ژئوشیمی، خاستگاه ژئودینامیکی و منشأ احتمالی ارتوگنیس‌های کوهستان بلقیس، شمال باختری ایران

نوع مقاله: مقاله پژوهشی

نویسندگان

1 دانشیار، گروه زمین شناسی، دانشگاه تربیت مدرس، تهران، ایران

2 دانشیار، پژوهشکده علوم زمین، سازمان زمین شناسی و اکتشافات معدنی کشور، تهران، ایران

چکیده

منطقه­ مورد مطالعه در شمال­ خاوری تکاب به عنوان بخشی از مجموعه دگرگونی تکاب شامل سنگ­های دگرگونی مختلف همانند شیست­های رسی، ارتوگنیس، متابازیت­ها و متااولترامافیک­ها و مرمر است. این سنگ­ها بخشی از پوسته­ قاره­ای ایران زمین هستند که بر اثر عملکرد فازکوهزایی پان­آفریکن در پروتوروزوییک پایانی- کامبرین آغازین دگرگون و دگرریخت شده­اند و بر اثر عملکرد فاز کوهزایی آلپ پایانی در ترشیاری، توسط توده­های نفوذی بسیاری با ترکیب حدواسط تا اسیدی مورد هجوم قرار گرفته­اند. بر پایه داده­های ژئوشیمیایی، سنگ مادر ارتوگنیس‌های کوهستان بلقیس پتاسیم‎بالاست و به سری ماگمایی کالک­آلکالن تعلق دارد و فرایند­های تفریق بلوری و آمیختگی ماگمایی نقش بسزایی در تحول آن داشته است. الگوی تغییرات عناصر کمیاب بهنجار شده با کندریت و گوشته اولیه نشان‌دهنده غنی­شدگی این سنگ­ها از LREE و LILE، تهی­شدگی  آنها از HREE و HFSE و وجود بی‌هنجاری­های منفی Eu, Nb, Ti, Ba, Sr, P  در بیشتر نمونه­هاست که این در کنار موقعیت نمونه­ها در نمودارهای تمایز محیط زمین‌ساختی و نیز میزان پایین TiO2 و P2O5  همراه با نسبت پایین Nb/Zr  نشان از شکل‌گیری سنگ­های مورد مطالعه در محیطی مرتبط با فرورانش و حاشیه فعال قاره­ای تا محیطی برخوردی دارد. همچنین شواهدی همچون نسبت La/Nb بزرگ‌تر از 1 نمونه­های مورد مطالعه، مقدار به نسبت بالای نسبت­هایZr/Nb (14.7), Th/Nb (1.5), Ba/Nb (58), La/Nb (3)  و Ti/Zr (14.9)، نسبت­های (La/Sm)n, Nb/Ce  که به مقادیر ارائه شده برای پوسته (به­ترتیب برابر با 23/0 و 52/4) بسیار نزدیک است (به‌طور میانگین برابر با 24/0 و 76/3)؛ همگی منشأ گوشته­ای محض را برای نمونه­های مورد مطالعه رد می‌کنند و به نظر می­رسد تلفیقی از ماگماهای پوسته­ای و گوشته­ای در تکوین این نمونه­ها دخیل بوده است. همه این ویژگی­های ژئوشیمیایی در کنار رخنمون سنگ­های افیولیتی و زمین‌درزهای کهن (سرپانتینیت­ها، متامافیک و متا اولترامافیک­ها) در پیرامون این سنگ­های دگرگونی درجه متوسط تا بالا نشان می‌دهد که پس از پایان فرورانش سنگ‌کره اقیانوسی، رژیم برخوردی در منطقه حاکم شده است. 

کلیدواژه‌ها


عنوان مقاله [English]

Geochemistry, geodynamic setting and probable origin of orthogneiss in Belqeis Mountain (NW of Iran)

نویسندگان [English]

  • H. Rahbari 1
  • N. A. Rashidnejad Omran 1
  • M. Khalatbari-Jafari 2
1 Associate Professor, Department of Geology, Tarbiat Modares University, Tehran, Iran
2 Associate Professor, Research Institute for Earth Sciences, Geological Survey of Iran, Tehran, Iran
چکیده [English]

As a part of Takab Metamorphic Complex (TMC), Belqeis Mountain has a variety of ‎metamorphic rocks including orthogneiss, pelitic schists, meta-‎ultramafics and mafic rocks and dolomitic marbles. This complex was metamorphosed, faulted and folded ‎during Late Neoproterozoic-Early Cambrian Pan-African orogeny and invaded by ‎granitoid pertinent to the subduction of the Neo-Tethys oceanic crust beneath the ‎Iranian crust during Tertiary. Geochemically, the protolith of orthogneiss of ‎Belqeis Mountain is ‎high K and has calc-alkaline affinity and both fractional crystallization and magma mixing have been incorporated significantly to produce the protolith of them. The pattern of trace elements normalized to chondrite and primitive mantle points to enrichment in LILE and LREE, depletion of HFSE and HREE and shows negative anomalies in Eu, Ba, ‎Nb, Sr, Ti and P. Depletion in HFSE was accompanied with enrichment of LILEs and ‎LREEs indicating the generation of protolith in subduction setting related to active ‎continental margins. Tectonic discrimination diagrams suggest a combination of volcanic ‎arc and continental collision settings for the studied samples. There are many evidence including La/Nb>1 and relatively high values of several ratios such as Zr/Nb (14.7), Th/Nb (1.5), Ba/Nb (58), La/Nb (3), Ti/Zr (14.9), Nb/Ce (0.24) and (La/Sm)n (3.76),close to crust values, confirming that mantle was not incorporated solely but both crust and mantle sources were contributed  in origin of these rocks. All aforementioned ‎points in addition to occurrence of paleo-suture zone and ophiolitic ‎rocks (i.e. serpentinites, meta-mafic and meta-ultramafic rocks) around these medium to ‎high grade metamorphic rocks confirm that subduction of an oceanic lithosphere followed ‎by collision in the study area.  ‎
 

کلیدواژه‌ها [English]

  • Belqeis Mountain
  • Takab Metamorphic Complex
  • Orthogneiss
  • Northwest of ‎Iran

کتابنگاری

باباخانی، ع. و قلمقاش، ج.، 1371- نقشه زمین­شناسی چهارگوش تخت سلیمان، مقیاس 1:100000، سازمان زمین­شناسی کشور. 

علوی نائینی، م.، عمیدی، م.، طاطاوسیان، ش.، حاجیان، ج.، بلورچی، م.، آقانباتی، ع. و پلی­سیر، ج.، 1361- نقشه زمین­شناسی چهارگوش تکاب ­ـ ­صائین­قلعه، مقیاس 1:250000، سازمان زمین­شناسی کشور.


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