شواهد بازشدگي ترياس نئوتتيس در زون سنندج- سيرجان با توجه به شواهد پتروگرافي، ژئوشيمي و مطالعات سن سنجي، منطقه اسفندآباد يزد

Evidence of triassic neotethys opening in Sanandaj_Sirjan zone, according to petrographic, geochemical and geochronological studies, in Esfandabad region of Yazd

تاريخچه تكاملي زون سنندج-سيرجان، تحت تاثير دو رژيم كششي و فشارشي بوده كه باعث ايجاد بازشدگي، فرورانش، تصادم و بسته شدن نهايي آن شده است. اين پژوهش با استناد به مطالعات پتروگرافي، ژيوشيمي و همچنين داده هاي سن سنجي سنگ هاي آتشفشاني منطقه اسفندآباد يزد، شواهدي از باز شدگي و ولكانيسم حاصل از آن را در اختيار قرار مي دهد. توالي مورد مطالعه شامل گدازه هاي تيره رنگ آندزيتي، ريوليتي و بازالتي همراه با ميان لايه هاي آذرآواري همراه با واحدهاي رسوبي است. در مقاطع ميكروسكوپي، كاني هاي اصلي تشكيل دهنده در سنگ هاي آندزيتي به ترتيب فراواني شامل پلاژيوكلاز، پيروكسن و كوارتز، در سنگ هاي ريوليتي پلاژيوكلاز، فلدسپار آلكالن و كوارتز و در بازالت ها شامل پلاژيوكلاز، اوليوين و پيروكسن است. براساس داده هاي ژيوشيميايي، سنگ هاي مورد مطالعه داراي تركيب شيميايي بازالتي و تراكي داسيتي با گرايش كالك آلكالن و توله ايتي مي باشند. همچنين نمودارهاي تعيين جايگاه تكتونيكي محيط درون صفحه اي را در تشكيل آنان تاييد مي نمايد. مطالعات ژيوشيميايي نشان داد كه سنگ هاي تراكي داسيتي حاصل 5% ذوب بخشي يك منشا گارنت-لرزوليت و سنگ هاي بازالتي حاصل20 % ذوب بخشي يك منشا اسپينل-لرزوليتي مي باشند. مطالعات سن سنجي براساس زيركن هاي موجود در سنگ هاي تراكي داسيتي به روش اورانيوم-سرب سن 6/6 +240 معادل با ترياس زيرين را براي اين بازشدگي ارايه داده است كه با بازشدگي و تشكيل اقيانوس نيوتيس در پرموترياس مطابقت مي كند.

IntroductionThe evolutionary history of the Sanandaj-Sirjan zone has been influenced by two extensional and compressional regimes that have caused rifting, subduction, collision and its final closure.The study area is located in Esfandabad city of Yazd province. The sequence contains dark colored andesitic, rhyolitic and basaltic lava with interlayers of volcanoclastic and sedimentary units. The volcanic rocks detected contain aphanitic textures and have dark gray to black color with oxidized surface, fractures and pores. These volcanic rocks have a layered flow and are mostly seen as dome-shaped structures and are sometimes in the form of dikes. Another characteristic of this group of rocks is the green color obtained from the secondary minerals of chlorite and epidot, as a result of the alteration applied to them. The volcanoclastic rocks are mainly fine-grained and welded purple tufts. Materials and methodsIn order to achieve the aim of this study, after descriptive studies, field operations were performed in three stages and 100 samples were taken from the desired outcrops and thin sections were prepared. After petrographic studies, less altered samples were selected and sent to the GeoLeb Laboratory in Canada for geochemical analyzing using XRF, ICP-MS and ICP-IAS methods. Age studies have been conducted at the Caltech Institute in California. Results and discussionIn microscopic studies, the dominant texture in basaltic rock is porphyry, consisting of plagioclase, olivine and pyroxene in a fine-grained matrix composed of plagioclase, epidote, chlorite, sphene and opaque minerals. Andesitic rocks have microporphyric and seriate textures consisting of plagioclase, in a fine grained matrix consisting of plagioclase, pyroxene, quartz, epidote, carbonate and chlorite alteration products and opaque minerals. Rhyolite rocks have a microcrystalline to hyalline texture consisting of quartz, alkali feldspar and plagioclase. Evidence such as sosoritization of plagioclase, filling of cavities by chlorite and zeolite, idingitizated or chlorinated olivine and uralite pyroxene crystals indicate the performance of metamorphic phenomena in the studied rocks.ConclusionBased on geochemical data, the studied rocks have basaltic and trachydacite composition with tholeitic and calcalkaline affinity. The pattern of spider diagrams in the study area are almost the same but with a different frequency. The enrichment of Th and U in these diagrams can be attributed to crustal contamination and enrichment of LREE to low melting rates of mantle source (less than 15%), or contamination of magma with crustal materials. The negative Eu anomaly is related to the differentiation of plagioclase during magmatic crystallization or its persistence at the source when H2O activity is low. In the tectonic setting diagrams, which are used to separate the intra-plate basalts from other tectonic environments, the samples are located in the intra-plate field (WPB). Geochemical studies show an asthenospheric mantle source that has been affected by crustal contamination and crystalline differentiation. Petrographic evidence, such as the presence of porphyric texture in volcanic rocks in the area, indicate that magma has stopped in crustal chambers and contaminated it. Geochemical studies showed that trachydacite rocks form from5% melting of garnet-lorzolite and basaltic rocks produced compose from 20% melting of a spinel-lerzolite origin. U-Pb dating studies of trachydacite presents 240+ 6.6 ma age equivalent to the lower triassic which match the opening and formation of neotethys ocean in permotriass.