@article { author = {Monazzami Bagherzadeh, Reza and Karimpour, Mohammad Hassan and Farmer, G. Lang and Stern, Charles R. and Santos, Jose Francisco and Ribeiro, Sara and Rahimi, Behnam and Haidarian Shahri, Mohammad Reza}, title = {Zircon Geochronology (U-Pb), Petrography, Geochemistry and Radioisotopes of Bornaward Metarhyolites (Central Taknar Zone-Northwest of Bardaskan)}, journal = {Journal of Economic Geology}, volume = {8}, number = {1}, pages = {239-264}, year = {2016}, publisher = {Ferdowsi University of Mashhad}, issn = {2008-7306}, eissn = {2423-5865}, doi = {10.22067/econg.v8i1.46700}, abstract = {Introduction The Bornaward area is located in the Northeastern Iran (in the Khorasan Razavi province) 28 km northwest of the city of Bardaskan at 57˚ 46΄ to 57˚ 52΄ N latitude and 35˚ 21΄ to 35˚ 24΄E longitude. The Taknar structural zone, situated in the North central Iranian micro continent, is part of the Lut block (Forster, 1978). The Taknar zone is an allochthonous block bounded by the Darouneh and Taknar major faults. Much of this zone consists of metarhyolite-rhyodacite volcanic rocks, and rhyolitic tuff with interlayers of sandstone and dolomite (Taknar Formation). Analytical Results ICP-MS analysis of REE and minor elements of samples of the Bornaward metarhyolites was carried out at the ACME Laboratory in Vancouver, Canada. U-Pb dating of the metarhyolites was performed on isolated zircons in Crohn's Laser Lab, in Arizona (Gehrels et al., 2008). Measurement of Rb, Sr, Sm and Nd isotopes and (143Nd/144Nd)i and (87Sr/86Sr)i ratios took place in the radioisotope laboratory of the University of Aveiro in Portugal. Petrography The volcanic rocks are porphyritic, commonly containing phenocrysts of orthoclase and rarely sanidine, quartz and intermediate plagioclase in a groundmass of fine-grained quartz and feldspar. An alteration has produced oriented needles of sericite and clay minerals, clusters of fine-grained green biotite and clots of epidote and chlorite. Geochemistry The compositions of the volcanic rocks are calc alkaline and high K- calc alkaline. The obtained Shand index (Al2O3/( CaO+Na2O+K2O) is above 1.1, in the peraluminous S-type granite field (Chappell and White, 2001). Plotted on the TAS diagram (Middlemost, 1994), all the metarhyolite-rhyodacite samples are located in the sub-alkaline field and the majority fall into the rhyolite group. The metarhyolite-rhyodacites show enrichment of LREE with a moderately ascending pattern ((La/Yb)N=2.51-10.11 and La=46.45-145.48). Europium shows a negative anomaly (Eu/Eu*=0.23-0.71). U-Pb zircon geochronology Measurement of U-Pb isotopes of the Bornaward metarhyolite zircons of sample BKCh-103, indicates an age of 552.23+4.73,-6.62 Ma (Upper Precambrian). Sr-Nd isotopes The Sr ratios of the metarhyolites (87Sr/86Sr) were found to fall in the range of 0.688949 to 0.723435 and the Nd ratios (143Nd/144Nd)i were in the range of 0.511701 to 0.511855. These values indicate that the metarhyolites of samples BKCh-12, BKCh-103 and BKCh-177 were affected by hydrothermal alteration since their (87Sr/86Sr)I ratios are high. The Sr ratios suggest that the more negative Nd anomaly and the more negative ɛNd(552) of the samples BKCh-12, BKCh-103 and BKCh-177 indicate that these lavas originated in an enriched upper mantle source and/or lower continental crust. In contrast, two recent examples (Xua et al., 2005) can be related to sialic continental crust with significant contamination. Petrogenesis The Bornaward metarhyolite- rhyodacites show an enriched pattern for Rb, Th, U, K, Pb, Nd and Y relative to the primitive mantle, while Ba, P, Ti, Sr, Zr and Nb show a reduction as a result of fractional crystallization. Based on isotopic correlations of207Pb/204Pb vs 206Pb/204Pb, the primitive source of the Bornaward metarhyolite- rhyodacites is the lower continental crust. This part of the continental crust is only slightly depleted in Pb. Consequently, it has a low 87Sr/86Sr ratio (Samples BKCh-138 and BKCh-198). In contrast, the samples of BKCh-12, BKCh-103 and BKCh-177 have high 87Sr/86Sr ratios that could be the result of significant contamination to parts of the continental crust with very high 87Sr/86Sr (Karimpour et al., 2011). Results and Conclusions The calc-alkaline compositions of samples