روابط صحرایی، ژئوشیمی و خاستگاه ژئودینامیکی نهشته‌ی کرومیتی معدن بندان (شرق ایران)

نویسندگان

1 دانشگاه تربیت معلم تهران

2 دانشگاه فرارا

چکیده

معدن بندان اصلی‌ترین ذخیره­ی کرومیتی در زون جوش‌خورده­ی سیستان (شرق ایران) است. توده‌های کرومیتی به­صورت ساختارهای ورقه‌ای تا عدسی شکل تظاهر داشته و با پریدوتیت­های گوشته‌ای بیشتر دونیتی تا هارزبورژیتی میزبانی می‌شوند. رابطه­ی کرومیتیت- سنگ میزبان مشابه با کرومیت­های نوع آلپی با حضور یک غشای دونیتی پیرامون توده‌های کرومیتی مشخص می‌شود. کرومیت­ها بیشتر بافت توده­ای تا افشان و نیز برشی دارند. شیمی کرومیت­ها میزان TiO2 کم (کمتر از 2/0 درصد وزنی) در گستره­ی کرومیت­های افیولیتی نشان داده و نسبت Cr/Fe بیشتر از 2 است. مقدار Cr# (Cr# = Cr × 100/(Cr + Al)) در گستره­ی تقریبی 50 تا 52 آن­ها را در گروه کرومیت­های Al-بالا قرار داده و ترکیب محاسباتی شیمی ماگما­ی مادر با میزان  Al2O3 و نسبت FeO/MgO  به ترتیب حدود 15 تا 16 درصد وزنی و 1/1 تا 2/1 اشاره به سرشت نوع (Mid-ocean ridge basalt) MORB دارد. با اینکه درباره خاستگاه ژئودینامیکی کرومیت­های Al-بالا تردید وجود دارد ولی بنابر شواهد سنگ­شناسی- ژئوشیمیایی در توالی گوشته- پوسته افیولیتی، امکان ارتباط فرایند تشکیل کرومیت به جایگاه بالای زون فرورانش وجود دارد.

کلیدواژه‌ها


عنوان مقاله [English]

Field relations, geochemistry and geodynamic setting of the Bandan chromite mine, eastern Iran.

چکیده [English]

The Bandan chromite mine is the main chromite deposit in Sistan suture zone, eastrn Iran. The chromite deposits are structurally tabular to lens-shaped bodies hosted by dunitic to harzburgitic mantle peridotites. Similar to Alpine type podiform chromites, chromitite pods are enclosed within dunitic envelops. The chromites show mainly massive to disseminated and also brecciated textures. Chemically, the Cr/Fe ratio is higher than 2 and TiO2 content in accordance with ophiolitic chromites is low (< 0.2 wt. %). As a result of low Cr# (Cr#=Cr×100/(Cr+Al)) ranging from 50 to 52 the Bandan chromite deposit is high-Al type. Calculated parental melt chemistry shows MORB (Mid-ocean ridge basalt)-type signature with Al2O3 and FeO/MgO ratio contents of 15-16 and 1.1-1.2, respectively. Although the geodynamic setting of high-Al podiform chromites have been debated but petrographical- geochemical characteristics of ophiolitic mantle-crust sequences may relate chromite genesis to supra subduction zone setting.

کلیدواژه‌ها [English]

  • podiform chromite
  • high-Al type
  • Bandan mine
  • eastern Iran
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