نوع مقاله : مقاله پژوهشی
نویسندگان
1 دانشجوی دکتری گروه زمینشناسی، واحد علوم و تحقیقات، دانشگاه آزاد اسلامی، تهران، ایران
2 استادیار گروه زمین شناسی، دانشگاه آزاد اسلامی، دانشگاه علوم و تحقیقات، تهران، ایران
3 استادیار گروه زمینشناسی، واحد سواد کوه، دانشگاه آزاد اسلامی، سوادکوه، ایران
4 دانشیار گروه زمینشناسی، دانشگاه شهید بهشتی، دانشکده علوم زمین، تهران، ایران
5 استادیار گروه زمینشناسی، واحد کهنوج ، دانشگاه آزاد اسلامی، کهنوج، ایران
کلیدواژهها
عنوان مقاله English
نویسندگان English
Introduction:
The Central Alborz has sedimentary rocks from Precambrian to Quaternary. This area is part of the Alborz Mountain range, located in the eastern Central Alborz Zone. It lies between longitudes 50°15’ to 50°77’ and latitudes 35°50’ to 36°15’ North. Alborz’s volcanic activity was strong during the Tertiary period, peaking in the late Eocene and Oligocene. After a calm phase, intense activity resumed in the Pliocene. Eocene and Oligocene eruptions across Alborz, including the Qazvin highlands, Takestan, and areas around Tehran, mainly produced andesite, dacite, and rhyolite. These eruptions also created many ignimbritic and tuffaceous deposits, along with pyroclastic fall deposits and pumice lahar flows (Gholami, 2001).
Sampling Method: During fieldwork, we used a systematic sampling method, collecting 70 samples. We prepared thin sections from these samples for closer examination. To analyze the major, minor, and trace elements, we selected 18 whole-rock samples. These were analyzed using Inductively Coupled Plasma – Mass Spectrometry (ICP-MS) at Kansaran Binalood and Zar Azma companies in Iran.
Discussion:
After studying the thin sections, we identified four extrusive igneous rock groups: 1) Foliated Basaltic Trachyandesite, 2) Trachyandesite, 3) Olivine Basalt, and 4) Lamprophyres. Diagrams by Lobas et al. (1986) and Middlemost (1994) show that volcanic rock samples from Polur and the Haraz road fit within Trachybasalt, Basalt, and Trachyandesite categories. Based on the classification by Irvine & Baragar (1971), these rocks fall within the alkaline field and near the subalkaline boundary (Tchameni et al., 2006). In the Verma et al. (2006) diagram, the alkali basaltic samples are in the continental rift basalts and OIB (Ocean Island Basalt) fields. The Xu et al. (2015) diagram places these samples in the crustal contamination field, showing magma mixing with crustal components during ascent.
Some sources note that a high Ce/Pb ratio is a feature of mantle-derived ocean island basalts (OIB) (Lustrino, 2005). In the Ce/Pb diagram against Ce for the region's alkaline basalts and primary magma, samples fall within the mantle MORB/OIB range. However, some samples show continental crust characteristics due to crustal contamination and increased lead levels. In the Th/Yb diagram against Ta/Yb, the basaltic samples fit within the OIB range, indicating an asthenospheric origin with some enrichment.
Eskandari's (2016) geodynamic model for Damavand magmatism suggests that the lower crust, lithospheric mantle, and asthenospheric mantle all contribute in different amounts to forming primary magma. To determine the tectonic setting of the studied rocks, we used several diagrams. The samples indicate that these rocks have alkaline traits and formed in an intra-continental rift environment. Unlike the simple fractionation process, LREEs and some LILEs come from alkaline basalts to trachyandesites. Multi-element diagrams show that these elements and P are less in trachyandesites than in basalts. Rare element patterns in these diagrams don’t clearly show a tectonic environment. They show traits of both subduction and OIB environments. A look at some crustal contamination indicators, like elemental ratios, reveals that trachyandesites have more contamination than alkaline olivine basalts. High ratios of Ce/Pb, Nb/U, Ba/Nb, and Th/La, along with low ratios of Sm/La in alkaline olivine basalts, indicate these lavas are influenced more by crustal materials.
Conclusion
Along the Haraz road and near Polur, we find volcanic rocks ranging from alkali olivine basalt to trachyandesite. Instead of a simple differentiation trend, we observe a depletion of LREE (Light Rare Earth Elements) and some LIL (Large Ion Lithophile) elements from alkali basalts to trachyandesites. Multi-element diagrams also show a decrease in these elements and Phosphorus (P) in trachyandesites compared to basalts. The trace element patterns in these diagrams reflect characteristics typical of an OIB environment. Comparisons of crustal contamination indices suggest that trachyandesites have undergone more crustal contamination than alkali olivine basalts. Overall, these rocks show alkaline features and were formed in an intra-continental rift setting.
کلیدواژهها English
