باتولیت گرانیتوئیدی صاحب، شمال کردستان؛ شواهدی از ماگماتیسم کرتاسه-پالئوسن در پهنه سنندج-سیرجان

باتولیت گرانیتوئیدی صاحب در شمال استان کردستان و از دیدگاه تقسیم‌ بندی ساختاری ایران، در پهنه دگرگونی‌‌ماگمایی سنندجسیرجان قرار دارد. این باتولیت متشکل از چندین فاز نفوذی با ترکیب کوارتزمونزودیوریت، کوارتزمونزونیت و مونزوگرانیت است. یک دسته دایک ‌های دیابازی نیز باتولیت صاحب را قطع کرده‌ اند. بر اساس بررسی‌های پتروگرافی باتولیت گرانیتوئیدی صاحب از کانی ‌های اصلی ارتوکلاز، پلاژیوکلاز، کوارتز، آمفیبول و بیوتیت، با نسبت ‌های مختلف تشکیل‌شده است. بافت ‌های شاخص در این باتولیت گرانولار، پوئی ‌کیلیتیک و میرمکیتی هستند. ویژگی‌های کانی ‌شناسی باتولیت گرانیتوئیدی صاحب قابل‌مقایسه با گرانیتوئیدهای itype است. بر اساس نتایج سن ‌سنجی upb زیرکن از بخش ‌های مختلف باتولیت گرانیتوئیدی صاحب، سن ‌های 0.56‌± ‌62.03 تا 0.9‌±‌ 58.9 میلیون سال برای نمونه ‌های کوارتزمونزونیت، سن 1.3 ±  67.9 میلیون سال برای نمونه کوارتزمونزودیوریت و سن 0.56‌±‌‌61.10 میلیون سال  برای نمونه مونزوگرانیت به‌دست آمد. به‌طور‌کلی سن ‌های به‌دست آمده از این باتولیت گرانیتوئیدی بیانگر آن است که تشکیل باتولیت صاحب در زمان پالئوسن و مربوط به اشکوب ‌های دانین، سالاندین و تانشین است که به داخل واحدهای قدیمی‌ تر در منطقه تزریق‌شده است. علاوه‌بر این، سن ‌های به‌دست آمده از این باتولیت نشان‌دهنده آن است که باتولیت صاحب در طی یک فاز ماگمایی تشکیل‌نشده است و چندین فاز ماگمایی در زمان‌ های مختلف به داخل پوسته تزریق شده‌ اند و این باتولیت بزرگ را در زمان کرتاسه‌پالئوسن تشکیل داده‌ اند. ظهور توده ‌های گرانیتوئیدی با سن کرتاسه‌پالئوسن مربوط به آخرین مراحل ماگماتیسم مرتبط با فرورانش در پهنه سنندج‌سیرجان هستند.

Saheb Granitoid Batholith, North of Kurdistan: An Evidence of Cretaceous- Paleocene Magmatism in the Sanandaj-Sirjan Zone

