جایگاه زمین ساختی ـ ماگمایی سنگ‌های آتشفشانی پالئوژن رودبار (شمال ایران)

نوع مقاله : مقاله پژوهشی

نویسندگان

1 گروه زمین‌شناسی، دانشکده علوم، دانشگاه آزاد اسلامی واحد تهران شمال، تهران، ایران

2 گروه زمین‌شناسی، دانشکده علوم، دانشگاه آزاد واحد لاهیجان، لاهیجان، ایران

چکیده

سنگ‌های آتشفشانی پالئوژن رودبار (شمال ایران) دارای تنوع ترکیبی بین اولیوین بازالت، آندزیت بازالتی، هورنبلند آندزیت پیروکسن دار و آندزیت هستند. از نظر ژئوشیمیایی آنومالی‌های منفی  Ti، Nb ، Ta ، Zr، Ba، P و آنومالی مثبت Rb، Pb،Th ، K از شاخصه‌های آلودگی پوسته‌ای این سنگ‌ها هستند. مطالعات ژئوشیمیایی نشان می‌دهد که سنگ‌های بازیک منطقه با پوسته تحتانی و سنگ‌های حدواسط با پوسته فوقانی آلوده شده‌اند. تغییرات Y/Nb در برابر Zr/Nb و نیز Rb/Y در برابر Nb/Y و Ba/La در برابر La/Sm بیانگر آلودگی پوسته‌ای ماگماهای منشاً گرفته از منبع مورب می‌باشد. در نمودارتشخیص جایگاه‌های تکتونیکی تمامی نمونه‌ها در محدوده بازالت‌های ریفت درون قاره‌ای قرار گرفته‌اند. بررسی‌های ژئوشیمیایی نشان می‌دهد که سنگ‌های منطقه از یک منبع گوشته‌ای تیپ مورب با رخساره اسپینل در یک محیط ریفتی درون قاره‌ای حاصل شده که با سنگ‌های پوسته قاره‌ای آلوده شده‌اند.

کلیدواژه‌ها


عنوان مقاله [English]

Tectonic-magmatic position of Paleogene volcanic rocks of Roudbar (north of Iran)

نویسندگان [English]

  • zahra Shafeie 1
  • Mohammad Ali Arian 1
  • Shahrouz Haghnazar 2
1 Department of Geology, Faculty of Science, North Tehran Branch, Islamic Azad University, Tehran, Iran Department of Geology, Faculty of Science, Lahijan Branch, Islamic Azad University, Lahijan, Iran. Department of Geology, Faculty
2 Department of Geology, Faculty of Science, Lahijan Branch, Islamic Azad University, Lahijan, Iran
چکیده [English]

Introduction
The study area in north of Iran is located between 49 °, 30' and 50 °, 00' longitudes and 36 °, 30' to 37 °, 00' latitudes and in the 1.100000 sheet of Jirandeh. This area is considered as part of the structural zone of Iran in Alborz zone and is part of Tertiary zone. In this paper, with the help of geochemical evidence in Rudbar region, as part of Paleogene magmatism in Alborz, an attempt has been made to comment on the tectonic petrogenetic of the region.
Methodology
After field studies, considering lithological varieties of the volcanic units in the region, 100 samples were collected and thin sections were prepared and studied in terms of petrography using polarizing microscope. Then, among the mentioned samples, 22 samples with the lowest weathering and most lithological variety were selected to analyze the major elements using XRF method and the trace and REE elements by ICP-MS method at SGS laboratory in Toronto. In order to analyze data, the software Igpet 2007 and GCDkit are used.
Discussion
The studied area is located in the sheet of Jirande at a scale of 1:100000
,that are outcrops of volcanic and pyroclastic rocks of Paleogene age. Based on petrographic studies carried out on the lavas’ units, three rock units were distinguished: a) olivine basalts, b) andesitic basaltic and basaltic andesite, c) hornblende pyroxene andesites and andesites. In most investigated rocks, there are different types of xenoliths and xenocrysts.
Xenoliths are composed of gabbro, diorite and sometimes basalt. These xenoliths and xenocrysts are petrographic evidence for magmatic contamination.   The positive correlation of Na2O and K2O and the negative correlation of Fe2O3, MgO, CaO oxides with the increasing of SiO2 evidence, indicates fractional crystallization in the magma evolution trend of rocks in the area. The constant trends are also maintained through series, which were exposed to the AFC process and assimilation with fractional crystallization.
Comparing the pattern process of incompatible rare elements to crust values in mafic and intermediate terms indicates crustal contamination of mafic rocks to the lower crust and contamination of intermediate rocks towards upper crust. Linear correlation between the ratio of Y/Nb compared to Zr/Nb indicates the origination of magmas from MORB source mantle which were somewhat contaminated with the continental crust rocks.
Conclusion
Geochemical studies represent original relationship between all the studied rocks. This relationship indicates the crystal fractionation in the magma that forms these rocks. Investigations of the ratios of incompatible trace elements, suggest that the mafic samples of the region are close to MORB asthenosphereic mantle source. Also, the trends between primary and evolved samples indicate a linear arrangement between the MORB source mantle and the continental crust, representing an interaction of the MORB mantle-derived magmas with continental crust. All geochemical evidence indicates that
the volcanic rocks in the area were originated from melting of a MORB asthenosphere mantle source with spinel facies, which was contaminated with the continental crust rocks to some degree.
The crustal contamination of these basalts has caused, firstly, these rocks to follow exactly the elemental processes of the crust, and secondly misleadingly show the geochemical characteristics of rocks in subduction zones. This means generating magmas from mantle MORB source with crustal contamination are commonly seen in within-plate continental rift magmatisms.

