ژئوشیمی رسوبات پلایای سبزوار: کاربرد آن در تعیین موقعیت تکتونیکی و شرایط آب‌وهوای دیرینه

Introduction: Known as the dominant landform in arid and semi-arid regions, Playa offers the only evidence of past environmental conditions in semi-arid regions. in other words, considering the fact that Playa constitutes a region with negative water balance for more than half of the year and capillary properties close to the surface and sediments, playa sediments are, in some cases, considered as the only evidence of past environmental conditions in arid and semi-arid regions. On the other hand, geochemical methods are used to examine geochemical processes, tectonics, and the origin of playa sediments, the most important aspect of which is to discover the origins of changes in rocks, relief, climate, tectonic setting, transport history, and diagenesis. It should also be noted that the Quaternary period is considered responsible for the escalation of eolian processes, desert formation, and dryness of lakes, being characterized by drastic changes in climatic conditions worldwide.
 
Materials and method: Stretching over an area of approximately 120 km, the Sabzevar Playa is one of the most elongated depressions in the Khorasan Razavi province in northeastern Iran. The Playa is located between 35°55' – 36°25' north latitude and 56°15' – 57°45' east longitude, covering an area of 2648 km², which is typically classified along with Great Kavir and its surrounding playas (e.g. Damghan Kavir, Bajestan playa, Haj Aligholi Kavir) under the name of the “Dasht-e Kavir” basin. In addition, seasonal hydrological currents flows in the playa. However, according to krinsley (1970), the playa had been a closed basin throghout the Pliocene, which has then been converted to a semi-closed basin under the influence of the fault activities.
The geological nature of the playa comprises of alluvial and evaporation sediments belonging to the Quaternary period, including Windborne dunes, tertiary igneous rocks, and Cretaceous carbonates (dolomite and limestone) which are mainly found in neighboring mountain flanks. Moreover, some areas in the periphery of Sabzevar Playa constitute ophiolite sequences called Sabzevar ophiolites. On the other hand, while intrusive and volcanic units are mainly found around the northern and eastern parts of the Playa, carbonates and detrital sedimentary units containing conglomerates and sandstone are scattered along the playa, possessing abundant outcrops. It should be noted that there exists a metamorphic complex with pre-Jurassic sedimentary sequences near the western half of the playa, being characterized by a varying degree of metamorphism from green-schist to amphibolite.
This study examined one hundred sixty air-dried powder samples to identify both bulk and clay mineralogy using the X-ray diffraction (XRD) technique at the central laboratory of the Ferdowsi University of Mashhad and Razi Applied Science Foundation in Tehran. Moreover, the major concentrations of oxide and trace elements were determined via X-ray Fluorescence (XRF) method based on the procedure introduced by Abdi et al., (2018). Finally, the elemental ratios were, as major representatives of environmental changes (wet/dry periods), calculated at surface and depth levels. 
 
Results: The mineralogical results obtained via the application of the X-ray diffraction (XRD) technique and scanning electron microscopy (SEM-EDS) method revealed that silica oxide, clay, carbonate, and evaporite minerals were the most abundant minerals identified in the sediments of Sabzevar playa (Table 1). On the other hand, the results of the X-ray fluorescence (XRF) technique showed that silicon oxide (SiO₂) had the highest abundance (between 39.8 and 45.5% by weight) among the main oxides identified in the sediments. Furthermore, the amount of L.O.I (Loss on ignition) was 1.8 to 16.9 percent of the sediments’ weight in the samples . It was also found that minor elements in the above-mentioned samples comprised of As Ba Ce, Co, Cr, Cu, Nb, Ni, Pb, Rb, Sr, V, Y, Zr, Zn, and Cl. 
Discussion and Conclusion: Taking the Fe2O3/K2O oxide values ​​and the presence of clay minerals into consideration, it could be argued that according to Herron's classification (Herron, 1988), Sabzevar playa’s sediments are equivalent to wackes. On the other hand, compared to the values ​​of the upper continental crust and the negative trend of Na2O versus SiO2 and Al2O3, the enriched amounts of CaO, MgO, and Na2O oxides could be attributed to carbonate minerals such as dolomite, calcite, and the presence of halite minerals in the playa’s sedimentation environment. Moreover, the enriched amounts of Na2O, Al2O3, and K2O3 could be justified by the presence of feldspars and clay minerals. It was also found that the high amounts of Fe2O3 oxide in the sediments were due to the presence of magnetite. Also, compared to the playa’s other sediments and clay minerals, the relatively small amounts of siliceous sediments were found to be responsible for the positive correlation between SiO2 and Al2O3, the SiO2/Al2O3 ratio, and the depletion of these two oxides.
On the other hand, the low values ​​of TiO2 could be attributed to their derivation from intermediate rocks. Moreover, the enrichment of the Sr element could have occurred due to the replacement of Sr with K and Ca in potassium and calcium minerals, respectively. In addition, taking the positive trend of TiO₂ into consideration_, this study found that compared to Zr and Al2O3 and the ratios of minor elements such as Cr/V and Y/Ni, Sabzevar playa’s sediments are of intermediate to mafic igneous origin similar to ophiolites.
It was also revealed that based on the lithology around Sabzevar playa and the geochemical evidence, the Sabzevar ophiolite series in the northeast of the playa and the metamorphic complex in its western part had played a major role in forming the sediments of Sabzevar playa, whose geochemical data in the original area showed evidence of a dry climate. Furthermore, it was found that the sediments had been left in an active tectonic setting such as oceanic to continental magmatic arcs, which is consistent with the ophiolitic origin of the region and other results found in this regard.