ترکیب شیمیایی و ویژگی‌های نوری آمفیبول‌های سدیمی- کلسیمی و کلسیمی شیست‌های بازیک منطقه شاه‌نشین، جنوب غرب زنجان

نوع مقاله : مقاله پژوهشی

نویسندگان

دانشگاه زنجان

چکیده

منطقه شاه­نشین در جنوب غرب زنجان و در مرز استان­های زنجان و کردستان قرار دارد. بر اساس بررسی­های سنگ­نگاری، شیست­های بازی این منطقه شامل اکتینولیت شیست، اکتینولیت پیروکسن شیست، اکتینولیت پیروکسن گارنت شیست و اکتینولیت اپیدوت شیست هستند. اکتینولیت شیست­ها فراوان­ترین نوع شیست­های بازی هستند و از آمفیبول، موسکوویت، کلریت و آلبیت به عنوان کانی­های اصلی تشکیل شده­اند. در این شیست­ها، آمفیبول­ها در دو جایگاه بافتی مایل به شیستوارگی اصلی و هم راستا با شیستوارگی اصلی دیده می­شوند. بررسی ویژگی­های نوری آمفیبول­ها نشان می­دهد که از مرکز به سمت لبه بلور، مقادیر شکست مضاعف و زاویه خاموشی(nγᴧ c) کاهش یافته و رنگ غیر تداخلی از سبز مایل به آبی در راستا nγ در مرکز به سبز کمرنگ در لبه تغییر می­کند. از سوی دیگر، بررسی ویژگی­های شیمیایی آمفیبول­ها نشان می­دهد که مرکز آن دارای ترکیب وینچیت – ریشتریت است، درحالی که لبه­ها دارای ترکیب اکتینولیت هستند. بنابراین، تغییر ویژگی­های نوری ارتباط مستقیمی با تغییر ترکیب شیمیایی آمفیبول­ها دارد که بیشتر بر اثر عکس جانشینی گلوکوفانی CaM4 MgOct AlOctو جانشینی پلاژیوکلازی
(CaM4 AlT) رخ می­دهد. محاسبات فشار – دما برای ترکیب شیمیایی مرکز آمفیبول­ها نشان دهنده شرایط میانگین فشار kbar 14 و دمای OC 440 است درحالی که لبه آن­ها در شرایط میانگین  kbar4 و OC 520 تشکیل شده­اند. از این رو، ساختار منطقه­بندی نوری و شیمیایی در آمفیبول­های شیست­های بازی منطقه شاه­نشین برآمده از تغییر شرایط فشار – دمای دگرگونی است.      

کلیدواژه‌ها

عنوان مقاله [English]

Chemical compositions and optical characters of sodic–calcic and calcic amphiboles in basic schists of Shahneshin area, southwest of Zanjan

نویسندگان [English]

  • Derakhsh
  • Izadyar
  • Pouromidi
  • Hajabolfath
  • Tahmasebi
چکیده [English]

Shahneshin area is located in southwest of Zanjan at the border between Zanjan and Kurdestan provinces. Based on petrographic investigation, Shahneshin basic schists could be divided into actinolite schist, actinolite pyroxene schist, actinolite pyroxene garnet schist and actinolite epidote schist. Actinolite schists are the most abundant rocks among basic schists and are composed of amphibole, muscovite, chlorite and albite as major minerals. Amphiboles occurred in two different fabric including inclined and parallel to the main schistosity. Optical characters of amphibole changes from core towards rim in which birefringence and extinction angle decrease and ppl color changes from green-blue to the pale green. In the other hand, chemical analysis of the same amphibole from core towards rim shows that core has richerite-winchite composition while rim has actinolite composition. Therefore, there is clear relation between optical and chemical characters in which reverse glaucophane substitution (CaM4 MgOctAlOct) and plagioclase substitution (CaM4 AlT) occurred. P-T calculation using chemical composition of amphibole cores show that they formed under 14 kbar pressure and 440 OC temperature while their rims formed under pressure of 4 kbar and temperature of 520 OC. Thus optical and chemical zoning observed in amphiboles in Shahneshin basic schists could be formed due to changes in pressure and temperature of the metamorphism.

کلیدواژه‌ها [English]

  • Sodic–calcic and calcic amphiboles
  • basic schists
  • optical characters
  • Shahneshin
  • Zanjan

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سابقه مقاله

  • تاریخ دریافت: 10 دی 1398
  • تاریخ بازنگری: 03 بهمن 1398
  • تاریخ پذیرش: 14 اسفند 1398

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