کلیدواژه‌ها [English]

  • Crustal contamination
  • Paleogene
  • Within-plate continental rift
  • Volcanic rocks
  • MORB mantel source
-امامی، م.ه.، 1379. ماگماتیسم در ایران، سازمان زمین‌شناسی و اکتشافات معدنی کشور، تهران، 608 ص.
-قلمقاش، ج.، 1381. نقشه زمین­شناسی 100000/1 جیرنده، سازمان زمین‌شناسی و اکتشافات معدنی کشور، تهران، ایران.
-کلانتری، ک.، کنعانیان، ع.، آسیابانها، ع. و الیاسی، م.، 1387. بررسی منشا و محیط زمین ساختی سنگ­های آتشفشانی پالئوژن زرجه بستان (شمال قزوین) با استفاده از عناصر REE و HFSE. فصلنامه علمی علوم زمین، مجله علوم، دوره 17، شماره 68، ص 140-149.
-کنعانیان، ع.، حیدری، م. و آسیابانها، ع.، 1384. توده نفوذی شکرناب بخشی از ماگماتیسم ترشیری در البرز مرکزی، ایران، نشریه علوم دانشگاه تربیت معلم، علوم، دوره 5، شماره 384، ص 1631-1635.
-رحیمی، گ.، کنعانیان، ع. و آسیابانها، ع.، 1389. بررسی جایگاه زمین ساختی و سنگ­زایی سنگ­های آتشفشانی پسا ائوسن منطقه اباذر (شمال شرق قزوین)، مجله بلورشناسی و کانی­شناسی ایران، دوره 18، شماره 2، ص 167-180.
-معین وزیری، ح.، 1377. دیباچه­ای بر ماگماتیسم در ایران، چاپ اول، انتشارات دانشگاه تربیت معلم، تهران، 439 ص.
-ولی زاده، م.و.، عبدالهی، ح.ر. و صادقیان، م.، 1387. بررسی­های زمین­شناختی توده­های نفوذی اصلی البرز مرکزی، فصلنامه علمی علوم زمین، مجله علوم، دوره 17، شماره 67، ص 182-197.
 
 
 