Introduction The Iranian plateau is part of the AlpineHimalayan orogenic belt, which consists of several continental fragments separated from each other by major boundary faults and/or ophiolitic suture zones (Gansser, 1981). Generally, the tectonic evolution of Iran has been controlled by the opening and closure of the ProtoTethys, PaleoTethys and the NeoTethys during the PrecambrianCambrian, Paleozoic and Cenozoic, respectively. The study area is located in the northwest of Iran (the Kurdistan province) and 20 km northeast of the city of Saqez. This area is a part of the northern SanandajSirjan Zone (Aghanabati, 2005). This belt is response to opening and subduction of NeoTethyan oceanic crust beneath the Central Iran (Alavi, 1994). During CretaceousTertiary eras, numerous granitoid bodies were formed in this belt. The Saheb granitoid is one of these granitoid bodies which mainly consists of monzogranite, quartz monzonite and quartz monzodiorite. The aim of this research study is to discuss the evolution of the Late CretaceousEarly Paleocene Saheb granitoids in the SanandajSirjan zone based on geology, petrography and geochronology results.   Material and methods In this study, 70 rock samples were collected from different types of intrusive rocks from which 30 thin sections were prepared for petrographic studies. Furthermore, four samples from the granitoid bodies (quartz monzonite, quartz monzodiorite and monzogranite) were selected for UPb dating. Approximately 100 to 150 zircon grains were handpicked by a binocular microscope from each sample. Cathodoluminescence imaging and dating of zircon grains were examined at the China University Geosciences (Wuhan branch). Geochronological analysis were performed by using the (LA)ICPMS method at the China University Geosciences (Wuhan branch). The detailed analytical method is presented in Liu et al. (2010a, 2010b).   Geology of the study area The Saheb granitoid body is located in the SanandajSirjan zone. According to the geological map of Chapan (scale: 1/100000, Kholghi khosraghi, 1999), the Precambrian to Quaternary units are exposed in the study area. The oldest units are the Kahar, Bayandor and Soltanieh Formations with Precambrian to Cambrian age. The Permian sediments, the Ruteh and Doroud Formations, include sandstone, shale and carbonate. The Jurassic units are found in the northwest of the region, and include sandstones and shale. The Cretaceous sedimentary units are located in the south of the study area. These sediments contain sandstone, limestone, siltylimestone, shale and dolomitic limestone. During Late CretaceousEarly Paleocene era the Saheb granitoid intruded within the oldest units and caused Fe skarn type deposits in the Saheb area. The Saheb granitoid have been cut by a series of diabasic dikes.   Results The Saheb granitoid consists of several intrusive bodies containing quartz monzonite, quartz monzodiorite and monzogranite. The major minerals in the quartz monzodiorite consist of plagioclase (35 40%), quartz (15 20%), orthoclase (20 25%), and mafic minerals such as biotite and amphibole (1015%) with granular texture. The quartz monzonitic rocks show granular and poikilitic textures. Plagioclase (25 35%), quartz, orthoclase (30 40%), biotite and amphibole (1015%) are the main important minerals in the quartz monzonite. Plagioclase (2025%), quartz (2030%), orthoclase (3040%), biotite and amphibole (15%) are the major minerals in the monzogranite. Zoning in zircon crystals from all four samples is well developed representing their magmatic origin (Hancar and Miller, 1993). Measurements of UPb in the Saheb granitoid zircon grains of quartz monzonite samples show their ages to be 62.03±0.56 Ma and 58.9±0.9 Ma. The age of monzogranite is 67.9±1.3 Ma and the age of quartz monzodiorite is 61.1±0.56 Ma. Generally, the age of this granitoid body indicates that the Saheb granitoid has occurred during the Cretaceous Paleocene time.   Discussion Based on field and microscopic studies, the Saheb granitoid bodies have been divided into three types of quartzmonzonite, quartzmonzodiorite and monzogranite. The field and mineralogical studies suggest that the Saheb granitoid is an Itype granitoid. The mineralogical variations in this granitoid suggest that the fractional crystallization has played an important role in differentiation of different compositional phases in the Saheb granitoid. According to the geochronological results, during Late Cretaceous to Early Paleocene, the Saheb granitoid intruded within the Permian and Cretaceous units in the magmaticmetamorphic SanandajSirjan zone. These granitoids were formed by subduction of NeoTethys Ocean beneath the Iranian plateau. It should be mentioned that the intrusion of these granitoids into the Permian carbonates and Cretaceous carbonate and shale caused formation of skarn type iron oxide mineralization.   Acknowledgements The authors are grateful to the authorities at the University of Zanjan for their financial support. We also thank the authorities at the China University Geosciences (Wuhan branch) for their financial support to perform UPb zircon analysis.   References Aghanabati, A., 2005. Geology of Iran. Geological Survey of Iran, Tehran, 586 pp. (in Persian) Alavi, M., 1994. Tectonics of the Zagros Orogenic belt of Iran; new data and interpretations. Tectonophysics, 299(3): 211–238.  Gansser A., 1981. The Geodynamic History of the Himalaya. In: H.K. Gupta and F.M. Delany (Editors), Zagros Hindu Kush Himalaya Geodynamic Evolution, Vol. 3, Geodynamic Series. American Geophysical Union, Washington DC, pp. 111–121. Kholghi khosraghi, M.H., 1999. Geological map of Chapan, scale 1: 100000. Geological Survey of Iran. (in Persian) Liu, Y., Gao, S., Hu, Z., Gao, C., Zong, K. and Wang, D., 2010a. Continental and oceanic crust recyclinginduced meltperidotite interactions in the TransNorth China orogen: UPb dating, Hf isotopes and trace elements in zircons from mantle xenoliths. Journal of Petrology, 51(1–2): 537–571. Liu, Y.S., Hu, Z.C., Zong, K.Q., Gao, C.G., Gao, S., Xu, J.A. and Chen, H.H., 2010b. Reappraisement and refinement of zircon UPb isotope and trace element analyses by LAICPMS. Chinese Science Bulletin, 55(15):1535–1546.