ژئوشیمی و جایگاه زمین ساختی سنگ های آتشفشانی جنوب شرق میمه، شمال غرب اصفهان

برون‌زدهای آتشفشانی جنوب‌شرق میمه، در لبه غربی کمان ماگمایی ارومیه دختر و 80 کیلومتری شمال‌غرب اصفهان قرار گرفته اند.  این سنگ‌ها با طیف ترکیبی بازیک تا حدواسط دارای درشت بلورهای پلاژیوکلاز، پیروکسن، الیوین، آمفیبول و بیوتیت و بافت های میکرولیتی پورفیری و حفره ای هستند. غنی شدگی عناصر lil در مقایسه با hfses به ویژه nb و  tiهمراه با روند کاهشی شیب تغییرات عناصر کمیاب این سنگ ها، از ویژگی های شاخص ماگماتیسم مناطق فرورانشی است. بر اساس نمودارهای محیط زمین ساختی، نمونه ها در گستره حاشیه قاره پهنه های فرورانشی جای گرفته اند. داده های ژئوشیمیایی نشان می دهند که ماگمای اولیه از یک خاستگاه گوشته ا ی با ماهیت گارنت لرزولیتی که قبلا ًتوسط سیالات ناشی از ورقه فرورو غنی‌شده، پدید آمده است. به نظر می‌رسد فرورانش مداوم حجم بالای پوسته سرد اقیانوسی به داخل گوشته حین هم‌گرایی‌ ورقه های ایران عربستان، باعث برهم زدن ترازهای حرارتی گوشته‌شده و در نتیجه این آشفتگی، گوشته با فاز باقی‌مانده گارنت و نرخ ذوب‌بخشی پایین، ذوب‌شده و ماگمای آلکالن را به وجود آورده است. تفریق و آلایش پوسته ای ماگمای داغ بازیک حین صعود و جای‌گیری آن در پوسته، سنگ های حدواسط با ماهیت کالک‌آلکالن را پدید آورده است.

Geochemistry and tectonic setting of extrusive rocks in the southeast of Maimeh, northwest of Isfahan

