پتروگرافی و ژئوشیمی سنگ‌های رسوبی سیلیسی آواری سازند پادها در برش خوش‌ییلاق، البرز شرقی: کاربردی برای تحلیل خاستگاه

در مطالعۀ حاضر، پتروگرافی و ژئوشیمی ماسه‌سنگ‌ها و شیل‌های سازند پادها در برش خوش‌ییلاق واقع در البرز شرقی به‌منظور دستیابی به اطلاعات بیشتر دربارۀ خاستگاه آنها استفاده شد. سازند پادها در برش مطالعه‌شده با ضخامت 341 متر روی بازالت‌های سلطان‌میدان به سن اردویسین بالایی سیلورین قرار گرفته و سازند خوش‌ییلاق با مرز فرسایشی آن را پوشانده است. پس‌از مطالعه‌های صحرایی، تعداد 20 نمونه ماسه‌سنگ متوسط‌دانه برای مطالعه‌های نقطه‌شماری و 8 نمونۀ ماسه‌سنگی و 2 نمونۀ شیلی برای تجزیه‌وتحلیل‌های ژئوشیمیایی انتخاب شدند. باتوجه‌به درصدهای محاسبه‌شده از داده‌های نقطه‌شماری، ترکیب سنگ‌شناسی این ماسه‌سنگ‌ها کوارتزآرنایت، ساب‌آرکوز و ساب‌لیتارنایت تشخیص داده شد. بر اساس میزان q/q+f در ماسه‌سنگ‌ها و ویژگی‌های ژئوشیمیایی ماسه‌سنگ‌ها و شیل‌های سازند پادها ا‌زجمله نسبت al2o3/tio2 و نمودار tio2 در برابر zr، سنگ مادر غالب این نهشته‌ها را می‌توان سنگ‌های آذرین فلسیک در نظر گرفت. نمودارهای qmflt و qtfl خاستگاه زمین‌ساختی ماسه‌سنگ‌های سازند پادها را کراتون پایدار نشان دادند. داده‌های ژئوشیمی نیز اکثراً جایگاه تکتونیکی حاشیۀ غیرفعال قاره‌ای و ریفتی را برای این نمونه‌ها تایید کردند. با‌توجه‌به مطالعه‌های جغرافیای دیرینۀ انجام‌شده در منطقه می‌توان بالاآمدگی‌های تکتونیکی و فرسایش بستر گرانیتی کادومین و همچنین ورود مواد آواری از سنگ‌های گرانیتی کراتون عربستان را سنگ مادر دیگر رسوبات سازند پادها در نظر گرفت.

Petrography and geochemistry of siliciclastic sedimentary rocks of the Padeha Formation in Khoshyeilagh section; Eastern Alborz; implication for provenance

Abstract In this study, petrography and geochemistry of the sandstones and shales of Padeha Formation in Khoshyeilagh section in Eastern Alborz have been used to obtain more information on their provenance. The Padeha Formation in the studied section has a thickness of 341 meters and overlies the Late OrdovicianSilurian SoltanMeydan basalts and disconformably underlies the Middle to Upper Devonian Khoshyeilagh Formation. After field studies, 20 mediumgrained sandstone samples were selected for pointcounting and eight sandstone and two shale samples were selected for geochemical analysis. Based on the calculated percentages of point counting data, the composition of these sandstones has been detected as quartzarenites, subarkose and sub litharenites. Based on Q/Q+F in sandstones and geochemical properties of sandstones and shales, such as Al2O3/TiO2 ratio and TiO2 against Zr, the dominant parent rock of these deposits can be considered as felsic igneous rocks. The QmFLt and QtFL diagrams show the tectonic setting of the Padeha sandstones as a stable craton. The geochemistry has also largely confirmed the tectonic setting of passive continental and rifted margins. Considering paleogeography of the region, tectonic uplifts and erosion of the Cadomian granitic basement and also the entrance of clastic materials from granitic rocks of Arabian craton can be considered as other source rocks of the sediments of Padeha Formation.   Introduction     Petrography and the composition of siliciclastic sediment and sedimentary rock components provide important information on parent rock, which is one of the main factors in the reconstruction of tectonic evolution and sedimentation. In addition, by using the chemical composition (major, trace and rare earth elements) of siliciclastic sediments and sedimentary rocks, one can also estimate the provenance and other sedimentary processes such as weathering, transport and diagenesis (McLennan et al. 1993; ArmstrongAltrin et al. 2012; ArmstrongAltrin 2015).In this study, petrography and geochemistry of the sandstones and shales of the Padeha Formation in Khoshyeilagh section in Eastern Alborz have been used to obtain more information on their provenance. Material & Methods     The Padeha Formation in the studied section has a thickness of 341 meters and overlies the Late OrdovicianSilurian SoltanMeydan basalts and disconformably underlies the Middle to Upper Devonian Khoshyeilagh Formation. In this study, 30 samples of sandstones of the Padeha Formation in Khoshyeilagh section were selected after field sampling and petrographic studies. Twenty medium to finegrained sandstone samples were selected for modal analysis, and about 300 points were counted on every thin section using the GazziDickinson method (Ingersoll et al. 1984). According to the point counting, the main and accessory components of this sandstones are identified and quartz, feldspar and rock fragments modes are utilized for naming the sandstones according to the Folk classification (Folk 1980) as well as different diagrams of the provenance (Dickinson 1985). Based on the calculated percentages of point counting data, the composition of these sandstones has been detected as quartz­arenites, sub­arkose and sub­­­litharenites. Eight samples of mediumgrained sandstone samples with the lowest amount of carbonate cement and two samples of shales in the middle member of the Padeha Formation were selected, crushed and powdered with a mill and sent to the Zarazma Laboratory, Tehran for geochemical analysis (ICPMS method) to determine the major, trace and rare earth elements. Discussion of Results & Conclusions    According to the ratio of quartz to total feldspar and quartz in the sandstones of the Padeha Formation, which is 0.95 and geochemical properties of sandstones and shales, such as Al2O3/TiO2 ratio and TiO2 against Zr, the dominant parent rock of these deposits can be considered as felsic igneous rocks. The QmFLt and QtFL diagrams show the tectonic setting of the Padeha sandstones as a stable craton. Passive continental and rifted margins for Padeha Formation has also largely confirmed by geochemistry of sandstone and shale samples. Considering paleogeography of the region, tectonic uplifts and erosion of the Cadomian granitic basement and also the entrance of clastic materials from granitic rocks of Arabian craton can be considered as the other source rocks of the sediments of Padeha Formation.