پترولوژی (Apr 2024)
Mineral chemistry and geochemistry of Kabbarik syenite stock, North of Zanjan
Abstract
The study area, Tarom, is situated in northwestern Iran, within the Zanjan province. Geologically, it falls within the western Alborz zone, a volcanic-plutonic belt, oriented northwest-southeast. This region, extending from Qazvin (west of Taleghan) to the north-northwest of Miane and north of the Manjil embayment, is bounded by the Abhar-Zanjan-Miane axis to the south (Ghorbani, 2009). Within this mountain range, numerous intrusive masses, primarily granodiorite, have intruded into the Eocene volcanic rocks. These masses, including the Bakhtar Takhestan, Khorasanlu, Zaker, and Chal masses, follow a northwest-southeast trend, aligning with the overall orientation of the Tarom mountains. They are arranged in two parallel rows, cutting through the older volcanic formations.Geology of the areaThe most important rock units in the Tarom Mountains consist of Eocene pyroclastic rocks, including andesite, rhyolite, basalt, andesite basalt, and rhyodacite, as well as shear tuff and andesite tuff. The region's main elevations are composed of Eocene volcanic and volcano-clastic rocks, along with Oligocene intrusive masses, which form the bulk of the mountain unit. Oligocene trachytic deposits, which are deposited on the Eocene units at an angle of approximately 23 degrees, are exposed in the northern part of the Kabbarik syenitic porphyroid stock. The syenitic porphyroid stock has intruded into the Eocene volcanic and volcano-clastic rocks.This study is grounded in field surveys, sampling of the slightly intrusive mass, and the examination of thin sections. Additionally, chemical analyses were conducted on ten samples from Zarazma Company to determine major, minor, and trace elements using inductively coupled plasma mass spectrometry (ICP-MS). Two samples were also analyzed using electron microscopy. To analyze the mineral composition of Kabbarik rocks for major elements, a JEOL JXA-8100 Superprobe electron microprobe (EMP) equipped with an Oxford Instruments INCA EDS system was employed at the Institute of Geology and Geophysics, Chinese Academy of Science. A 5-micron spot size, a 20 nA beam current, and a 15 keV accelerating voltage were utilized during these analyses.PetrographyThe intrusive Kabbarik stock exhibits a porphyritic texture, characterized by euhedral to subhedral phenocrysts of alkali feldspar. Its primary mineral composition includes alkali feldspar (Orthoclase) constituting 65-70 Vol.%, followed by 10-15 Vol.% clinopyroxene (diopside), 5-7 Vol.% anorthoclase, and 3-5 Vol.% secondary minerals such as olivine, altered to iddingsite. Minor amounts of apatite, zircon, and dark minerals are also present. Orthoclase phenocrysts often display Carlsbad and polysynthetic twinning. Megacrysts of Orthoclase may contain tiny inclusions of olivine and clinopyroxene, exhibiting a poikilitic texture. Some crystals exhibit a sieve texture, and in certain cases, overgrowth on plagioclase results in a Rapakivi texture. Clinopyroxene (diopside) crystals are generally euhedral to subhedral, with some containing small dark mineral inclusions distributed along their edges and conforming to their shape. Subhedral crystals of iddingsitized olivine are scattered throughout the rock matrix in small quantities. The overall texture of the microlithic rock remains porphyritic.Mineral ChemistryThe chemical composition of clinopyroxene and alkali feldspar minerals from the syenitic stock was analyzed using a JEOL JXA-8200 electron microscope at the Department of Geology and Geophysics, Chinese Academy of Sciences.Based on relevant diagrams, the clinopyroxenes were classified as diopside-type (Eby et al., 1998; Morimoto, 1988). The chemical composition of the pyroxenes indicates a sub-alkaline, volcanic arc magmatic setting. EPMA analysis of the alkali feldspars revealed an anorthoclase composition (Deer et al., 1991).Whole Rock GeochemistryThe chemical analysis of significant, rare, and rare earth elements (ICP-MS) reveals a calc-alkaline composition for the intrusive mass of the syenite zone and its parent magma. Normalized spider diagrams compared to chondrite demonstrate a clear enrichment of light rare earth elements (LREE) and incompatible elements relative to heavy rare earth elements (HREE) in the examined rocks. Based on tectonic environment diagrams, the studied samples are classified as post-collisional.ResultsThe predominant rock formations in the Tarom Mountains consist of Eocene volcanic and volcaniclastic rocks, which constitute the bulk of the mountain range. The investigated syenitic stock, intruding into the overlying rocks, dates back to the Oligocene epoch. Based on spider diagrams, the absence of significant heavy rare earth element (HREE) depletion suggests a mantle that has been metasomatized by subduction-related fluids. Studies indicate that the analyzed samples originated in a setting associated with an arc and a post-collisional environment.
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