ویژگی های ژئوشیمیایی و ایزوتوپی پروتولیت اکلوژیت های شمال شهرکرد: شاهدی بر تکوین حوضه پشت‌ کمان نئوپروتروزوئیک در پهنه سنندجسیرجان

اکلوژیت های مورد بررسی بخشی از کمپلکس دگرگونی شمال شهرکرد هستند که به پهنه ساختاری سنندجسیرجان تعلق دارند. این اکلوژیت ها با ترکیب کانی شناسی غالب آمفیبول، گارنت، زوئیزیت و پیروکسن (امفاسیت) به‌صورت عدسی هایی همراه پاراگنایس های نئوپروتروزوئیک مشاهده می شوند. مجموعه کانیایی این سنگ ها نشان‌دهنده دگرگونی آنها در فشار بالا‌ست. با در‌نظر‌گرفتن سیستم نسبتاً بسته در تعادلات شیمیایی این سنگ‌ها در طی فرایندهای دگرگونی می‌ توان محیط تشکیل سنگ مادر این سنگ ها را مورد بررسی قرار داد. نمودار عناصر نادر خاکی بهنجار‌شده به کندریت، غنی شدگی جزئی از عناصر نادر خاکی سبک نسبت به عناصر نادر خاکی سنگین را نشان می دهد و نسبت la/yb)cn) در این نمونه‌ها بین 1.7 تا 2.7 متغیر است. نمودار عناصر نادر بهنجار‌شده به گوشته اولیه آنومالی منفی ti ،nb ،p و y را نشان می دهد. نمودارهای ژئوشیمیایی نشان‌دهنده تمایل زیاد این سنگ‌ها با بازالت های تولئیتی است که در محیط های کششی پشت‌کمان شکل گرفته‌ اند. بررسی‌های ایزوتوپی srnd نشان‌دهنده ماگمای اولیه این سنگ‌ها با ترکیب گوشته غنی‌ شده بالای زون فرورانش است. با توجه به الگوی پراگندگی عناصر و نسبت‌ های ایزوتوپی اولیه می توان تشکیل سنگ مادر اکلوژیت‌های منطقه را ناشی از کشش های پشت‌کمان در پهنه سنندجسیرجان در اواخر پروتروزوئیک پایانی (ادیاکاران) دانست.

Geochemical and Isotopic Characteristics of the Protolith of Eclogites from the North Shahrekord Metamorphic Complex: Evidence of Late Neoproterozoic Backarc Basin development in SanandajSirjan Zone

Introduction ;All of the major tectonic zones of Iran, except for the KopetDagh, contain PanAfrican crystalline basement. Subduction of the ProtoTethys at about 630 Ma caused Cadomian arc– and backarc magmatism in the northern margin of Gondwana (Hassanzadeh and Wernicke 2016). SanandajSirjan Zone with NWSE trend is the most active zone in Iran that extends to the southeast of Turkey and then to Bitlis Massif. There are many similarities between the Precambrian basement in the zone from Iran with Bitlis and Menderes massifs from Turkey.;The study area is a part of a largescale ductile shear zone, containing a wide range of metamorphic rocks with sedimentary and magmatic origins. Metasedimentary rocks comprise of paragneiss, various schists and metacarbonates that have cropped out through the shear zone which extends along the ZayandehRood River. Metabasic rocks of the North Shahrekord Metamorphic Complex (NSMC) are composed of amphibolites, garnet amphibolites and eclogites as lenses in metagranitoid bodies and other metamorphic rocks.;The work is focused on the origin of the eclogites based on field geology, petrography, geochemistry and SrNd isotopic ratios. The new petrological and geochemical analyses are presented to show a geodynamic model for the protolith of the eclogites and their relations to ProtoTethys subduction during Late Neoproterozoic.; ;Materials and methods;After microscopic studies, nine fresh samples were selected for wholerock geochemical analysis (XRF and ICPMS analysis) for determining the major trace elements, and REE contents. Six samples were analyzed at Nagoya University (Japan) and three samples were analyzed for XRF at Salzburg University (Austria) and ICPMS, in ACME lab (Vancouver, Canada).; ;Results ;Chondrite normalized REE diagrams display minor enrichment of LREE in comparison with HREE. (La/Yb)cn ratio varies between 1.7 to 2.7 without Eu anomaly. Primary mantle normalized diagram of trace elements show negative anomaly in P, Ti, Nb and Zr. Initial magma had been a basalt to andesitebasalt composition. Tholeiitic magma are revealed by relatively flat REE patterns and geochemical diagrams for their protolith. The geochemical data of the NSMC eclogites shows compositional characteristics of EMORB which is composed of a mix of lithospheric and asthenospheric mantle, and final melt segregation has occurred at depths between 10 to 30 km. Spinel was aluminum bearing phase in the mantle. Tectonic discrimination diagrams display that magma is formed in a backarc basin environment. Studying SrNd isotopes specifies a range of 0.707 to 0.711 for 87Sr/86Sr and 0.5129 as an average of 143Nd/144Nd. ɛNdt varies between 3 to 7. Moreover, the initial 87Sr/86Sr ratios of the samples vary from 0.705–0.709 (MalekMahmoudi et al., 2017). The isotopic evidences indicate that the initial magmas are formed from the mixture of EMII and MORB reservoir with a 0.4 to 0.8 influence of the subduction component.; ;Discussion ;Iranian basement rocks that were affected by the Cadomian orogeny are reported from different zones of Iran with an age range between 630 to 514 Ma (Hassanzadeh and Wernicke 2016). Neoproterozoic rocks of the Gondwana supercontinent, which were formed in a backarc basin setting, have been reported from some zones of Iran, such as Alborz, Central Iran and Zagros Hormoz complex (e.g. Etemad Saeed et al., 2015; Faramarzi et al., 2015; Hosseini et al., 2015( and also from Turkey (e.g. Gürsu and Göncüoglu, 2005). Ages of the backarc successions range from 570 to 530 Ma (e.g., Abbo et al., 2015; Shafaii Moghadam et al., 2016).;Our geochemical evidences indicate the formation of a continental backarc basin in SanandajSirjan Zone during Late Neoproterozoic. The association of eclogites with metasedimentary rocks including paragneiss, schist, quartzite, metadolomite and metasandstone, display a shallow marine sedimentary environment. The combination of field observations and chemical composition of the eclogites shows that the protolith of the rocks are formed at a rifted backarc basin at the Gondwana during late Neoproterozoic to early Cambrian. Then, highpressure metamorphic phase was affected on the rocks during Early Jurassic (Davoudian et al., 2016).; ;Acknowledgements;The authors would like to thank the Shahrekord University for providing the budget for this research.; ;References;Abbo, A., Avigad, D., Gerdes, A. and Güngör, T., 2015. Cadomian basement and Paleozoic to Triassic siliciclastics of the Taurides (Karacahisar dome, southcentral Turkey): paleogeographic constraints from U–Pb–Hf in zircons. 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