IntroductionThe studied area is situated 15 km away from the southwest of Maimeh at the western part of UrumiehDokhtar magmatic arc. This zone is a part of the Zagros orogenic belt formed due to the subduction of the NeoTethyan oceanic crust under the Central Iran block. The magmatic activity in the UrumiehDokhtar magmatic arc has begun in Eocene (Alavi, 2004) and continued until Quaternary (Ghasemi and Talbot, 2006). In the middle part of the studied area, several outcrops of the postEarly Cretaceous volcanic rocks with basaltic to andesitic composition have been seen (Vahdati Daneshmand, 2006). Until now, no petrological and geochemical data about these rocks are present. Therefore, in this study, petrographic and the geochemical features of these rocks are discussed in detail. This study aims to reveal a better understanding of the petrology and petrogenesis of volcanic rocks in the southeast of Maimeh and the middle part of the UrumiehDokhtar magmatic arc as a part of the Zagros orogenic belt. Materials and methodsTo reach the goal of the research, after collecting basic information using geological maps and works done in the study area, all volcanic outcrops systematically sampled, and more than 50 fresh samples were chosen and studied. Afterward, seven samples were chosen for geochemical analyses by using inductively coupled plasma mass spectrometry (ICPMS) at the ACME Laboratories, Vancouver, Canada. The results of chemical analyses are listed in table 1. Discussion Based on the field observations, the volcanic rocks have basaltic to andesitic composition with plagioclase, clinopyroxene, olivine, amphibole, biotite, and opaque microphenocrysts. Clinopyroxene (probably augite) is the main minerals as phenocrysts and small mineral in the groundmass. Olivine phenocryst has undergone limited alteration to iddingsite and amphiboles show burned margin. Opacitization in amphibole occurs due to a decrease in water pressure with magma rising or as a result of the increase in temperature (Plechov et al., 2008). These rocks have microlithic porphyry, glomeroporphyry and vesicular textures. According to geochemical analysis, intermediate rocks have calcalkaline nature and basalt is alkaline. They display enrichment in LILEs (Rb, Ba, K, Sr, U, and Th) relative to HFSEs (especially Nb, Ti, and P) and coherent REE patterns characterized by enrichment in LREEs relative to HREEs without negative Eu anomaly. These features are characteristics of subductionrelated magmatism (Woodhead et al., 1993). U and Th enrichment may be due to crustal contamination (Kuscu and Geneli, 2010) or the addition of pelagic sediments and/or altered oceanic crust to the source of magma (Fan et al., 2003). The tectonic discrimination diagrams show an active continental arc setting for these rocks. Geochemical evidence shows that the volcanic rocks were originated from low degree partial melting (<0.1) of the enriched mantle with Cpx garnet lherzolitic composition in 80 km depth. Mantle enrichment is due to the addition of aqueous fluids derived from dehydration of the subducted oceanic crust. It seems that the continuous subduction of cooled oceanic crust into the mantle along with convergence between Arabia and Central Iran plates led to low degree partial melting of the mantle and producing alkaline magmas. The ascending parental magma was differentiated and undergone AFC processes until rising from the crust. In these processes, the alkaline basalt under the influence of fractional crystallization and crustal contamination turned into intermediate compositions of calcalkaline andesite. It seems that these rocks were formed from the subduction of NeoTethyan oceanic crust under the Iranian microplate in an arc magmatic zone. ResultsThe postEarly Cretaceous volcanic rocks in the southeast of Maimeh is situated in the western part of UrumiedDokhtar magmatic arc and includes most basic to intermediate associations. The rocks have the porphyritic texture with basalt to andesite composition and are characterized by alkaline to calcalkaline affinity and enrichment in LIL elements (Rb, Ba, Th, U and …) relative to HFSE with negative Ti and Nb anomalies and highly differentiated pattern of rare earth elements, as evident in spider diagrams normalized to primitive mantle and chondrite. The significant features are mainly a result of subductionrelated magmatism. Tectonomagmatic diagrams suggest an arcrelated tectonic setting for these rocks. Based on the geochemical evidence, the volcanic rocks originated from low degrees (>1) partial melting of a garnet lherzolitic mantle source that enriched by slabderived fluids. The magma has undergone AFC processes during ascending and alkaline affinity changed to calcalkaline nature. The volcanic rocks occurred as a result of the subduction of the NeoTethyan oceanic crust beneath the Central Iran microplate. References Alavi, M., 2004. Regional stratigraphy of the Zagros foldthrust belt of Iran and its proforeland evolution. American Journal of Science, 304(1): 1–20. http://doi.org/10.2475/ajs.304.1.1Fan, W.M., Guo, F., Wang, Y.J. and Lin, G. 2003. Late Mesozoic calcalkaline volcanism of postorogenic extension in the northern Da Hinggan Mountains, northeastern China. Journal of Volcanology and Geothermal Research, 121(1–2): 115–135. http://doi.org/10.1016/S03770273(02)004158Ghasemi, A. and Tabatabaei Manesh, S.M. 2015. Geochemistry and petrogenesis of Ghohroud igneous complex (Urumieh–Dokhtar zone): Evidence for Neotethyan subduction during the Neogene. Arabian Journal of Geosciences, 8(11): 9599–9623. http://doi.org/10.1007/s1251701518837Kuscu, G.G. and Geneli, F., 2010. Review of post collisional volcanism in the Central Anatolian volcanic province (Turkey), with special reference to the Tepekoy volcanic complex. International Journal of Earth Sciences, 99(3): 593–621. https://doi.org/10.1007/s0053100804024Plechov, P.Y., Tsai, A.E., Shcherbakov, V.D. and Dirksen, O.V., 2008. Opacitization conditions of hornblende in Bezymyannyi volcano andesites (March 30, 1956 eruption). Petrology, 16‌(1): 19–35. http://doi.org/ 10.1134/S0869591108010025Vahdati Daneshmand, F., 2006. Geological map of Maimeh, Scale 1:100000, Sheet No. 6256. Geological Survey of Iran. Tehran, Iran.Woodhead, J., Eggins, S. and Gamble, J., 1993. 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