. The Aroud intrusive body exhibits diverse textures, including perthite and various intergrowths, reflecting a complex petrogenesis. Magmatic differentiation, water fugacity, decompression, fluid activity, and metasomatism are inferred as the primary factors influencing textural variability. Whole-rock and mineral chemistry data classify the Aroud samples within the shoshonitic series. Trace element and rare earth elements (REE) patterns, normalized to chondrite, reveal enrichment in LREEs and elements such as Pb, K, La, Rb, and Th, coupled with relative depletion in Ti, P, and Y. These geochemical characteristics indicate a complex magmatic history influenced by various petrogenetic processes. The geochemical signature is marked by enrichment in large ion lithophile elements (LILEs) and light rare earth elements (LREEs), coupled with relative depletion in high field strength elements (HFSEs) and a relatively steep HREE slope. These geochemical characteristics suggest a petrogenetic model involving delamination and partial melting of a mafic lower continental crust in the Aroud region, resulting in the generation of continental-type adakites (C-type adakites). Adakites are a specific type of subduction-related melt distinguished from typical granitoids of arc settings by their unique geochemical signature. Simplest models posit that adakites are predominantly generated through partial melting of the subducting oceanic crust. |