Petrology and Geochemistry of Sill, Dikes and Intercalated Lavas of Karaj Formation, East of Tehran

Document Type : Research Article

Authors

Asal Hasan Harati ¹

Amir Ali Tabbakh Shabani ²

Morteza Delavari Kooshan ²

¹ Ph.D. Student, Department of Geochemistry, Faculty of Earth Science, Kharazmi University, Tehran, Iran

² Associate Professor, Department of Geochemistry, Faculty of Earth Science, Kharazmi University, Tehran, Iran

10.22067/econg.2026.1166

Abstract

In the east of Tehran and the south of the Central Alborz Subzone, subvolcanic rocks are exposed as sills, dykes, and intercalated lavas within the Karaj Formation. The age of this complex is middle to upper Eocene and, based on petrographic and geochemical analyses, they have a basaltic composition. The main minerals in all the investigated samples are olivine, plagioclase, and clinopyroxene, set in a microcrystalline matrix of similar minerals. Minerals resulting from alteration include chlorite, sericite, calcite, epidote, and zeolite. Their textures are diverse and include intergranular, microlithic-porphyritic, intersertal, and ophitic to subophitic in subvolcanic rocks. The magmatic nature of the rocks is high-potassium calc-alkaline. All samples display similar rare-earth element (REE) patterns, supporting a genetic relationship, which may reflect varying degrees of partial melting or fractional crystallization from a common mantle source. Geochemical modeling based on incompatible element concentrations indicates that ~30–40% fractional crystallization of plagioclase, clinopyroxene, and olivine can account for the observed compositions and suggests that the samples are genetically related through this process .The multi-element spider diagram normalized to the primitive mantle shows depletion of immobile incompatible elements such as Nb, Ta, Zr, and Ti, as well as enrichment of mobile incompatible elements, which are the main characteristics of subduction zones. The Alborz is considered the back-arc of the Urumieh-Dokhtar magmatic belt, developed during Paleogene slab rollback of the Neo-Tethyan oceanic plate. The rocks originated from a phlogopite-bearing spinel lherzolite mantle source, with a minor contribution from garnet lherzolite, at depths of ~90–100 km.
 
Introduction
The extrusive igneous rocks, located in the east of Tehran, are one of the several Middle to late Eocene bodies intruded into the volcano-sedimentary Karaj Formation in south of Central Alborz zone. The evolution of the Cenozoic Alborz magmatic arc belt is regarded as the back arc or rear arc of the Urumieh-Dokhtar magmatic belt which is related to the Neo-Tethys subduction and the continental collision between the Arabian and Eurasian plates (e.g., Asiabanha and Foden, 2012; Maghdour-Mashhour et al., 2015; Sepidbar et al., 2019). One of the most considerable episodes of the magmatism of Iran was an extensive magmatic flare-up that occurred principally in the UDMB and the AMAB during the Paleogene (e.g., Verdel et al., 2011). This magmatism is characterized by basic to intermediate rock compositions from calc-alkaline to shoshonitic affinity occurred in an extensional arc setting (Verdel et al., 2011; Shafaii Moghadam, et al., 2018). In Central Alborz several Late Eocene intrusive bodies intruded Karaj Formation such as Mobarakabad gabbro, Lavasan syenite, Shekarnab monzonite and Karaj Dam basement gabbro to monzoite sill. In the Bumehen and Rudehen areas outcrops of volcanic rocks hosted by Karaj Formation is investigated to clarify the petrological and geochemical characteristics.
 
Materials and Methods
After field sampling and thin section preparation at Kharazmi University, ten least-altered samples were selected for geochemical analysis. Sample preparation was carried out using the lithium borate fusion method at Zarazma Mineral Studies Company. Major elements were analyzed by ICP-OES, and trace and rare earth elements were determined by ICP-MS. LOI of the samples was determined using electrical oven at about 1000 °C. Iron oxidation ratios were determined by the method of Middlemost (1989). The rock analysis results are presented in Table 1.
 
