نقش ترکیبات مرتبط با فرورانش در منشأ ماگمای سازنده توالی پوسته ای افیولیت جنوب ده شیر

نوع مقاله: مقاله پژوهشی

نویسندگان

1 استادیار، پژوهشکده علوم‌زمین، سازمان زمین‌شناسی و اکتشافات معدنی کشور، تهران، ایران

2 کارشناس ارشد، دانشگاه آزاد اسلامی، واحد علوم و تحقیقات، تهران، ایران

چکیده

افیولیت تکتونیزه جنوب ده­شیر در انتهای شمالی کمربند افیولیتی شهربابک- ده­شیر، در حاشیه جنوبی خرده قاره ایران مرکزی رخنمون دارد و دربردارنده پریدوتیت­های گوشته­ای و توالی پوسته­ای است. پریدوتیت­های گوشته­ای، سرپانتینی‌شده­ و دربردارنده هارزبورژیت کلینوپیروکسن­دار و لرزولیت هستند که توسط دایک­های دیابازی منفرد قطع شده‌اند. توالی پوسته­ای گابروی لایه‌ای ندارد و دربردارنده گابروهای ایزوتروپ با ویژگی‌های کومولایی است که به­صورتی تدریجی و در بالا به کمپلکس دایک­های ورقه­ای دیابازی تبدیل و توسط ادخال‌های ورلیتی، دایک­ها و پاکت­های کوارتزدیوریت-کوارتزمونزودیوریت و پگماتیت­گابرو قطع شده­اند. توالی آتشفشانی این مجموعه نیز شامل تناوبی از چرت، رادیولاریت، سنگ­آهک پلاژیک دارای ریززیاهای (میکروفوناهای) کرتاسه بالایی در قاعده و برش هیالوکلاستیک، هیالوکلاستیت، توف، روانه صفحه­ای و گدازه­بالشی در بالای توالی­است. بر اساس تجزیه‌های شیمیایی، سنگ­های توالی پوسته­ای تنوع ترکیبی و روند ماگمایی تولئیتی تا کلسیمی-  قلیایی نشان می­دهند. در بررسی نمودارهای تکتونوماگمایی، این سنگ­ها بیشتر در محدوده کمان­های آتشفشانی و گاهی در قلمرو اقیانوسی جای گرفته­اند. الگوهای بهنجارشده با مقادیر گوشته اولیه، ویژگی‌های افیولیت­های فرافرورانش مانند تهی­شدگی از عنصر Nb در بیشتر تجزیه‌ها و Ti در برخی از آنها را دارد. این الگوها در ترازهای مختلفی جای گرفته‌اند و غنی­شدگی متفاوت از عناصر LILE نشان می­دهند که  می­تواند  نشان دهنده ناهمگونی منبع گوشته­ای و درجات متفاوت ذوب بخشی باشد. بررسی رفتار عناصر در نمودارهای مختلف و مقایسه آنها با مناطق تکتونوماگمایی شاخص، در قلمروهایی به دور از مورب و نزدیک به کمان و بررسی مقادیر برخی از عناصر مانند Ba، Th، Nb و Yb و نسبت­های آنها نشاندهنده تأثیر متفاوت سیال‌های برخاسته از صفحه فرورونده بر گوه گوشته‌ای  دارد. این ویژگی‌های ژئوشیمیایی و رخنمون ادخال‌های(اینتروژن) ورلیتی و توده­های کوچک کوارتز دیوریتی- کوارتزمونزودیوریتی هورنبلنددار، نشان می­دهد که افیولیت جنوب ده‌شیر، مراحل متوالی از حوادث شامل تولد، جوانی و بلوغ یک افیولیت را پشت سر گذاشته است که پیامد طبیعی تشکیل افیولیت‌ها در سامانه‌های فرافرورانش است. به نظر می‌رسد که افیولیت جنوب ده‌شیر در محدوده‌ای نزدیک به پیشانی کمان با فاصله اندکی از لبه آن تشکیل شده که با سناریوی فرورانش نئوتتیس، طی کرتاسه بالایی قابل توجیه است.
 

کلیدواژه‌ها


عنوان مقاله [English]

The Role of Subduction Components in the Generation of the Crustal Sequence of Dehshir Ophiolite

نویسندگان [English]

  • M. Khalatbari Jafari 1
  • H. Sepehr 2
1 Assistant propessor, Research Institute for Earth Sciences, Geological Survey of Iran, Tehran, Iran.
2 M.Sc., Islamic Azad University, Science and Research Branch, Tehran, Iran.
چکیده [English]

The south Dehshir tectonized Ophiolite located at the extreme northern part of Sharbabak-Dehshir Ophiolite belt comprise of mantle peridotite and crustal sequence. The mantle peridotites are well serpentinized and consist of cpx- bearing harzburgite and lherzolite, cross cut by isolated diabasic dikes. There is no layered gabbro in the crustal sequence and that comprise of high level isotrope gabbro with cumulative character which gradually change to diabase sheeted dike complex on top of the sequence intruded by wherlitic intrusion, quartz diorite, pegmatite gabbro and plagiogranite- tronjemite. The volcanic sequence of this association consist of alternation of chert, radiolarite, pelagic limestone have Upper Cretaceous microfaunas at base and hyaloclastic breccia, hyaloclastite, tuff and pillow lava on top the sequence. The sheeted dike complex oriented in N- S direction with couples of inclinations toward either the east or the west. Listvenite and rodingite are the sporadic metasomatic rocks that expose inside the tectonized assemblage. A few dislocated amphibolites and calc-silicate blocks are tectonically exposed. Base of chemical analyses the crustal sequence show variety in composition and have tholeitic to calc-alkaline trends. In the tectonomagmatic diagrams, these rocks are often placed in the regions associated with the volcanic arcs and in some occasions they are plotted in the oceanic basin regions. The primitive mantle-normalized spider diagrams show depletion of Nb in most of analysis and depletion of Zr and Ti in some of them which indicating a supra- subduction origin. These types of patterns located in different levels and characterized by enrichment of LILE, might be related to diversity in mantle source and different degree of partial melting. The behavior of elements in different diagrams and comparation with the different tectonic setting, show that they tend located far from MORB type and near by the Arc type. The values of the elements such as Ba, Th, Nb, Yb, indicate the influence of the fluids releaved from the subducted slab on the mantle wedge. These geochemical characters and the expose of wherlitic intrusions and the small intrusions of quartz diorite-quartze monzodiorite indicate that the South Dehshir tectonized Ophiolite display a consistent sequence of events during their formation and evolution, includes birth, youth and maturity stages which is a natural consequence of the Supra- subduction zones. It seam that the Dehshir Ophiolite formed in a forearc basin with a distance from margin could be interpret with the downgoing of Neo-Tehytis in Late Cretaceous.

کلیدواژه‌ها [English]

  • Ophiolite
  • Peridotite
  • Isotope Gabbro
  • Crustal Sequence
  • Supra-Subduction
خلعت‌بری، م. و سپهر، ه.، 1390- زمین‌شناسی و سنگ‌شناسی افیولیت تکتونیزه کهدوئیه، فصلنامه علوم‌زمین، سال بیست و یکم، شماره 82، صفحه 103 تا 112.
 
 
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