نشریه علوم زمین خوارزمی

نشریه علوم زمین خوارزمی

سنگ‌شناسی داسیت‌ها و ریولیت‌های ائوسن کوه گوره (شمال غرب انارک، استان اصفهان)

نویسندگان
دانشگاه اصفهان
چکیده
کوه گوره در حدود 55 کیلومتری شمال ‌غرب شهر انارک (شمال شرق اصفهان) و در حاشیه غربی خرد قاره شرق - ایران مرکزی (غرب بلوک یزد) قرار دارد. بررسی‌های صحرایی نشان می‌دهد که سنگ‌های آتشفشانی ائوسن در این منطقه دارای دو لیتولوژی داسیت (خاکستری) و ریولیت (روشن رنگ) هستند. گدازه‌های داسیتی، ریولیت‌ها را قطع کرده و از آن‌ها جوان‌تر هستند. مطالعات پتروگرافی حاکی از آن است که داسیت‌ها، دارای کانی‌های اصلی پلاژیوکلاز (لابرادوریت و بیتونیت)، کلینوپیروکسن (اوژیت) و کوارتز بوده، و ریولیت‌ها دارای کانی‌های اصلی پلاژیوکلاز، سانیدین و کوارتز می‌باشند. کلریت، کلسیت، هماتیت و لیمونیت کانی‌های ثانویه­ موجود در این دو گروه سنگی هستند که از دگرسانی کلینوپیروکسن، پلاژیوکلاز و مگنتیت حاصل شده‌اند. بافت‌‌های اصلی این سنگ‌ها پورفیریتیک، گلومروپورفیریتیک، آنتی ‌راپاکیوی، غربالی و پوئی‌کیلیتیک می‌باشد. در داسیت‌ها، ترتیب تبلور کانی‌ها به صورت مگنتیت، کلینوپیروکسن، پلاژیوکلاز و کوارتز است و در ریولیت‌ها نیز ترتیب تبلور به صورت پلاژیوکلاز، سانیدین و کوارتز می‌باشد. نتایج آنالیزهای ژئوشیمیایی نمونه‌های سنگ کل نشان می‌دهد که سنگ‌های ولکانیک ائوسن کوه گوره دارای ترکیب داسیت و ریولیت هستند. ماگمای سازنده این سنگ­ها ماهیت ساب­‌آلکالن داشته و جزء سری کالک‌­آلکالن می‌باشد. نمودارهای بهنجارسازی سنگ‌های ولکانیک کوه گوره نسبت به ترکیب کندریت و گوشته اولیه، نشان می‌دهد که این سنگ‌ها از نظر LREEها و LILEها غنی بوده و فقیر از عناصر HFS مانند Ti, Nb, Ta (TNT) می‌باشند. آنومالی منفی TNT در این سنگ‌ها احتمالا بیانگر ماگماتیسم مرتبط با کمان‌های آتشفشانی و مناطق فرورانش است. این سنگ‌ها، از نظر ترکیب شیمیایی شبیه سنگ‌های معمول یک کمان ماگمایی هستند. تصور بر این است که با آبزدایی اسلب فرورونده و ذوب بخشی پریدوتیت‌های گوه گوشته‌ای، یک مذاب بازیک ایجاد شده که با صعود به سمت بالا و جایگیری در افق‌های زیرین و میانی پوسته قاره­ای باعث ذوب آمفیبولیت‌ها گردیده است. مذاب‌های اسیدی حاصل شده در نهایت در مسیر گسل‌ها و زون‌های برشی به صورت گدازه به سطح زمین رسیده است.
کلیدواژه‌ها

عنوان مقاله English

Petrology of Eocene dacites and rhyolites from the Gooreh Mountain (NW of Anarak, Isfahan Province)

نویسندگان English

Raziyeh Khalili
Ghodrat Torabi
University of Isfahan
چکیده English

The Gooreh Mountain is located about 55 km northwest of Anarak (northeast of Isfahan) and in the western margin of the Central-East Iranian microcontinent (west of the Yazd block). The field studies show that the Eocene volcanic rocks in this area present two lithologies of gray-colored dacite and light-colored rhyolite. Dacite lavas cross-cut the rhyolite ones and are younger. Petrography indicates that dacites contain the major minerals of plagioclase (labradorite and bytownite), clinopyroxene (augite) and quartz, and rhyolites are mainly composed of plagioclase, sanidine and quartz. Chlorite, calcite, hematite and limonite are the secondary minerals in both rock units, formed by the alteration of clinopyroxene, plagioclase and magnetite. The main textures of these rocks are porphyritic, glomeroporphyritic, anti-rapakivi, sieved and poikilitic. The crystallization order of minerals in dacites is magnetite, clinopyroxene, plagioclase and quartz, and in rhyolites, is plagioclase, sanidine and quartz. Whole rock geochemical analyses show that the studied volcanic rocks of the Gooreh Mountain are sub-alkaline (calc-alkaline magma series) in nature. Moreover, the chondrite and primitive mantle-normalized diagrams are characterized by the enrichment of LREEs and LILEs and depletion of HFS elements (e.g., Ti, Nb and Ta). The negative TNT (Ti, Nb and Ta) anomalies probably indicate subduction-related volcanic arc magmatism. The dehydration of subducting slab and partial melting of the mantle wedge spinel peridotites produced a basic magma that, during ascent through the lower and middle continental crust, led to melting of amphibolites and formation of acidic magmas.

کلیدواژه‌ها English

Petrology
Volcanic rocks
Eocene
Dacite
Rhyolite
Gooreh Mountain
Anarak
Central Iran
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