پتروگرافی و ژئوشیمی دولومیت‌های بخش زیرین سازند لار در البرز مرکزی، جنوب خاوری بهشهر

محورهای موضوعی : پژوهش های نوین در زمین لرزه

محبوبه اصغری ¹ , نادر کهنسال قدیم وند ² , حمید رضا ناصری ³ , حسینعلی مختارپور ⁴ , عیسی متاجی ⁵

1 - دانشجوی دکتری رسوب شناسی و سنگ شناسی رسوبی- دانشگاه آزاد اسلامی واحد تهران شمال
2 - عضو هیئت علمی دانشگاه آزاد اسلامی واحد تهران شمال
3 - عضو هیئت علمی دانشگاه شهید بهشتی
4 - عضو هیئت علمی مجتمع عالی پیامبر اعظم(ص)
5 - عضو هیئت علمی دانشگاه آزاد اسلامی- واحد چالوس

تاریخ دریافت : 1393/09/01 تاریخ پذیرش : 1393/11/01 تاریخ انتشار : 1393/12/01

کلید واژه: بهشهر, پتروگرافی, ژئوشیمی, دولومیت, سازند لار,

چکیده مقاله :

بررسی‌های پتروگرافی سازند لار در ارتفاعات جنوبی بهشهر در استان مازندران نشان می‌دهد که فرایند دولومیتی شدن به طور گسترده بخش زیرین این سازند را تحت تأثیر قرار داده است. در این پژوهش انواع دولومیت‌های این سازند بر اساس اندازه و فابریک تفکیک شده و سپس ویژگی‌های ژئوشیمیایی و نحوه تشکیل آنها مورد بررسی قرار گرفته است. در برش‌های زیرسطحی مورد مطالعه، پنج نوع دولومیت شامل دولومیت های زنوتاپیک A، ایدیوتاپیک S، ایدیوتاپیک E، ایدیوتاپیک C و زنوتاپیک C شناسایی شد. دولومیت نوع اول تقریباً همزمان با رسوبگذاری و در اثر پمپاژ آب دریا تشکیل شده است. سایر دولومیت‌ها به ترتیب درشت شدن اندازه بلور آنها، در محیط‌های کم عمق تا عمیق تدفینی تشکیل شده‌اند. تغییر ترکیب سیال های دولومیت ساز در طی مراحل اولیه تا تاخیری دیاژنز سبب چنین تنوعی در تشکیل دولومیت های این سازند شده است. با توجه به نتایج آنالیز عنصری، میزان عناصر Ca و Sr از دولومیت نوع اول تا پنجم کاهش یافته و میزان عناصر Fe، Mn و Mg افزایش می یابد. تمرکز Na نیز در دولومیت‌های دیاژنتیک اولیه روندی کاهشی دارد ولی در دولومیت دیاژنتیک تأخیری به دلیل تدفین عمیق و احتمالا تأثیر شورابه‌های حوضه‌ای افزایش می‌یابد.

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