شیمی کانی گارنت در پگماتیت ها و سنگهای دگرگونی منطقه همدان

زمین شناسی اقتصادی

مقاله 3، دوره 7، شماره 2 - شماره پیاپی 13، دی 1394، صفحه 243-258   

نوع مقاله: علمی- پژوهشی

شناسه دیجیتال (DOI): 10.22067/econg.v7i2.31041

نویسندگان

احمد احمدی خلجی^* ؛ زهرا طهماسبی

لرستان

چکیده

محدوده همدان به‌طور عمده متشکل از سنگهای دگرگونی و پلوتونیک مزوزوئیک است. در این منطقه از غرب به شرق به‌ترتیب واحدهای متمایزی از توده‌های نفوذی، سنگهای دگرگونی مجاورتی و رگه‌های پگماتیتی و آپلیتی رخنمون دارند که در برخی مناطق این سنگها حاوی گارنت می‌باشند. بر اساس مطالعات انجام شده ترکیب اصلی گارنت در شیست‌های کمری و پگماتیت‌های ابرو از نوع آلماندین (گارنت غنی از آهن) و به مقدار کمتر اسپسارتین (گارنت غنی ازمنگنز) است که این دو نوع گارنت به گروه پیرالسپیت معروفند. الگوی منطقه‌بندی گارنت در سنگهای دگرگونی (شیست‌ها و هورنفلس‌ها) با سنگهای آذرین (پگماتیت) تغییرات متفاوتی از مرکز به حاشیه نشان می‌دهد. به‌طوری که ترکیب گارنت به‌ترتیب از مرکز به حاشیه در شیست‌ها از Alm0.63, Prp0.07, Sps0.24, Grs0.05 تا Alm0.71, Prp0.09, Sps0.13, Grs0.05در هورنفلس‌ها از Alm 0.79, Prp0.14, Sps 0.06, Grs0.07تا Alm0.8, Prp0.13, Sps 0.05, Grs0.01 و در پگماتیت‌ها، از Alm0.73, Prp0.015, Sps0.24, Grs0.07 تا Alm0.7, Prp0.011, Sps0.28, Grs0.00 تغییر می‌کند. شواهد پتروگرافی نظیر همراهی تورمالین با گارنت پگماتیت‌ها و آندالوزیت با گارنت سنگهای دگرگونی مجاورتی و شیمی‌کانی گارنت (افزایش Mn در هسته گارنت شیست‌ها و کاهش آن در هسته گارنت پگماتیت، کاهش عنصر Mg در مرکز گارنت پگماتیت و افزایش آن در هسته گارنت شیست‌ها، خطی بودن Ca, Al از حاشیه به مرکز در هورنفلس‌ها) و تقریباً 10 درصد گارنت پیروپ حاکی از منشأ گرفتن گارنت موجود در شیست‌ها و هورنفلس‌ها از یک سیستم بسته (دگرگونی مجاورتی) و ماگمایی بودن آن در پگماتیت‌هاست.

کلیدواژه‌ها

همدان؛ دگرگونی؛ گارنت؛ منطقه‌ بندی

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