تفسیر توالی دیاژنتیکی و ژئوشیمی سازند پروده (ژوراسیک میانی) در بلوک طبس، شرق ایران مرکزی

سازند پروده از نهشته‌های ژوراسیک میانی به عنوان اولین سازند از گروه مگو پس از رخداد سیمرین میانی در بلوک طبس، شرق ایران مرکزی، برجای گذاشته شده است. به منظور تفسیر توالی دیاژنزی و تاریخچة پس از رسوب‌گذاری سنگ‌های کربناته این سازند، دو برش مزینو و کلشانه با ضخامت‌های 69 و 54.9 متر در بلوک طبس، مورد مطالعه قرار گرفته است. فرآیندهای دیاژنزی موثر بر این سنگ‌ها شامل میکریتی شدن، سیمانی شدن، نئومورفیسم، فشردگی، شکستگی و پرشدگی رگه‌ها، سیلیسی شدن، پیریتی شدن، دولومیتی شدن و هماتیتی شدن است. روند مثبت تغییرات نمودارهای آهن در برابر منگنز، آهن در برابر سدیم و همچنین پایین بودن نسبتsr/mn  در نمونه های مورد مطالعه و رابطه خطی معکوس آن با مقادیر mn نشان دهنده بیشترین تاثیر دیاژنز متئوریک و تا اندازه ای دیاژنز تدفینی در این سازند است. همچنین تغییرات ((wt)1000*sr/ca) در برابر mn نشانگر باز بودن سیستم دیاژنتیکی و ترسیم نمودار sr/na در برابر mn، ترکیب نزدیک به نمونه‌های کربناته و آب و هوای مناطق نیمه‌گرمسیری معتدل عهد حاضر است و با نقشه‌های جغرافیای دیرینه ژوراسیک میانی نیز هم‌خوانی دارد. بر اساس شواهد پتروگرافی و آنالیز عنصری، توالی پاراژنزی سنگ آهک‌های سازند پروده در چهار محیط دریایی، آب شیرین، تدفینی و بالاآمدگی تفسیر و طی سه مرحله ائوژنز، مزوژنز و تلوژنز، رسوبات را تحت تاثیر قرار داده‌اند.

Interpretation of diagenetic sequence and geochemistry of Parvadeh Formation in East-Central of Iran (Tabas Block)

IntroductionAfter the MidCimmerian tectonic event in the CentralEast Iranian Microcontinent (CEIM), the marine sediments of the Parvadeh Formation were deposited in the Middle Jurassic, with a significant expansion in the north of the Tabas block and mainly formed from dark gray fossiliferous limestone (Aghanabati, 2014; SeyedEmami et al., 2004). In this study, two sections of Parvadeh Formation in RavarMazino subblock (92 km west of Tabas) and Kalshaneh syncline in Kalmard block (83 km north of Tabas) have been selected for diagenesis and geochemistry studies. Diagenetic events are an important key in sequence stratigraphy studies, identifying surface exposure phases and changes in environmental conditions (Moore and Wade, 2013). Analysis of trace and major elements also leads to a better understanding of environmental conditions characteristics and better identification of fluid systems associated with diagenetic processes (Nader, 2017; Oliveira and Truckenbrodt, 2019). A combination of field, petrographic and geochemical data was used to investigate the effect of diagenesis on the studied deposits. Materials and MethodsAfter completion of field studies, 113 rock samples were selected and studied by Cathodoluminescence (CL) and polarizing microscope after the thin sections were prepared, while Dickson (1966) method was used to detect calcite from dolomite. Also, 11 limestone samples were selected for geochemical analysis and then tested by Zarazma Mineral Studies Company using the ICPOES Varian model to determine trace and major elements. DiscussionPetrographic studies indicate that the Parvadeh Formation has been affected by micritization, cementation (Isopachus, Granular, Drusy, Blocky, and Poikilotopic), neomorphism, replacement (dolomitization, silicification, and pyritization), physical and chemical compaction, iron oxide, and fracturing processes. Calcite was pink to red in staining with red alizarin, while dolomite was not stained. In addition, elemental analysis of samples from the Parvadeh Formation showed that the amount of Sr varied from 269 to 678 ppm, and the amount of Na varied from 604 to 1189 ppm. The changes ranged between 134 to 443 ppm for Mn, 1978 to 13183 ppm for Fe, 33.15 to 38.24% for Ca, and 2785 to 4639 ppm for Mg. Positive trend changes in the Fe/Mn, Fe/Na, the low Sr/Mn ratio in the studied samples, and its inverse linear relationship with Mn values showed the highest effects of meteoric diagenesis and, to some extent, burial diagenesis. Based on the Sr/Ca plot versus Mn, it is possible to figure out the open and closed diagenetic system, the ratio of watertorock interactions, and the oxidation and reduction conditions of the environment (Brand and Veizer, 1980).The bivariate plot of Sr/Ca versus Mn shows that fluids in an open diagenetic system have stabilized the limestone of the Parvadeh Formation. Modern and ancient tropical carbonates differ from their nontropical counterparts by their Sr/Na ratio and Mn contents (Rao, 1991; Winefield et al., 1996). In the Parvadeh limestone, Sr/Na concentrations range from 1.44 to 0.3 (mean: 0.6), similar to carbonate samples and the subtropicaltemperate climate of the recent regions. The plotting of Sr/Mn with Mn provides a valuable measure of the degree of dissolution in limestone (Rao, 1991). The average Sr/Mn ratio in the Parvadeh Formation carbonates is 1.61. The low Sr/Mn ratio in the studied samples and its inverse linear relationship with Mn values show the highest effects of meteoric diagenesis and, to some extent, burial diagenesis. ConclusionMicritization, cementation, neomorphism, physical and chemical compaction, dissolution, Viens filling and fracturing, dolomitization, silicification, and pyritization are the most important diagenetic processes of the Parvadeh Formation. These processes have been carried out in four marine, meteoric, burial, and uplift diagenetic environments, with meteoric and burial diagenesis having the most significant impact on the Parvadeh Formation, respectively. Micriticization processes, formation of microcrystalline dolomites, and some cement (overgrowth) have occurred in the stage of marine diagenesis. In contrast, dissolution, neomorphism, and appearance of some cement (Blocky, Granular, Drusy) have taken place in the stage of meteoric diagenesis. Also, physical compaction, stylolite, formation of some cement (Blocky and Drusy), and medium crystalline dolomites have occurred in the burial diagenesis stage, while the development of fractures and their filling has taken place in the uplift stage. The positive trend of changes in Fe versus Mn and Fe versus Na graphs indicates the effect of meteoric diagenesis. The low Sr/Mn ratio in the studied samples and its inverse linear relationship with Mn values show an increase in burial depth and a decreasing trend of Sr/Na versus an increase in Mn, indicating a close combination of recent temperate carbonates, whose geochemistry results are consistent with subtropical to temperate climates obtained during the Middle Jurassic. Also, changes (1000 * Sr/Ca (wt)) versus Mn indicate the open diagenetic system in the Parvadeh Formation. ReferencesAghanabati, S.A., 2014. Stratigraphy Lexicon of Iran. Volume 3 (Jurassic). Geological Survey of Iran, 545 p. (In Persian)Brand, U., Veizer, J., 1980. Chemical diagenesis of the multicomponent carbonate system1: trace elements. Journal of Sedimentary Petrology, 50: 12191236.Dickson, J.A.D., 1966. Carbonate identification and genesis as revealed by staining. Journal of Sedimentary Petrology, 36: 491505.Moore, C.H., Wade, W.J., 2013. 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