پتروگرافی و ژئوشیمی ایزوتوپی دولومیت‌های سازند بایندور در برش نمونه، جنوب‌غرب زنجان

سازند بایندور به سن پرکامبرین پسین در برش نمونه واقع در جنوب‌غرب زنجان 402 متر ضخامت دارد و از شیل و ماسه‌سنگ با میان‌لایه‌های دولومیتی (با ضخامت حدود 2 تا 8 متر) تشکیل شده است. بر اساس مطالعه‌های صحرایی، رخساره‌های سنگی مربوط به پهنۀ ساحلی و دریای کم‌عمق در سازند بایندور شناسایی شده‌اند. گسترش و تشکیل دولومیت‌ها در سازند بایندور به موازات لایه‌های رسوبی است و لایه‌های دولومیتی گسترش جانبی درخور توجهی را نشان می‌دهند. توالی‌های کربناتۀ این نهشته‌ها درنتیجۀ تاثیر سیالات دولومیت‌ساز به‌طور کامل دولومیتی شده‌اند؛ به‌طوری‌که شواهد بافت رسوبی اولیه به میزان کم حفظ شده است. این دولومیت‌ها حاوی مقادیر درخور توجهی ندول‌های چرت و آثار جلبک‌های سبز آبی (استروماتولیت‌ها) به موازات لایه‌بندی‌های رسوبی‌اند. بر اساس مطالعه‌های پتروگرافی، دولومیت‌های سازند بایندور بر اساس فابریک و اندازۀ بلور به سه گروه دولومیت‌های ریز، متوسط و متوسط تا درشت‌بلور تقسیم می‌شوند. میزان ایزوتوپ کربن 13 در این نمونه‌ها بین 63/1 تا ‰pdb 02/2 و مقادیر ایزوتوپ اکسیژن 18 بین 94/5 تا ‰ pdb15/8 متغیر است. این مقادیر تقریباً مشابه با مقادیر ایزوتوپ‌های کربن و اکسیژن آب دریای پرکامبرین پسین است. با‌توجه‌به نتایج مطالعه‌های صحرایی، پتروگرافی و ژئوشیمیایی و همچنین دمای تشکیل دولومیت‌های ریز و متوسط‌بلور (حدود 38 درجۀ سانتی‌گراد)‌، احتمالاً آب دریا منشا یون منیزیم در این دولومیت‌ها بوده است و این دولومیت‌ها تحت‌تاثیر مدل دولومیتی‌شدن نفوذ و تراوش در محیط‌های تدفین کم‌عمق شکل گرفته‌اند. با ادامۀ روند تدفین و افزایش دمای سیالات دولومیت‌ساز، دولومیت‌های متوسط تا درشت‌بلور از دولومیت‌های ریز و متوسط‌بلور حاصل شده‌اند. داده‌های ایزوتوپ اکسیژن در دولومیت‌های متوسط تا درشت‌بلور نیز تفاوت مشهودی بین این نوع دولومیت با دولومیت‌های نوع اول و دوم نشان نمی‌دهد که احتمالاً بیان‌کنندۀ تشکیل این دولومیت‌ها در شرایط تدفین کم‌عمق و در‌نتیجۀ فرایند تبلور مجدد دولومیت‌های ریزبلور است. این نتایج به همراه محیط رسوبی کم‌عمق زمان ته‌نشست سازند بایندور، تشکیل‌شدن دولومیت‌ها در‌نتیجۀ سازوکار نفوذ و تراوش توسط آب دریا و طی تدفین کم‌عمق را تایید می‌کنند.

Petrography and isotopegeochemistry of the Bayandor Formation dolomites at the type section, southwest of the Zanjan

