مطالعه زادگاه و دگرگوني درجه پائين ميكاشيست هاي حاشيه قاره‌اي مرتبط با آميزه رنگين گيسيان- جنوب اروميه

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

شيست­ هاي منطقه گيسيان در جنوب اروميه از اجزاء افيوليت سيلوانا در منتهي ­اليه شمال غربي ايران بوده و در بخش آميزه رنگين در نزديكي مرز با تركيه و عراق برونزد دارند. كاني­ شناسي ساده اين واحد سنگي (موسكويت، بيوتيت، كلريت، كوارتز، فلدسپار و دانه اپك) و فقدان كاني­هاي شاخص متاپليت ­ها در آنها مانعي در جهت تخمين شرايط دگرگوني ناحيه ­اي در اين سنگ­ها مي­ باشد. ژئوشيمي سنگ­ها حاكي از يك سنگ والد رسوبي اسيدي (شبيه پوسته قاره­اي فوقاني) با تغذيه كننده آذرين در حد تراكيت به همراه درجه بالايي از شاخص­ هاي هوازدگي و تغيير تركيب در آن مي­ باشد. اين رسوبات نابالغ در يك محيط تكتونيكي نظير حاشيه فعال قاره تكوين يافته و سپس طي تصادم قاره ­ها دچار دگرگوني ناحيه­اي درجه پايين (LT/LP-MP) با دو مرحله دگرشكلي شده ­اند. تخمين دما و فشار به روش مختلف از جمله ترسيم شبه­برش دما- فشار براي تركيب سنگ­ كل معين شيست­ هاي گيسيان، برآورد متوسط دما و فشار با واكنش ­هاي احتمالي و نيز قطع تعادل­هاي چندگانه واكنش­ هاي محاسبه شده توسط نرم ­افزار THERMOCALC صورت گرفته و نتيجه براي شرايط اوج دگرگوني ناحيه­اي دماي كمتر از 20± 550 درجه سانتيگراد و فشار كمتر از 1 ± 5/5 كيلوبار را به دست داده است.

Introduction Superposition 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 wedge-shaped mass. It is usually called an active-margin wedge which develops along the boundary of the non-subducted plate. This tectonic unit is commonly reported to occur in a collisional regime with low-grade metamorphic symptoms. Matrials and Method Mesoscopic samples collected from the region were selected by random sampling from Gysian to Kachaleh villages (Iran's border with Turkey) and surrounding heights by re-examining 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, ICP-MS methods. Results and discussion Petrography 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 56-70% silica. The frequency of aluminum oxide was high in the spectrum of 14-20%. 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 10-15 km, which matches with the subduction conditions and active margins of the continent. Conclusions The 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.