پتروگرافی، ژئوشیمی و سازوکار دولومیتی‌ شدن توالی‌ های پرکامبرین پسین در برش چپقلو، جنوب‌‌شرق زنجان

برش چپقلو در فاصلۀ 90 کیلومتری جنوب‌شرق شهر زنجان و در کوه‌های سلطانیه قرار دارد. مطالعۀ حاضر دربارۀ نهشته‌های پرکامبرین پسین شامل سازند بایندور به ضخامت 605 متر و متشکل از شیل با میان‌لایه‌های دولومیتی و بخش دولومیت زیرین سازند سلطانیه به ضخامت 100 متر انجام شده است. مطالعه‌های پتروگرافی و ژئوشیمیایی (xrd icp-oes) دولومیت‌های این دو سازند نشان داد این برش دارای 5 نوع دولومیت به شرح زیر است: 1. دولومیت‌های بسیار ریزبلور؛ 2. دولومیت‌های ریزبلور؛ 3) دولومیت‌های متوسط‌بلور؛ 4. دولومیت‌های متوسط تا درشت‌بلور؛ 5. سیمان دولومیتی. نتایج پژوهش حاضر نشان می‌دهند بیشترین تنوع دولومیت‌ها در برش یادشده به انواع دولومیت‌های ریزبلور و متوسط‌بلور مربوط است. دولومیت‌های بسیار ریزبلور به دیاژنز اولیه مربوط هستند. دولومیت نوع دوم از تبلور دوبارۀ دولومیت نوع اول تشکیل شده است. دولومیت‌های نوع سوم تا پنجم در مراحل آخر دیاژنز و در اثر سیالات دولومیت‌ساز (احتمالاً آب‌های بین‌حفره‌ای) تشکیل شده‌اند. سازوکار دولومیتی‌شدن برای دولومیت نوع اول الگوی سبخا، دولومیت‌های نوع دوم از نوع تدفینی کم‌عمق و دولومیت‌های سوم تا پنجم از نوع تدفینی عمیق است. مطالعه‌های ژئوشیمیایی نشان می‌دهند مقدار کم استرانسیم در دولومیت‌های درشت‌بلور این برش نسبت به آب دریا و دولومیت‌های عهد حاضر و حتی نسبت به دولومیت‌های بسیار ریزبلور نشان از درشت‌بلورشدن و عمق تدفین بیشتر این دولومیت‌ها دارد.مقدار فوق‌العاده زیاد آهن و منگنز در دولومیت‌های برش چپقلو نسبت به دولومیت‌های عهد حاضر و آب دریا نشان‌دهندۀ تشکیل این دولومیت‌ها در شرایط احیایی و حضور باکتری‌های احیاکنندۀ سولفات است.

Petrography, geochemistry and mechanism of dolomitization of late Precambrin succession in Chopoghlu Section, South East of Zanjan

Introduction      Dolomite is mainly composed of internal structures and geochemical properties that usually indicate the conditions of formation environments and can be formed through the different mechanisms. For all these mechanisms, fluid flow and suitable amount of the magnesium in the fluid are required. During recent years, studies of dolomites have been one of the most important issues in the international sediment researches. Most of the Precambrian carbonate sequences are affected by dolomitization. Due to the spread amount of dolomites in the late Precambrian rocks of the Alborz Basin, carbonate systems of Chopoghlu section which are located in the southeast of Zanjan into the Soltanieh Mountains have been discussed in this research. Despite the significant presence of dolomite in the late Precambrian carbonate platforms of the Chopoghlu section, no work has been done up to now related to the texture, geochemical properties and also the origin of the late Precambrian dolomitization in this section. Therefore, this study was carried out to determinate the textural, mineralogical and geochemical characteristics of the late Precambrian dolomites in Chopoghlu section to determinate its dolomitization mechanism and the nature of dolomite fluids. Material & Methods     Different fieldwork and laboratory methods were used in this research. During the fieldwork studies, 50 rock samples from carbonate deposits (limestone and dolomite) have been taken for petrography and geochemical studies of these dolomites to determinate dolomitization mechanisms. The thickness of the layers is measured and the stratigraphic column of this section drowns. For the petrography study, 18 thin sections of these dolomites have been prepared. After petrography studies, 14 samples of these dolomites were selected for geochemical analysis to determinate the mechanisms of dolomitization by ICPOES method. Also, 4 samples were analyzed for mineralogy studies and the determination of major and minor minerals by XRD method. Discussion of Results & Conclusions     The study of dolomite in the Chopoghlu section identified five types of dolomite including: very finecrystalline, finecrystalline, mediumcrystalline, medium to coarsecrystalline and also dolomitic cement. It should be noted that the finecrystalline dolomites is the most abundant type of dolomite in the region. In this section, dolomite cements which filled the vugs and fractures have been distinguished into the some of the studied samples. XRD results from dolomite samples in the region indicate that the most abundant mineral is dolomite, and calcite, quartz and muscovite minerals are present as minor and trace minerals in the studied rocks.   The results of this study clearly show that the very finecrystalline dolomite is related to primary diagenesis and is formed under a sabkha dolomitization model in a shallow and saline environment (average of sodium is around 478 ppm). The low amount of the strontium content into the coarsecrystalline dolomite in this section (average of 43 ppm) relative to sea water and the presentday dolomites, and even related to the very finecrystalline dolomites, clearly shows that the coarse crystalline dolomites are formed during greater burial depths. Second type of dolomite or finecrystalline dolomite is formed under shallow burial depth. These dolomites are formed via marine fluids with normal salinity and are interpreted from the progressive recrystallization of previous dolomite. The presence of stylolite is the main characteristic of this type of dolomite into the studied section. Third to fifth type dolomites formed during the last stages of diagenesis and are related to deep burial and impact of hypersaline interstitial fluids. The higher amount of iron and manganese into the chopoghlu late Precambrian dolomites in comparison with modern dolomites and seawater probably indicate that the formation of these kinds of dolomites may be associated with reduction conditions and the presence of sulfatereducing bacteria. In general, the dolomitization mechanism for the firsttype of dolomite is related to Sabkha model, for the secondtype of dolomite is related to shallow burial dolomitization and finally for the third to fifthtypes of studied dolomites are related to deep burial dolomitization.