جایگاه تکتونوماگمایی دیابازها و جریان‌های بازالتی افیولیت شمال مکران، جنوب شرقی ایران
محورهای موضوعی :عزیزالله تاجور 1 , محمدمهدی خطیب 2 , محمدحسین زرین¬کوب 3
1 - دانشگاه بیرجند
2 - دانشگاه بیرجند
3 - دانشگاه بیرجند
کلید واژه: افیولیت شمال مکران, تولئیت, جریان‌های بازالتی, دیاباز, کالک¬آلکالن.,
چکیده مقاله :
افیولیت شمال مکران در جنوب شرقی ایران، به‌عنوان بخشی از افیولیت های تتیس، در حدفاصل بلوک های قاره ای لوت و باجکان-دورکان قرار دارد. از جمله سنگ‌های تشکیل‌دهنده این توالی افیولیتی، دیابازها و جریان‌های بازالتی هستند که بیشترین برونزد را در شرق منطقه دارند. شواهد ساختاری، سنگ نگاری و زمین شیمیایی بیانگر شکل گیری این سنگ‌ها در جایگاه های زمین ساختی متفاوت است. بر اساس ویژگی های زمین شیمیایی، دیابازها و بازالت‌ها در دو گروه جای می گیرند: در یک گروه، دیابازها و جریان‌های بازالتی با ترکیب تولئیتی قرار دارند که ویژگی‌های شبیه به پشته های میان‌اقیانوسی (MORB) را به نمایش می گذارند. گروه دوم شامل جریان‌های بازالتی تا داسیتی با ترکیب کالک آلکالن است که ویژگی‌های کمان های آتشفشانی را به نمایش گذاشته و نشانه-های محیط‌های فرورانش در آنها دیده می شود. دو رخداد ماگمایی منجر به تشکیل این دو گروه از سنگ‌ها شده است: 1) ماگماتیسم نوع MORB حاصل از کشش و بازشدگی بین دو بلوک قاره ای که منجر به شکل گیری دیابازها و بازالت‌های تولئیتی در کرتاسه پیشین شده است. غنی شدگی این سنگ‌ها نسبت به عناصر نادر خاکی سبک(LREE) و مقادیر پایینLa/Yb و به نسبت بالای U/Th نمایانگر تاثیر ترکیبات قاره ای در مذاب به وجود آورنده آنها است. 2) ماگماتیسم مرتبط با فرورانش که گدازه های بازالتی، آندزیتی و داسیتی دارای ویژگی‌های کالک آلکالن را در کرتاسه پسین برجای گذاشته است. غنی شدگی LREE و LILE، ناهنجاری منفی Nb و Ta، نسبت بالای Pb/Ce و مقدار تمرکز اندک TiO2 در این سنگ‌ها، نمایانگر تاثیر ورقه فرورانشی در ترکیب آنها است.
The north Makran ophiolite in southeast of Iran, as a part of Tethyan ophiolites, is located between Lut and Bajkan-Durkan continental blocks. Among the rocks of this ophiolite sequence, diabase and basalt flows are present more abundant in the outcrops in the eastern part of the studied north Makran ophiolite. Structural, petrographic and geochemical evidences suggest distinct geodynamic setting for the formation of these rocks. Based on geochemical characteristics, diabase and basalts fall into two groups: In the first group, tholeiitic diabase and basalt flows represent MORB-like affinity, and the second group include calc-alkaline basaltic to dacitic lavas with arc environment and supra-subduction affinities. These two lava types represent two major magmatic events: 1) MORB-type magmatism resulted from Early Cretaceous rifting/opening between two continental blocks and resulted in the formation of tholeiitic diabase and basalt. LREE enrichment, low La/Yb and relatively high U/Th ratios suggest continental influence in their melt source, and 2) subduction-related magmatism, that formed calc-alkaline basaltic, andesitic and dacitic lavas in Late Cretaceous. LILE, LREE enrichment, Nb and Ta negative anomaly, low TiO2 concentrations and relatively high Ce/Pb ratio document subduction influence in their composition.
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