شواهد کانی‌شناسی و ژئوشیمیایی آمیختگی ماگمایی در برونبوم‌های موجود در سنگ‌های گرانودیوریتی منطقه آستانه (جنوب غرب اراک)

نویسندگان

1 دانشگاه لرستان

2 دانشگاه اصفهان

3 دانشگاه تهران

چکیده

توده گرانودیوریتی آستانه واقع در جنوب غرب اراک حاوی برونبوم­های ریزدانه کروی و به ندرت بیضوی با ترکیب گابرو، دیوریت و دیوریت کوارتزدار است. قطر این برونبوم­ها ازحدود چند میلی­متر تا 40 سانتی‌متر در تغییر است. اندازه کانی‌های تشکیل دهنده برونبوم­ها در بخش خارجی، ریزتر از مرکز آن­هاست. ریز دانه بودن برونبوم­ها حاکی از سرد شدن سریع آبگون سازنده آن هنگام ورود به درون ماگمای گرانیتوئیدی میزبان است. حضور برونبوم­های ریز دانه مافیک همراه با شواهدی از وجود بافت‌های ناتعادلی مانند فلدسپار با بافت‌های پوئی‌کیلیتیک، کوارتزهای مدور و چشمی شکل که با کانی‌های کوچک با دمای بالاتر مانند پلاژیوکلاز، بیوتیت و آمفیبول به صورت منطقه­ای در بر گرفته شده‌اند، حضور انبوهه­های مافیک، آپاتیت‌های سوزنی، تیغه‌های پلاژیوکلاز کوچک درون پلاژیوکلازهای بزرگتر، همگی آمیختگی ماگمایی را تأیید می‌کنند. بر اساس نتایج آنالیز ریزپردازشی، ترکیب پلاژیوکلاز در گرانودیوریت‌ها از الیگوکلاز تا آندزین و در برونبوم­ها از آندزین تا لابرادوریت متغیر است و غالباً منطقه­بندی عادی نشان می‌دهند که حاکی از آمیختگی ماگمایی است. برونبوم­ها و سنگ‌های میزبانشان از نظر ژئوشیمیایی با یکدیگر تفاوت‌های قابل توجهی نشان نمی‌دهند فقط این که برونبوم­ها نسبت به گرانودیوریت‌ها از Cr, Ni, Co و به میزان کمتر از‌ عناصر HREE غنی شده‌اند. با توجه به الگوی مسطح HREE در برونبوم و سنگ میزبان به نظر می­رسد که هر دو از پوسته زیرین ریشه می­گیرند ولی غنی شدگی بیشتر برونبوم­ها از HREE عمق بیشتری را برای برونبوم­ها تداعی می­کند. برونبوم­ها گلبول­هایی [گویچه­هایی] از ماگمای مافیک هستند که با ماگمای فلسیک آمیختگی جزئی داشته­اند. 

کلیدواژه‌ها


عنوان مقاله [English]

Geochemical and mineralogical evidence for magma mixing in enclaves of Astaneh area (south western Arak)

چکیده [English]

Astaneh granodiorite intrusion, located at south west of Arak, includes an abundant rounded and rarely ellipsoid microgranular enclaves composed of gabbro, diorite and quartz diorite. The diameter of enclaves ranges from a few millimeters to 40 centimeters. These enclaves are very fine-grained and show chilled margins. The size of crystals decreases gardually from center toword margin. The chilled margin is an evidence of rapid magma cooling once the enclaves were surrounded by host felsic magma. Occurrence of the mafic microgranular enclaves within the felsic rocks with disequilibrium textures including: poikilitic texture of K-feldspar megacrystals, ocelli quartz rimmed by zones of fine-grained accumulations of early formed minerals, zoned plagioclase, mafic clots, acicular apatite, small lath-shaped plagioclase within large plagioclase, all indicates the mixing of two magmas. Based on microprobe analyses, the plagioclase crystals show a variable composition from oligoclase to andesine in granodiorite and from andesine to labradorite in enclaves. They have essentially normal zoning. The enclaves and their host rocks, geochemically, are not different from each other and enclaves are enriched in Cr, Ni, and Co. In addition, the enclaves display the HREE enrichment compared to their host granodiorites. Therefore, it seems that the enclaves and granodiorites have been originated from two various depths of magmas with different depths which were located adjacent to each other by magma mixing process.

کلیدواژه‌ها [English]

  • Magma mingling
  • microgranular enclaves
  • Astaneh intrusion
  • Arak
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