كاني شناسي و ژئوشيمي تپه‌هاي ماسه‌اي جنوب سه قلعه شهرستان سرايان، استان خراسان جنوبي

Mineralogy and geochemistry of aeolian sand dunes in south of Seh-Qaleh, Sarayan, Southern Khorasan province

يكي از نمودهاي اصلي مناطق بياباني آثار فرسايش بادي مي باشد. تپه هاي ماسه اي از جمله اشكال شاخص اين گونه مناطق محسوب مي شوند.تاكنون مطالعات تفصيلي در مورد ژئوشيمي تپه هاي ماسه اي و از جمله برخانوييد در ايران صورت نگرفته است. از اين رو در اين پژوهش تعداد 27 تپه ماسه اي بادي در 4 زون در محدوده اي با مختصات´28°33 تا´34 °33عرض شمالي و ´21 °58 تا ´22 °58 طول شرقي، در فاصله 13 كيلومتري جنوب سه قلعه شهرستان سرايان در شرق ايران مورد نمونه برداري و مطالعه قرارگرفته‌اند. بدين منظور، نخست تركيب كاني شناسي تپه هاي ماسه اي سرايان بررسي شده و سپس به منظور انجام مطالعات ژئوشيميايي تعداد 12 نمونه با  استفاده از روش XRFمورد تجزيه و تحليل قرار گرفته اند. داده هاي حاصل از مطالعات ژئوشيميايي اكسيدهاي مختلف و ترسيم نمودارهاي مربوطه نشان داد كه يك منشأ آذرين حد واسط تا فلسيك با ميزان كوارتز متوسط و محيط تكتونيكي حاشيه فعال قاره اي را براي تپه هاي ماسه اي سرايان بايد در نظر گرفت. شاخص هوازدگي شيميايي (CIW)در تپه هاي ماسه اي مورد مطالعه از 82/81 درصد تا 84/96 درصد در نوسان است كه اين مقدار بيانگر آب‌ و هواي گرم و خشك مي باشد. ميزان اكسيدهاي اصلي TiO2Fe2O3 و MgOمي تواند نشانه فراواني يا فقدان كاني هاي سنگين باشد كه با بررسي اين اكسيدها شاهد كاهش اين عناصر از غرب به شرق منطقه و در نتيجه شاهد كاهش كاني هاي سنگين از غرب به شرق منطقه مي باشيم. اين موضوع با وجود باد شمال غرب كه داراي قدرت فرسايندگي و جابجايي زياد مي باشد هم‌خواني دارد.

Iran is located in arid belt of the earth that the effect of the wind erosion is one of the main phenomena in this area. In global scale, it can be said that the risk and importance of wind erosion would not be less than water erosion. Sand dunes are the common and dominant shapes in these regions. So far, detailed geochemical analysis of these dunes, e.g. barchanoids, has not been carried out in Iran; thus, research on this subject can answer to fundamental questions about the geochemistry of these sand dunes in this area of Iran. Therefore, totally 27 sand dunes within 4 different zones were sampled and studied in this research that geographically coordinates N33 28 to 33 34 latitude and E58 21 to 58 22 longitude which are located in 13 Km south of SehQaleh, Sarayan city in east of Iran that is a part of northern Lut Block and is greatly affected by wind erosion due to the climatic condition, several droughts, wind blow and lack of vegetation. For this purpose, firstly mineralogical compositions of Sarayan sand dunes have been studied by Rooney and Basu (1994) method and then 12 samples were collected and analyzed using XRF method for geochemical investigations. The mineralogical study in sand dunes of Sarayan indicates that the rock fragments has the highest frequency with 50 to 62 percent abundance (mean 60.11 percent), then quartz with 33 to 46 percent (mean 36.62 percent) and finally feldspar has 1 to 5 percent (with an average of 3.14 percent) abundance. Study on major oxides shows that the SiO2 with 64.20 percent is mostly frequent oxide in comparison with the other major oxides of the studied samples. The resultant data of different oxides and relevant diagrams indicate an igneous, intermediate to felsic origin with normal quartz content and tectonically an active continental margin environment for Sarayan sand dunes. Calculated chemical index of Weathering (CIW) ranges between 82.81 to 84.96 percent which indicates an arid and warm climate. Also, slight chemical weathering and strong physical weathering in this region were approved. The study of heavy minerals can provide some evidences for recognition of probable source of sediments. Also, the values of major oxides such as MgO, Fe2O3 and TiO2 can provide evidence of abundance or lack of heavy minerals. For this reason, firstly the amounts of Fe2O3, TiO2 and MgO major oxides in 12 samples of sand dunes from four zones in a northsouth direction were analyzed and no remarkable trend were found. Then the values of these major oxides in samples from four zones within a westeast direction were examined and revealed an eastward decreasing trend. This issue related to northwestern winds that have a significant erosional influence and is in agreement with the direction of main northwestern wind which is responsible for the formation of barchanoid sand dunes in this region. The analysis of major oxides indicates that SiO2 and Al2O3 are the most abundant oxides in the studied samples. The Al2O3 values plotted versus Na2O and SiO2 show a positive relationship whereas Al2O3 versus the other major oxides such as Fe2O3, K2O, MgO, TiO2 and MnO reveal negative trend that it can be related to lack of notable amounts of clay minerals and feldspar within the studied samples. On the other hand, positive relationship of Al2O3 and NaO2 might be caused due to the presence of Na within the structure of aluminosilicate minerals. Moreover, the values of TiO2 in the studied samples indicate a felsic to intermediate composition for source rock. The study of most important trace elements in the studied sand dunes showed high values of barium and zircon and also low amount of strontium which could be caused by frequency of original minerals such as Kfeldspar and also deficiency of plagioclase within the source rocks. The low values of Sr might be related to negligible amounts of carbonate minerals, e.g. calcite and dolomite, within the studied samples. In addition, the amounts of trace elements such as Ba and Zr suggest a granitic and/or reworked sedimentary source rocks, associated with high concentration of heavy minerals. Furthermore, the application of weathering indices and graphs showed that plagioclase are slightly weathered and also weathering determination graphs suggest a negligible effect of chemical weathering in this area. Consequently, the main type of weathering in Sarayan region could be considered as physical weathering