-Asiabanha, A. and Foden, J., 2012. Post- collisional transition from an extensional volcano–sedimentary basin to a continental are in the Alborz Ranges, N-Iran. Lithos v. 148, v. 98-111.
-Barry, T.L., Saunders, A.D., Kempton, P.D., Windly, B.F., Pringle, M.S., Dorjnamjaa, D. and Saandar, S., 2003. Petrogenesis of Cenozoic basalts from Mongolia: evidence for the role of asthenospheric versus metasomatized lithospheric mantle sources. Journal of Petrology, v. 44, p. 55-91.
-Cox, K.J. and Howkeworth, C.J., 1985. Geochemical Stratigraphy of Decan Traps, At Mahabalshwar, Westrn Ghats, India, With Implications for Open System Magmatic Processes. J, Petrol, v. 26, p. 355-377.
-Dellenbach, J., 1964. Contribution a` l'e´tude ge´ologique de la re´gion situe´e a l'est de Tehran. Fac.sci, uni, Strasbourg(France), 117 p.
-Emami, M.H., 2000. Magmatism in Iran. Geological Survey of Iran, Tehran (in Persian).
-Fitton, G., Hardarson, B.S., Saunders, A.D. and Norry, M.J., 1996. The chemical distinction between depleted plume and N-MORB mantle sources. Abstract 1996 Goldschmidt conference, J. Conf. Abstr., v. 1, 167 p.
-Hoffman, A.M., 1997. Mantle geochemistry: The messages from Oceanic volcanism. Nature, v. 385, p. 219-229.
-IInicki, S., 2010. Petrogenesis of continental magfic dykes from the Izera complex karkonosze – Izera Block (West sudetes, SW Poland). Iternational Journal of Earth Sciences (Geologische Rundschau), v. 99, p. 745-773.
-Kurt, H., Asan, K. and Ruffet, G., 2008. The relationship between collision-related calcalkaline, and withinplate alkaline volcanism in the Karacadag Area (KonyaTurkiye, centeral Anatolia), Chemie der Erde, v. 68, p. 155-176.
-Pearce, J.A., 1982. Trace element characteristics of lavas from destructive plate boundaries. In: Thorpe, R.S. (Eds.): Andesites. Wiley, chichester, New York, USA, p. 528-548.
-Pearce, J.A., 1983. Role of sub-continental lithosphere in magma genesis at active continental basalts and mantle xenoliths, shiva, Nantwich, p. 230-249.
-Rooney, T.O., 2010. Geochemical evidence of Lithospheric thinning in the southern main Ethiopian rift. Lithos, v. 117, p. 33-48.
-Roy, A., Sarkar, A., Jeyakumar, S., Aggrawal, S.K. and Ebihara, M., 2002. Sm-Nd age and mantel source characteristics of the Dhanjori volcanic rocks, Eastern India. Geoehemical Journal, v. 36, p. 503-518.
-Rudnick, R.L. and Fountain, D.M., 1995. "Nature and composition of the continental crust: A Lower crustal perspective."Rev. Geophys, v. 33, p. 267-309.
-Ryan, J., Morris, J., Bebout, G. and Leeman, B., 1996. Describing chemical fluxes in subduction zone: insights from “Depth-Profiling” studies of arc and foreac rocks in: subduction top to bottom (Eds. Bebout, G.E., Scholl, D.W., Kirby, S.h., and platt, J.p.) 263-268 p. American Geophisical Union, Washington DC.
-Shuging, S., Yunliang, W. and Chengjian, Z., 2003. Discrimination of the tectonic setting of basalts by Th, Nb and Zr[J], Geol, Rev., v. 49, v. 40-47(in chinese with English abstact).
-Stocklin, J., 1974. Northern Iran: Alborz mountain. Geological Society London Special Publications, v. 4, p. 213-234.
-Sun, S.S. and McDonough, W.F., 1989.Chemical and isotopic systematics of oceanic basalts: implication for mantel composition and processes in: Saunders, A.D. and Norry, M.J.(eds), Magmatism in ocean basalts. Geological. Society. London, Special Publications, v. 42, p. 313-345.
-Taylor, S.R. and McLennan, S.M., 1985. The continental crust: its composition avdevolution, Blackwell, Oxford, 312 p.
-Verma, S.P., 2009. Continental rift setting for the central part of the Mexican volcanic belt: A statistical approach. The Open Geology Journal, v. 3, p. 8-29.
-Wang, K., Plank, T., Walker, J.D. and Smith, E.I., 2002. A mantle melting profile across the Basin & Range, SW USA. Journal of Geophysical Research, v. 107, p. 1-21.
-Wang, Y., Zhang, C. and Xia, S., 2001. Th/Hf-Ta/Hf identification of tectonic setting of basalts. Acta Petrologica Sinica, v. 17, p. 413-421(in Chinese with English abstract).
-Weaver, B.L., 1991. Trace element evidence for the origin of ocean-island basalt. Geology, v. 19, p. 123-126.
-Wilson, M., 1989. Igneous Petrogenesis a global tectonic approach. Unwin Hyman, London, 466 p.
-Zhao, J.H. and Zhou, M.F., 2007. Geochemistry of Neoproterozoic mafic introsions in the panzhihua district (Sichuan Province, SW China): Implication for subduction related   metasomatism in the uppermantle Precambrian Research, v. 152, p. 27-47.