Result
The studied rocks are divided into three main groups: subvolcanic rocks, intercalated lavas of the Karaj Formation, and younger post-Middle Eocene lavas. All are dark gray to black and mainly consist of plagioclase, olivine, and clinopyroxene with microlitic-porphyritic and intergranular textures. Some plagioclase crystals show sieve textures, indicating magma mixing or rapid pressure decrease (Fig. 3D) (Tsuchiyama, 1985; Nelson and Montana, 1992). Vesicles in the intercalated lavas are filled with secondary minerals such as calcite and zeolite. Geochemically, the samples are predominantly basaltic (Fig. 4A).  Their magmatic series, based on the K₂O–SiO₂ diagram, vary: the post-Middle Eocene lavas are mainly high-K calc-alkaline, the subvolcanic samples fall within both calc-alkaline and high-K calc-alkaline fields, one sample is in the shoshonitic field, and the intercalated lavas of the Karaj Formation also fall within the shoshonitic field. Considering the general northward increase in potassium across the Iranian and Alborz magmatic arc (Asiabanh, 2001; Asiabanha et al., 2009), it is likely that samples with calc-alkaline and shoshonitic characteristics were originally derived from a high-K calc-alkaline source. Multi-element spider diagrams and REE patterns show enrichment relative to primitive mantle, enrichment in incompatible lithophile elements (LILE), and depletion in high-field-strength elements (HFSE), reflecting the influence of subduction in arc magmatism. HREE depletion suggests the presence of garnet in the source. Geochemical modeling based on fractional crystallization demonstrates that some magmatic suites could be derived from a similar parent melt. The overlap between REE patterns and multi-element normalized diagrams of basaltic samples and the modeled melt confirms the genetic relationship of certain samples through crystal fractionation, although not all samples display this connection.
 
Discussion
Geochemical and mineralogical data suggest that the parental magma was not primary but derived from a metasomatized, enriched mantle source affected by subduction fluids. The low Mg#, Ni, and Cr contents further confirm its non-primary nature and indicate modification of the mantle source before magma generation. The Zr/Y diagram (Sun and McDonough, 1989) places the samples within the enriched mantle field, likely reflecting the presence of a hydrous phase such as phlogopite or amphibole. The Rb/Sr versus Nb/Th diagram (Furman and Graham, 1999), further supports phlogopite as the dominant hydrous phase, indicating a relatively deep magma source consistent with thickened continental arc lithosphere (Tabbakh Shabani et al., 2017). The Ce/Yb versus Ce diagram (Ellam, 1992) estimates the mantle source depth at approximately 90–100 km, and the high Ce/Yb ratios (>10) suggest low-degree partial melting with garnet in the residue (Mattsson and Oskarsson, 2005). Geochemical modeling using Sm/Yb and La/Yb ratios indicates that the parental magma was likely derived from a mixed spinel-garnet lherzolite source, with 3–5% partial melting required for magma generation (Zhao and Zhou, 2009). From a tectonic perspective, the Alborz region lies along the Alpine-Himalayan orogenic belt, and the studied samples show strong similarities to other Paleogene magmas in the region (Harangi et al., 2006; Irannezhadi, 2009). The Nb/La versus La/Yb diagram shows that most samples fall at the boundary between alkaline arcs and continental arcs, with only a few samples fully within the alkaline arc field (Hollocher et al., 2012). Previous studies also report that post-Eocene volcanism exhibits continental arc characteristics (Asiabanha and Foden, 2012). Magma formation occurred during a post-subduction extensional phase following the rollback of the Neotethyan slab in the Eocene, which promoted asthenospheric upwelling and provided the heat necessary for partial melting of the lithospheric mantle (Maghdour-Mashhour et al., 2015; Heidari et al., 2022). Accordingly, the observed geochemical transition from calc-alkaline to more alkaline compositions during the Eocene can be attributed to this process (Verdel et al., 2011; Nabatian et al., 2014; Maghdour-Mashhour et al., 2015; Ashrafi et al., 2018; Shafaii Moghadam, et al., 2018; Ahmadvand et al., 2021). Paleogene magmatism initially occurred in the forearc region through explosive eruptions and intercalated lava flows, and subsequently migrated to the back-arc region (~45–40 Ma) (Sepidbar et al., 2019). Overall, the temporal sequence and geochemical characteristics of the samples indicate a coupled forearc–backarc magmatic process associated with lithospheric extension and Neotethyan slab rollback, which played a critical role in the evolution of Paleogene magmatism in NW Iran.
 
Conclusion
Three main groups of subvolcanic rocks, intercalated lavas of the Karaj Formation, and younger post-Middle Eocene lavas with the basaltic composition outcropped in the east of Tehran. Multi-element spider diagrams and REE patterns show enrichment relative to primitive mantle, enrichment in incompatible lithophile elements (LILE), and depletion in high-field-strength elements (HFSE), reflecting the influence of subduction in arc magmatism. Magmatism occurred during a post-subduction extensional phase following the rollback of the Neotethyan slab in the Eocene, which promoted asthenospheric upwelling and provided the heat necessary for partial melting of the lithospheric mantle.
 
Acknowledgments
The authors sincerely appreciate the reviewers of the Journal of Economic Geology for their valuable comments and constructive suggestions, which significantly improved the quality of this paper.

Keywords

Dike

Sill

Lava

Geochemistry

Central Alborz

Karaj Formation

East of Tehran

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