دياژنز و كاني شناسي اوليه سنگ‌هاي آهكي ژوراسيك فوقاني در بلوك طبس

Diagenesis and primary mineralogy of the upper Jurassic carbonate rocks in the Tabas block

سنگ­ هاي سري­­ هاي راور و سازند بيدو با سن آكسفوردين – كيميريجين متعلق به ژوراسيك پسين در جنوب بلوك طبس گسترش دارند. در اين بخش از بلوك طبس چهار برش زمين­شناسي مورد بررسي قرار گرفته است. سنگ­شناختي غالب اين برش­ها، آهك و شيل بوده كه قسمت فوقاني آنها با لايه ­هاي تبخيري پوشيده شده است. مهم­ترين فرآيندهاي دياژنزي مشاهده شده در مقاطع نازك مورد مطالعه آهن­دار شدن، ميكريتي شدن، دولوميتي شدن، سيماني شدن، ندول­هاي تبخيري و تراكم مي­باشند. تفسير توالي پاراژنتيكي سنگ­هاي آهكي مورد مطالعه نشان دهنده تاثير فرآيندهاي مختلف دياژنزي در مراحل دريايي، متائوريكي و تدفيني مي­باشد. در اين پژوهش، نمونه­هاي آهكي از برش­هاي حورجند، لكركوه شمالي و جنوبي، و خورند مورد تجزيه ژئوشيميايي قرار گرفتند. ژئوشيمي عناصر فرعي (Na, Sr, Mn) نظير بالا بودن مقدار Sr/Na با ميانگين بيش از 2 حاكي از تركيب كاني­شناسي اوليه آراگونيتي است. بالا بودن مقادير Mn حاكي از تاثير دياژنز متائوريكي روي اين نمونه­ها مي­باشد. پايين بودن نسبت Sr / Mn و Sr/ Ca در مقابل Mn حاكي از دگرساني بيشتر در اين نمونه­ ها است كه يك سيستم دياژنتيكي عمدتاً باز با نسبت تبادل آب به سنگ بالا را نشان مي­دهد.

IntroductionThe Oxfordian – Kimmeridgian rocks of the Ravar series and the Bido Formation with the age of Upper Jurassic, are distributed in the south of Tabas Block. The most important diagenetic processes in the studied thin sections are namely hematitization, micritization, dolomitization, cementation, evaporite nodules and compaction. Interpretation of paragenetic sequences of studied samples show the effect of different processes in the marine, meteoric and burial diagenetic environments. Major and minor elements and comparison with previous studies, indicate that aragonite was an original carbonate mineralogy for the Upper Jurassic carbonate rocks in the Ravar area. Low Sr / Mn and Sr/ Ca versus Mn show higher alteration in the studied samples which mainly show an open diagenetic system with high water- rock interaction.Material and MethodsAfter field observations, 78 samples of carbonate rocks were selected from four geological sections of north of Lakar- Kuh -1, 2, Horjond, Baghamshah, and Khorand. After microscopic observations, 21 carbonate samples from studied intervals were selected for elemental investigations in the geochemistry lab of the Shahid Beheshti University. In order to determine original carbonate mineralogy, these data were compared with Mozduran Formation carbonates, Gordon warm water tropical limestones of Tasmania, the Ilam, Fahliyan Formations and Qaleh- Dokhtar carbonates with original aragonite mineralogy. Low Sr/Mn ratio show high water – rock interaction.Results and discussionActivity of microscopic organisms in these environments is associated with micritization in the lagoon environments. Bladed cement forms in the marine environments and equant, blocky and drusy calcite cements are formed in meteoric and mainly in burial environments. With increasing depth of burial, chemical compaction has formed dissolution seams and stylolite. Hemititization and dolomitization processes are formed in the late diagenetic stages.Geochemistry of major elements (Na, Sr, and Mn) such as high Sr/Na ratio with the average of > 2 show aragonite was an original carbonate mineralogy. High Mn values suggest the effect of meteoric diagenesis on the studied samples. Higher Sr values in some of the samples may indicate original aragonite mineralogy and low impact of diagenetic processes. The high Sr/Ca and Mn variations in the studied carbonates represent the effect of meteoric diagenesis in closed and open diagenetic systems. ConclusionSr/Mn versus Mn variations in samples fall within and very close to aragonite field of the Mozduran Formation due to higher impact of meteorite diagenesis. This probably shows the effect of meteoric diagenesis with higher water - rock interaction (W/R) in an open diagenetic system. Relatively low Sr/Mn values show an open diagenetic system for studied carbonate samples. The Sr/Ca ratio in the studied carbonates show a relatively open diagenetic system with high water- rock interaction.