Abstract The Late Precambrian Bayandor Formation has a thickness of 402 m and consists of shale and sandstone with dolomite interlayers (with a thickness of about 2 to 8 m) in the type section of the southwest of Zanjan. Based on fieldwork studies lithofacies of the Bayandor Formation belong to a coastal plain and shallow marine environments have been considered for the Bayndor Formation. Distribution and expansion of the dolomites in this formation are parallel with the sedimentary layers and has considerable lateral extension in the field. Carbonate sequences of these deposits have all been thoroughly dolomitized, due to the effects of dolomitic fluids and it is difficult to recognize the original depositional textures. In these dolomites, the abundant chert nodules and bluegreen algaes (stromatolites) clearly observed along with the sedimentary bedding. During the petrographic study, the dolomites of Bayandor Formation based on fabric and crystal size divided into the three groups including fine, medium and medium to coarse crystalline dolomites. In these samples the contents of 13C isotope are varies between 1.63 to 2.02 PDB‰ and the contents of 18O isotope are varies between 5.94 and 8.15 PDB‰. These values are almost similar to the carbon and oxygen values of the Late Precambrian seawater. Based on fieldwork, petrography and geochemical results and also by determining of temperature formation (around 38°C), the source of magnesium for fine and medium crystalline dolomites are seawater, and likely the conditions for the formation of these dolomites are seepagereflux dolomitization model during the shallow burial environments. With increasing temperature during the burial process, medium to coarse crystalline dolomites also formed from the previous ones. The oxygen isotope data for the medium to coarse crystalline dolomite does not show a significant difference with the fine and medium crystalline dolomites. This can show that these types of dolomites also formed during the shallow burial conditions and perhaps from the recrystallization process of the fine crystalline dolomites. These results in addition to a shallow marine sedimentary environment of the Bayndor Formation show that the formation of these dolomites might be because of seepagereflux model by seawater during the shallow burial. Keywords: Dolomitization, Carbon and Oxygen isotope, Late Precambrian, Bayandor Formation, Zanjan    Introduction Dolostonecapped shallowwater carbonate successions have been reported throughout the geologic record (Geske et al. 2012). Amongst these, pervasive secondary dolomitization of shallowwater carbonates is the most abundant dolostone type, but also one that is genetically the least understood (Warren 2000). This is because many shallowwater carbonates, formed under different depositional and diagenetic environments worldwide, are characterized by similar fabrics and geochemical features. On the other hand the dolomites are more common in Precambrian than in Phanerozoic and these dolomites are still one of the puzzles of geology. The Bayandor Formation dolostone case example represents one of the so frequent case examples of late Precambrian dolomite deposits worldwide. Due to the importance and also spread amount of dolomites in the Late Precambrian rocks of the Alborz Basin, dolomite layers of the Bayanodr Formation in the type section which are located in the southwest of Zanjan into the Soltanieh Mountains have been discussed in this research. In this paper we aim to provide a comprehensive characterization of the petrography, isotope geochemistry and spatial architecture of Bayandor Formation dolostones.   Material & Methods For the present study, one major outcrop exposing the Bayandor Formation in the southwest of the Zanjan region (at the type locality) were logged and studied bedbybed. The section measured a total length of 402 m and consists of shale and sandstone with dolomite interlayers. During the fieldwork studies, 50 rock samples from carbonate deposits (limestone and dolomite) have been taken for petrography and geochemical studies of these dolomites. Almost 25 thin sections of pervasively dolomitized intervals were studied using transmitted light microscopy. The aim was to assess precursor depositional fabrics, general dolomite crystal properties and textures. In order to differentiate ferroan and nonferroan calcite from ferroan and nonferroan dolomite in thin sections, the staining method of Dickson (1965) was applied. According to Sibley and Gregg (1987) and based on petrographic characteristics (e.g., crystal size and shape, extinction, characteristics of allochems and fabric), several main phases of dolomite development are recognized. Oxygen and carbon isotope analyses were performed on 10 bulkrock dolostone at the Erlangen University, Germany.   Discussion of Results & Conclusions Stratigraphically, the Bayador Formation consists of the 9 lithostratigraphic units and starts with red shale beds with some sandstone intercalations. The thirteen dolomites intercalations (with a thickness of about 2 to 8 m) are found through this succession. Dolomite layers in this formation contain microbial structures. The uppermost 30 m of the succession is made up of greencolored shale beds with dolomite interbeds. The upper contact with Soltanieh Formation is sharp and conformable. Here in this research for the first time, we have documented several lines of evidence all pointing to an early diagenetic origin of the vast majority of the Bayabor Formation limestone dolomitization. Moreover it should be noted that, fieldwork studies of the Bayandor Formation are indicative a coastal plain and shallow marine environments sensitive to sealevel change. Distribution and expansion of the Bayandor dolomites are parallel with the sedimentary layers and has considerable lateral extension in the field. Carbonate sequences of these deposits have all been thoroughly dolomitized, due to the effects of dolomitic fluids and it is difficult to recognition the original depositional textures. During the petrographic study, the dolomites of Bayandor Formation divided into the three groups including fine, medium and medium to coarse crystalline dolomites. In these samples the contents of 13C isotope are varies between 1.63 to 2.02 PDB‰ and the contents of 18O isotope are varies between 5.94 and 8.15 PDB‰. Based on fieldwork, petrography and geochemical results and also by determining of temperature formation (around 38°C), the source of magnesium for fine and medium crystalline dolomites are seawater, and likely the conditions for the formation of these dolomites are seepagereflux dolomitization model during the shallow burial environments. With increasing temperature during the burial process, medium to coarse crystalline dolomites also formed from the previous ones. These results in addition to a shallow marine sedimentary environment of the Bayndor Formation show that the formation of these dolomites might be because of seepagereflux model by seawater during the shallow burial.