کیانیت در هاله‌های دگرگونی

نویسندگان

1 دانشگاه علوم پایه

2 لیورپول

چکیده

مدلهای مختلفی جهت تفسیر علت حضور کیانیت در فاز پایداری در هاله‌های دگرگونی مجاورتی بخصوص آنهایی که دارای سایر انواع سیلیکاتهای آلومینیوم نظیر آندالوزیت و سیلیمانیت یا فیبرولیت هستند ارائه شده‌است. تبلور متوالی کیانیت←آندالوزیت←سیلیمانیت در چنین هاله‌هایی را می‌توان به افزایش درجه حرارت در فشار ثابت (پایین تر از نقطه سه گانه سیستم 5SiO2Al) نسبت داد. تبلور متوالی کیانیت←سیلیمانیت (فیبرولیت) می‌تواند حاصل افزایش درجه حرارت در فشار ثابت (بالاتر از نقطه سه گانه سیستم  5SiO2Al) باشد. تشکیل کیانیت  را می‌توان حاصل شکسته شدن کانی پیروفیلیت یا به احتمال بیشتر مجموعه کلریت بعلاوه مسکویت دانست. عدم حضور کیانیت در برخی از هاله‌های دگرگونی را می‌توان به عدم وجود ترکیب شیمیایی مناسب در سنگهای مادر یا افزایش نرخ سریع درجه حرارت و در نتیجه اوراستپینگ واکنشهای تشکیل دهنده کیانیت نسبت داد.

کلیدواژه‌ها


عنوان مقاله [English]

Kyanite in thermal aureoles

چکیده [English]

Different models are discussed to interpret the presence of kyanite as a stable phase in contact aureoles especially in the andalusite-, sillimanite-bearing aureoles. In such aureoles the polymorphic sequence kyanite → andalusite → sillimanite can be explained in terms of an essentially isobaric path during which kyanite initially crystallises from the breakdown of pyrophyllite or more likely muscovite + chlorite beneath the pressure of the triple point (3-4 kbar  10-14 km depth), followed by andalusite and sillimanite crystallisation.  The sequence kyanite-sillimanite and/or fibrolite can be seen in thermal aureoles subjected  to  isobaric metamorphism  above  the  triple  Al2SiO5  point (5-8 kbar 17-28 km depth). The absence of kyanite in some thermal aureoles can be explained on the basis of either compositional constraint or overstepping of low-temperature reactions due to high heating rates.

کلیدواژه‌ها [English]

  • kyanite
  • Aluminium silicates
  • contact aureole
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