BKCh-12, BKCh-103 and BKCh-177, the high K- calc alkaline of samples BKCh-138 and BKCh-198 of the Bornaward metarhyolites and the higher temperature overgrowth of plagioclase on lower temperature microcline phenocrysts can be a reason for entrance lavas with different generations. The distinct isotopic characteristics of the two groups of rhyolitic samples are the reasons for two different sources for the production of these lavas: 1) partial melting of an enriched mantle reservoir or lower continental crust, and 2) sialic continental crust with high contamination. With respect to the Bornaward metarhyolite- rhyodacites with (143Nd/144Nd)i ratios from 0.511701 to 0.511855, geochemical characteristics and the high volume of volcanic rocks in the area, their formation can be attributed to a continental rift environment. This rift system can be formed by initiation of a plume in the upper mantle beneath East Iran during Neoproterozoic time. References Chappell, B.W. and White, A.J.R., 2001. Two contrasting granite types. Australian Journal of Earth Sciences, 48(4): 489-499. Forster, H., 1978. Mesozoic – cenozoic metallogenesis in Iran. Journal of the Geological Society, 135(4): 443-455. Gehrels, G.E., Valencia, V.A. and Ruiz, J., 2008. Enhanced precision, accuracy, efficiency, and spatial resolution of U–Pb ages by laser ablation–multicollector–inductively coupled plasma-mass spectrometry. Geochemistry, Geophysics, Geosystems, 9(3): 1-13. Karimpour, M.H., Farmer, G.L., Stern, C.R. and Salati, E., 2011. U-Pb zircon geochronology and Sr-Nd isotopic characteristic of Late Neoproterozoic Bornaward granitoids (Taknar zone exotic block), Iran. Iranian Society of Crystallography and Mineralogy, 19(1): 1-18. Middlemost, E.A.K., 1994.Naming materials in the magma igneous rock system. Earth Science Reviews, 37(3- 4): 215-224. Xua, B., Jianb, P., Zhenga, H., Zouc, H., Zhanga, L. and Liub, D., 2005. U–Pb zircon geochronology and geochemistry of Neoproterozoic volcanic rocks in the Tarim Block of northwest China: implications for the breakup of Rodinia supercontinent and Neoproterozoic glaciations. Precambrian Research, 136(2): 107–123.}, keywords = {Isotope,Zircon,Geochronology,Neoprotrozoic,Bornaward,Taknar,Bardaskan}, title_fa = {ژئوکرونولوژی (U-Pb) زیرکن، پتروگرافی، ژئوشیمی و رادیو ایزوتوپهای متاریولیت‌ های برنورد (مرکز زون تکنار- شمال غرب بردسکن)}, abstract_fa = {منطقه مورد بررسی، در بخش میانی زون ساختاری تکنار در شمال خرد قاره ایران مرکزی قرار گرفته است. این زون ساختاری از سازند تکنار و کمپلکس پلوتونیک برنورد تشکیل شده است. تشکیلات ولکانوسدیمنت سازند تکنار متشکل از سنگهای آتشفشانی فلسیک (متاریولیت- ریوداسیت و توف ریولیتی) با ضخامت قابل توجه و به‌همراه مرمر و ماسه‌سنگ کمی دگرگون‌شده، فیلیت، اسلیت و شیست است. کانی‌شناسی و شیمی اولیه این سنگها بر اثر فرآیندهای ثانویه (دگرگونی ناحیه ای و دگرسانی هیدروترمالی) مورد تغییر قرار گرفته است. بیشترین حجم سنگهای آتشفشانی مورد بررسی در بخشهای زیرین و میانی سازند تکنار به‌همراه توفهای لایه‌ای سبز روشن تشکیل شده است. در برخی از نقاط، این مجموعه ولکانوسدیمنت در کنتاکت با کمپلکس گرانیتوئید برنورد مشاهده می‌شود. سن‌سنجی کانی زیرکن به‌روش (U-Pb) متاریولیتهای سازند تکنار، سن 6/62- ،4/73+ 552/23 میلیون سال قبل (اواخر نئوپروتروزوئیک) را نشان می دهد. εNd(552) زمان تشکیل متاریولیتهای برنورد (مرکز زون تکنار) در گستره 6/98- تا 1/5- قرار می‌گیرد. عناصر نادر خاکی سبک (LREE) به جز Eu نسبت به عناصر نادر خاکی سنگین (HREE) با شیب ملایمی غنی‌شدگی نشان می دهند. همچنین، در این متاریولیت‌ها، غنی شدگی در بیشتر عناصر کمیاب ناسازگار (Th,U,Y,K,Rb,Pb, Nd) دیده می شود در حالی که عناصر (Ba,P,Ti,Sr,Zr, Nb) فقیرشدگی شاخصی را در مقایسه با گوشته اولیه نشان می دهند. مشخصه -ایزوتوپی (143Nd/144Nd)i متاریولیت‌های منطقه مقدار عددی 0/511701 تا 0/511855 به‌دست آمده است. مقدار عددی نسبت i(87Sr/86Sr) متاریولیتهای منطقه برنورد در گستره 0/688949 تا 0/723435، بیانگر دو منشأ مختلف برای گدازه های ریولیتی این منطقه است. با توجه به مطالب بالا و همچنین حجم بالای ولکانیسم در منطقه تکنار، می توان تشکیل آنها را به یک محیط ریفت قاره‌ای مربوط دانست. این ریفت می تواند با ایجاد یک زبانه در گوشته بالایی واقع در زیر پی‌سنگ شرق ایران در مدت زمان نئوپروتروزوئیک تشکیل شده باشد.}, keywords_fa = {ایزوتوپ,زیرکن,سن سنجی,نئوپروتروزوئیک,برنورد,بردسکن,تکنار}, url = {https://econg.um.ac.ir/article_31494.html}, eprint = {https://econg.um.ac.ir/article_31494_70f3d1b8db0b1acf003ce96296153ce1.pdf} }