سازوکار تشکیل سیلیس و دولومیت در سنگ‌های کربناتی ژوراسیک فوقانی (آکسفوردین – کیمریجین- تیتونین؟) در غرب بجنورد و جاجرم

نویسندگان

دانشگاه فردوسی مشهد

چکیده

توالی­های کربناتی ژوراسیک فوقانی در کوه­های بینالود، غرب بجنورد و شمال­شرقی و جنوب­غربی جاجرم رخنمون کامل داشته و ضخامت آنها به 500 متر می­رسد. گرهک­های سیلیسی به اشکال مختلف کروی، بیضوی، کشیده، دیسکی و بی­شکل در این توالی­ها وجود دارند. سیلیسی­شدن در این نهشته­ها بیشتر به صورت جانشینی بوده که حضور باقیمانده­های کربناتی در درون گرهک­ها و قسمت­های سیلیسی­شده مؤید این جانشینی است. سیلیس بیشتر به اشکال دانه­ای، دروزی و شعاعی و بیشتر به صورت ریزبلور است. این نوع سیلیس بیشتر در رخساره­های عمیق دارای رادیولر و سوزن اسفنج تشکیل شده است که نشان دهنده خاستگاه آلی سیلیس است. دولومیت در این کربنات­ها بصورت اولیه و ثانویه و به اشکال مختلف قابل مشاهده است. این دولومیت­ها شامل چهار نوع اولیه ریز بلور (D1) فاقد هرگونه دانه، دولومیت­های ثانویه جانشینی ریز تا میانگین بلور (RD1 و RD2)، و سیمان دولومیتی (CD) است. با توجه به شواهد موجود دولومیت­های مورد بررسی به صورت همزمان با رسوبگذاری و کمی پس از آن تا مراحل تدفین کم عمق تشکیل شده اند. مقادیر پایین استرانسیم و سدیم در دولومیت­ها و گرهک­های چرتی نسبت به گل آهکی میزبان نشانه­ی تأثیر فرایندهای دیاژنتیکی تدفینی بر این نهشته­هاست. همچنین مقادیر پایین­تر منگنز و آهن گرهک­های چرتی و دولومیت­ها نسبت به  سنگ آهک­های مورد بررسی تشکیل شده در محیط­های کم اکسیژن می­تواند حاصل تمرکز پایین آن در محلول­های دولومیتی و سیلیسی­کننده و نیز شرایط نیمه اکسیدی هنگام تشکیل آنها باشد. مقادیر ایزوتوپ اکسیژن سنگ آهک­ها (با میانگین 1/3- ‰ VPDB)، دولومیت­ها (با میانگین 91/3- ‰ VPDB) و گرهک­های سیلیسی (با میانگین 82/14- ‰ VPDB) نیز تأثیر فرایندهای دیاژنتیکی تدفینی بر این نهشته­ها را نشان می-دهد. دمای تشکیل دولومیت­های مورد بررسی با توجه به مقادیر ایزوتوپ اکسیژن با دمای 54 تا 68 درجه سانتگراد محاسبه شده است.

کلیدواژه‌ها


عنوان مقاله [English]

Mechanism of silica and dolomite formation in the Upper Jurassic carbonate sediments (Oxfordian-Kimmeridgian-Tithonian?) in the west Bojnurd and Jajarm

چکیده [English]

Upper Jurassic carbonate successions have excellent exposure in Binalud Mountains, west of Bojnurd, northeast and southwest Jajarm area with thickness of about 500m. Chert nodules are present in various shapes such as spherical, elliptical, elongated, discoid and irregular. Silicifications in these deposits are mostly replacement for the carbonate minerals. The presence of carbonate residues in silicified parts of nodules supports this idea. Silica has granular, drusy, radial and mostly microcrystalline fabrics. Microcrystalline quartz in deep facies is mostly containing radiolaria and sponge spicule that indicated organic orgin of silica. Dolomite in these carbonates observed as early and secondary dolomites with different shapes. Four types of dolomites have been identified including fine crystalline (D1) with no allochem and low quartz grains, secondary fine to medium crystalline replacement dolomites (RD1 and RD2) and cements dolomites (CD). On the basis of these data, the studied dolomites have formed penecontemporaneous and early post depositional to shallow burial conditions. Low amount of Na and Sr in dolomites and chert nodules relative to host lime mud indicate the influence of burial diagenetic process on these deposits. In addition, the lower amount of Mn and Fe in chert nodules and dolomites relative to studied limestones that formed in low oxygen conditions, can be related low concentration in dolomitized and silicified fluids and suboxic condition during their formation. Oxygen isotope values of lime mudstones (average -3.1 ‰ VPDB), dolomites (average -3.91 ‰ VPDB) and silica nodules (average -14.82 ‰ VPDB) indicate the influence of burial digenetic process. Based on oxygen isotope values, the formation temperature of studied dolomites is about 54 to 68◦ C.

کلیدواژه‌ها [English]

  • Silicification
  • dolomitization
  • nodule
  • Upper Jurassic
  • west Bojnurd
  • Jajarm
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