پترولوژی و زمین‌ شیمی سنگ‌ های آتشفشانی ائوسن جنوب‌ شرق خور (استان اصفهان، ایران مرکزی)

سنگ های آتشفشانی ائوسن با ترکیب تراکی بازالت و تراکی آندزیت بازالتی در جنوب‌شرق شهر خور (استان اصفهان) در راستا و نزدیک گسل ترکمنی اوردیب، رخنمون خوبی دارند. این منطقه در زون ساختاری خرد قاره ایران مرکزی و حاشیه شرقی بلوک یزد واقع‌شده است. بافت غالب این سنگ­ها پورفیریتیک، میکرولیتیک پورفیریتیک، پوئی کیلیتیک و گلومروپورفیریتیک است. الیوین، پلاژیوکلاز، کلینوپیروکسن و ارتوپیروکسن کانی های اصلی سازنده این سنگ‌ها بوده که به صورت فنوکریست دیده می‌شوند. این سنگ‌ها کالک آلکالن هستند. در این منطقه علاوه بر گدازه‌ها،  نهشته های آذرآواری که اغلب شامل توف است، نیز وجود دارند. تشکیل افق بنتونیتی منطقه خور، محصول دگرسانی این توف هاست. الگوی عناصر مختلف بهنجارشده به کندریت و گوشته اولیه بیانگر غنی‌شدگی از lree و lile و تهی شدگی از عناصر گروه hfs (نظیر ti، ta و nb) است که از ویژگی‌های ماگماهای مرتبط با فرورانش است. الگوهای بسیار مشابه این سنگ‌ها از نظر عناصر نادر خاکی و ناسازگار در نمودارهای بهنجارسازی، گویای پتروژنز یکسان این سنگ‌هاست. محیط زمین‌ساختی سنگ های آتشفشانی احتمالاً یک کمان ماگمایی وابسته به فرورانش در حاشیه خرد ­قاره ایران مرکزی بوده است. ویژگی زمین‌شیمیایی نشان می­دهد که ماگمای سازنده سنگ های منطقه احتمالاً در اثر ذوب بخشی گوه گوشته ای متاسوماتیزه شده در بالای اسلب فرورونده ایجاد شده و محصول ذوب بخشی یک اسپینل لرزولیت گوشته لیتوسفری بوده است.

Petrology and geochemistry of Eocene volcanic rocks from southeast of Khur (Isfahan province, Central Iran)

IntroductionSubductionrelated magmas are characterized by enrichment of large ion lithophile elements (LILEs), light rare earth elements (LREEs) and depletion in high field strength elements (HFSEs) (Harangi et al., 2007). These geochemical signatures of magmatic rocks are commonly explained by the addition of hydrous fluids from subducting oceanic lithosphere combined with the flux of melts from subducted sediments to the mantle wedge, lowering the mantle solidus and leading to magma generation (Aydınçakır, 2016).Asthenospheric mantle, subcontinental lithospheric mantle and/or lower crust may be the principal source of these rocks (Eyuboglu et al., 2018). In addition, magma differentiation processes, such as fractional crystallization, crustal contamination, and magma mixing may also play an important role in the genesis of these rocks.This research study presents new petrological and geochemical data from the volcanic rocks with NW–SE trending, which are situated in the northwestern margin of the Central –East Iranian Microcontinent (CEIM) (southeast of Khur, Isfahan Province) which have been formed during the peak activity of Eocene. Study of this typical small volume subduction related magmatism will be useful in understanding the origin and geological evolution of the Central Iran in Cenozoic. Analytical MethodsThe petrographic investigations on Eocene volcanic rocks from the SE of Khur area were carried out with an optical microscope (OlympusBH2) in the petrology Laboratory of the University of Isfahan, Iran. Major and trace element concentrations of samples from whole rocks were obtained by a combination of inductively coupled plasma mass spectrometry (ICPMS) and inductively coupled plasma atomic emission spectroscopy (ICPAES) at the Als Chemex Laboratory of Ireland. The chemical compositions of 4 samples (B865, B866, B867, and B868) were determined by Neutron Activation Analysis (NAA) in the Isfahan Activation Center.The detection limit was 0.01% for all major element oxides and 0.01 ppm for rare earth elements.Mineral abbreviations were adopted from Whitney and Evans (2010). Results and DiscussionEocene volcanic rocks with trachybasalt and trachybasaltic andesite composition are exposed in the northwestern part of the CentralEast Iranian Microcontinent (CEIM) (SE of Khur, Isfahan Province, Central Iran). These rocks which have a dominant northwestsoutheast trend crosscut the Cretaceous sedimentary rocks.Petrography and mineral chemistry analyses indicate that the predominant rockforming minerals of volcanic rocks are olivine, plagioclase, clinopyroxene and orthopyroxene. Phenocrysts set in a fine to medium grained matrix of the same minerals plus sanidine with minor amounts of opaque minerals. Secondary minerals are chlorite and calcite. The most common textures of these rocks are porphyritic, microlitic porphyritic, poikiolitic and glomeroporphyritic.Geochemical analyses of whole rock samples show that these rocks have been enriched in alkalies and large ion lithophile elements (Cs, K, Rb, Sr, Ba,), and have been depleted in high field strength elements (HFSE) (Ta, Nb, Ti). All samples indicate moderate to high fractionation in LREE patterns. These geochemical signatures point out to the subductionrelated calcalkaline nature of these rocks and their similarity to volcanic rocks of continental arcs or convergent margins (Yu et al., 2017).Pb enrichment and low values of Nb/La, Nb/U and Ce/Pb ratios reveal that crustal contamination has played an important role in magma evolution (Srivastava and Singh, 2004; Furman, 2007).The large volume of hydrous fluids coming from the subducted slab rather than sediments have caused enrichment and metasomatism of the subcontinental lithospheric mantle source.The geochemical characteristics of the studied rocks suggest that the parental magma have been derived from partial melting of a metasomatized spinel lherzolite of lithospheric mantle, which was previously modified by dehydration of a subducting slab. The tectonic environment, in which these rocks were formed has probably been a volcanic arc.Subduction of oceanic crust around the CentralEast Iranian Microcontinent (CEIM) is the most reasonable mechanism which can be used to explain enrichment in volatiles of the mantle, and the calcalkaline magmatism of the study area in Eocene times. AcknowledgmentsThe authors thank the University of Isfahan for financial supports.