مطالعه زادگاه و دگرگونی درجه پائین میکاشیست های حاشیه قاره‌ای مرتبط با آمیزه رنگین گیسیان جنوب ارومیه

شیست های منطقه گیسیان در جنوب ارومیه از اجزاء افیولیت سیلوانا در منتهی الیه شمال غربی ایران بوده و در بخش آمیزه رنگین در نزدیکی مرز با ترکیه و عراق برونزد دارند. کانی شناسی ساده این واحد سنگی (موسکویت، بیوتیت، کلریت، کوارتز، فلدسپار و دانه اپک) و فقدان کانی های شاخص متاپلیت ها در آنها مانعی در جهت تخمین شرایط دگرگونی ناحیه ای در این سنگ ها می باشد. ژئوشیمی سنگ ها حاکی از یک سنگ والد رسوبی اسیدی (شبیه پوسته قاره ای فوقانی) با تغذیه کننده آذرین در حد تراکیت به همراه درجه بالایی از شاخص های هوازدگی و تغییر ترکیب در آن می باشد. این رسوبات نابالغ در یک محیط تکتونیکی نظیر حاشیه فعال قاره تکوین یافته و سپس طی تصادم قاره ها دچار دگرگونی ناحیه ای درجه پایین (lt/lpmp) با دو مرحله دگرشکلی شده اند. تخمین دما و فشار به روش مختلف از جمله ترسیم شبه برش دما فشار برای ترکیب سنگ کل معین شیست های گیسیان، برآورد متوسط دما و فشار با واکنش های احتمالی و نیز قطع تعادل های چندگانه واکنش های محاسبه شده توسط نرم افزار thermocalc صورت گرفته و نتیجه برای شرایط اوج دگرگونی ناحیه ای دمای کمتر از 20± 550 درجه سانتیگراد و فشار کمتر از 1 ± 5/5 کیلوبار را به دست داده است.

The provenance studies and metamorphic conditions of the Gysian colored mélange low grade active continental margin schists south of Urmia

IntroductionSuperposition of tectonic and metamorphic proceedings at the margins of the earth’s plates produce some rather complex geological terranes. During the subduction of an oceanic plate, marine sediments are generally frayed off from the subducting plate and accumulate as a wedgeshaped mass. It is usually called an activemargin wedge which develops along the boundary of the nonsubducted plate. This tectonic unit is commonly reported to occur in a collisional regime with lowgrade metamorphic symptoms.Matrials and MethodMesoscopic samples collected from the region were selected by random sampling from Gysian to Kachaleh villages (Iran’s border with Turkey) and surrounding heights by reexamining and ensuring less weathering for microscopic sections and chemical analysis. 10 samples were analyzed in Zarazma Zangan laboratory for obtaining total rock geochemistry with XRF, ICPMS methods.Results and discussionPetrography of metapelites in the Gysian colored mélange combination is very simple and there is no porphyroblast in it. Paragenesis of mica, feldspar, and quartz is common in all samples. Due to the existence of foliation in the rocks, which is also well evident in the hand sample. These can be called mica schists in which white mica is more than biotite. Also, graphite has been seen in some samples. Most of the schists of Gysian region have 5670% silica. The frequency of aluminum oxide was high in the spectrum of 1420%. The total reported iron oxide changes between 8 4 % (bearing in mind iron minerals such as chlorite and biotite). Magnesium oxide often varied from 2 to 4 %. With the aim of determining the rock classification, the diagrams based on the major oxide base and trace elements were used. On this basis, the schist’s parent sedimentary rock has a name between grey wake to lihtarenite and wake to arkoses. The igneous feeder of the sedimentary basin is trachyte to alkali rhyolite. Therefore, in most of the plots of this study, the samples related to these alluvial deposits are produced from an acidic to intermediate igneous parent. The temperature range between 400 and 550 °C at maximum pressure from 3.5 to 5.5 kbar, indicates the conditions of low temperature/medium pressure metamorphism in the active continental margin. The sediment should at least be buried at a depth of more than 1015 km, which matches with the subduction conditions and active margins of the continent.ConclusionsThe studied schists are outcropped in the southern part of Urmia at the Gysian area, a part of the colored mélange of Silvana. The simple mineralogy of the lithological unit and lack of index minerals in them is a problem in estimating the metamorphic conditions of the region and conversely, the total rock chemistry of these rocks is possible to estimate their sedimentary provenance. The geochemistry indicates an acidic sedimentary parent (resembling the upper continental crust) and igneous feeder of trachyte to alkali rhyolite with a high degree of weathering. These sediments were formed in the active continental margin tectonic setting and then metamorphosed during the collision of the continents, the low grade (LT/MP) under two deformation stages.