شیمی کانی و ژئوشیمی عناصر جزئی/ نادرخاکی درالترامافیک¬های غرب ماکو
محورهای موضوعی :
1 - دانشگاه ارومیه
کلید واژه: افیولیت, الگوی عناصر نادر خاکی, سرپانتینیت, شیمی بلور, ماکو.,
چکیده مقاله :
سنگ‌های الترامافیک کرتاسه فوقانی در ناحیه غرب ماکو در نزدیکی مرز با ترکیه متعلق به افیولیت نئوتتیس و به‌صورت توده¬های کوچک و پراکنده، توزیع وسیعی دارند. این بخش از خاک ایران بخشی از بلوک ارمنستان جنوبی و گندوانایی و نزدیک به خطواره سیوان-آکرا است. در این مطالعه با استفاده از مشاهده دقیق روابط صحرایی، بررسی شیمی بلور و داده¬های سنگ¬کل که شامل عناصر جزئی و نادر خاکی است، به طبقه¬بندی این گروه سنگی، شناسایی ترکیب فازهای باقیمانده و تشخیص جایگاه تشکیل سنگ گوشته¬ای پرداخته شده است. سنگ‌های الترامافیک این ناحیه به‌شدت سرپانتینی شده، کم کلسیم و هیدراته هستند. این سنگ‌های گوشته¬ای درجه بالایی از ذوب بخشی را تجربه کرده و از عناصر ناسازگار تهی هستند. بنابراین اندازه¬گیری مقادیر این عناصر با روش خاصی صورت گرفت. فازهای اصلی شامل فورستریت، انستاتیت، دیوپسید و کروم اسپینل است و در یک متن غالب سرپانتینی از نوع کریزوتیل و لیزاردیت به چشم می¬خورند. الگوی عناصر نادر خاکی سنگ‌ها زنگوله¬ای است و نشانه¬ی آلایش با پوسته قاره¬ای را ثبت کرده است. بعلاوه سنگ‌های الترامافیک غرب ماکو بر اساس ژئوشیمی سنگ¬کل و با استفاده از نمودارها و نسبت¬های بین اکسید عناصر اصلی و جزئی، از دسته سرپانتینیت¬های فرورانده شده ارزیابی شده و درجات بالایی از باروری مجدد آنها توسط فرایندهای تبادل سنگ/سیال روی داده است. بنابراین فراوانی نسبی بالای عناصر محلول در سیال مانند اورانیم، سرب و استرانسیم در آنها حاصل فرایندهای ثانوی می‌باشد.
Late Cretaceous ultramafic rocks of the Neo-Tethyan ophiolite are located in the western Maku near the Iran-Turkey border as small, scattered bodies in vast areas. This part of Iran's territory belongs to the South Armenian Block (SAB), is Gondwanan, and is near the northern Neo-Tethyan suture. This study is focused on rock classification, residual phase composition, and mantle rock formation setting, using exact field relation observation, crystal chemistry studies, and whole rock chemistry accompanied by trace and rare earth elements investigations. The studied rocks were highly serpentinized, Ca-poor, and hydrated. These mantle rocks experienced a high degree of partial melting and were completely depleted from the incompatible elements. Therefore, the abundance of them was measured using a special method. The main phases are forsterite, enstatite, diopside, and chromian-spinel which are seen in the chrysotile-lizarditic serpentinized matrix. The U-shape rare earth elements pattern of the western Maku serpentinites is recorded as crustal contamination evidence. Additionally, the studied ultramafic rocks of western Maku based on the whole rock chemistry, diagrams and relations between major oxide and trace elements belong to the subducted serpentinites group. Also, they practiced high refertilization values by fluid/rock interactions on them. Hence, the enrichment of fluid mobile elements (FMEs) such as U, Pb, and Sr results from secondary events occurring during exhumation.
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