Petrography, Geochemistry, and Petrogenesis of Plutonic Rocks in Vineh Area, South of Central Alborz

The intrusive body in the south of Vineh, located in the north of Karaj city, is one of the several late Eocene plutons that intruded into the volcano-sedimentary Karaj Formation in the south of Central Alborz. This intrusive body comprises monzogabbro, monzodiorite, monzonite, and syenite with an alkaline shoshonitic nature and geochemically is cogenetic, evolved through fractional crystallization. The rocks are medium to coarse-grained with a dominant hypidiomorphic granular texture and consist of plagioclase, olivine, clinopyroxene, amphibole, orthoclase, and quartz. Titanite, apatite, biotite, and opaque occur as accessory minerals, whereas, epidote, chlorite, calcite, and iddingsite as secondary minerals. Geochemical data such as LREE enrichment relative to HREE, Pb positive anomaly, and depletion of Nb, Ta, Zr, Ti, as well as major, minor, and trace element data indicate that primary magma of these rocks formed in an active continental margin under the influence of Neo-Thetys subduction components beneath Central Iranian microplate. Alternatively, based on tectonic discrimination diagrams, the study of plutonic rocks is mainly attributed to the post-collision tectonic regime. Therefore, it seems that the magma originated from a low degree of partial melting (3 to 5 percent) of phlogopite-spinel peridotite source at a depth of about 60 to 65 km in an extensional back-arc basin as a result of slab rollback in the late Eocene, following the subduction of Neo-Thetys in Central Iran. The generated melt during the ascent underwent assimilation and fractional crystallization in lower depth magma chamber. 

The intrusive body in the south of Vineh, located in the north of Karaj city, is one of the several Late Eocene bodies that intruded into the volcano-sedimentary Karaj Formation in the south of Central Alborz zone. The evolution of the Cenozoic Alborz Magmatic Arc Belt (AMAB) is regarded as the back arc of the Urumieh-Dokhtar Magmatic Belt (UDMB) 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., 2021). One of the most considerable episodes of magmatism in Iran was an extensive flare-up magmatism that developed principally in the UDMB and the AMAB throughout the Eocene-Oligocene (Berberian and King, 1981; Verdel et al., 2011; Asiabanha and Foden, 2012). This magmatism is distinguished by intermediate rock compositions from calc-alkaline to shoshonitic nature occurring in an extensional arc setting (Verdel et al., 2011; Agard 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 monzonite sill. In the south of Karaj Dam basement sill in Vineh village area, an outcrop of monzogabbro to syenite sill hosted by the Karaj Formation is investigated to clarify the petrological and geochemical characteristics. To achieve this purpose, field relationships, rock textures, and chemical analyses for different rock types are presented. The data and the findings of previous studies (e.g., Asiabanha and Foden, 2012; Maghdour-Mashhour et al., 2015; Sepidbar et al., 2021) are subsequently employed to infer the type of the geodynamic regime of the Alborz throughout the Cenozoic.

The study area is located in the north of Karaj city between the northern geographical latitude of 35° 51´ 02" and 35° 54´ 10", eastern longitude of 51° 00´ 23" and 51° 03´ 17", and geologically in southern-central Alborz structural zone (Figure 1). The dominant rock types in the area consist of basic lavas, tuffs, and clastic rocks accumulated from the Middle to Late Eocene, creating 3 to 5 km thick Karaj Formation in Central Alborz (Dedual, 1967). The Karaj Formation in the study area was intruded by Vineh sill that seems to be coeval with the Karaj Dam basement sill during Late Eocene-Early Oligocene (Maghdour-Mashhour et al., 2015). The contact of the intrusion with country rocks is sharp in the field.

Thirty rock specimens from different outcrops were collected from Vineh sill. Subsequently, based on field evidence and thin section petrography, 8 specimens were selected and analyzed by ICP-OES (major elements) and ICP-MS (trace elements) techniques at Zarazma Company. The accuracy of measurements was within 5% for major and 10%-15% for trace elements. Table 1 shows the results of chemical analyses.

Vineh intrusive rocks with mainly hypidiomorphic texture are composed essentially of plagioclase, clinopyroxene, and olivine as well as minor amphibole, biotite, and K-feldspar. In monzonite and syenite, amphibole, biotite, and K-feldspar are dominant in addition to plagioclase and clinopyroxene. Chlorite, calcite, epidote, and iddingsite are secondary minerals, whereas apatite, titanite, zircon, and opaque are minor. The rocks are porphyritic at the margin and have medium to coarse-grained equigranular texture in the center of the sill. The geochemistry of eight specimens plotted on the rock classification diagram of Middlemost (1994) shows monzodiorite, monzonite, and syenite (Figure 5A), and on the normative diagram of Streckeisen and LeMaitre (1979), monzogabbro, monzodiorite, monzonite, syenite, and alkali feldspar syenite (Figure 5B). This intrusive body with an alkaline shoshonitic affinity is geochemically cogenetic and evolved through fractional crystallization.

Geochemical data show that the rocks are alkaline and shoshonitic in nature and the chondrite-normalized REEs diagram, exhibits LREE enrichment relative to HREEs. In the primitive mantle-normalized multi-element diagram (Sun and McDonough, 1989), the patterns of rocks show enrichment of LILE (e.g., Ba and Rb), Pb positive anomaly, and depletion of HFSE (Nb, Ta, Ti, Zr), the outstanding characteristics of subduction-related magmatism. It seems that the magma originated from a low-degree partial melting (3 to 5 percent) of a phlogopite spinel-lherzolite mantle at a depth of 60 to 65 km in an extensional back-arc basin due to slab rollback following the subduction of Neo-Thetys beneath Central Iran. In other words, the parent magma formed as a result of fluids and sediments derived from the Neo-Tethys oceanic crust caused metasomatism of subcontinental lithospheric mantle followed by extension-related decompression melting of phlogopite spinel-lherzolite mantle by the heat supplied by rising of the asthenosphere. The generated melt during the ascent underwent assimilation and fractional crystallization in lower depth magma chambers.

The authors are very grateful to the reviewers of the Journal of Economic Geology for their constructive ideas in improving the scientific structure of the article.