ولكانيسم كواترنري طبس و نقش گسل بزرگ ناي بند

Quaternary volcanism in Tabas and the role of the Nayband great fault

در جنوب شرق شهر طبس منطقه اي به وسعت حدود 400 كيلومتر مربع از بازالت هاي مربوط به كواترنري پوشيده شده است كه از نظر زمين شناسي جزء زون لوت محسوب مي گردند . براساس مطالعات ، بافت اين سنگها عمدتاً پورفيريك است . فنوكريست هاي اين بازالتها عمدتاً اليوين از نوع كريزوليت و اوژيت و پلاژيوكلاز (لابرادوريت ) مي باشند كه در خميره اي از ميكروليتهاي پلاژيوكلاز و بلورهاي ريز پيروكسن و گاهي همراه با شيشه جايگزين شده اند . براساس نمودار هاي مقدار مجموع آلكالن در مقابل مقدارSiO2 ماهيت ماگماي اين سنگها آلكالن تا ساب آلكالن تعيين شد مطالعات ايزوتوپهاي Sr و Nd اين آغشتگي پوسته اي را تاييد نموده و بعلاوه نشاندهنده اين است كه منشاء ماگماي اوليه بازالتها از بخشهاي فوقاني جبه مي باشد . براساس مطالعات صحرايي و نمونه هاي تكتونوماگمايي اين بازالتها از نوع درون قاره اي مي باشند كه در كواترنري در اثر فعاليت گسل بزرگ ناي بند و بازشدگي در محل تقاطع با شاخه هاي فرعي اين گسل به سطح زمين راه يافته اند .

Introduction: Quaternary basalts are one of the last signs of magmatism in Iran، some of which are related to deep fractures and active faults in the Quaternary. Quaternary basalts of Tabas are very important due to their high expansion and location in eastern Iran، and are located 140 km south east of Tabas city and according to the proposed divisions are part of the Lut zone. The aim of this study was to investigate the petrogenesis of these basalts and determine the tectonomagmatic environment of the region، in which the role of the great Nayband fault should be considered. The great Nayband fault is one of the deep and large fractures in Iran's general building، which is about 500 kilometers long and extends to the Bam city. The operation of this fault is straight and displaces Quaternary sediments up to 20 meters. Materials and methods: More than 180 thin sections were prepared and their petrographic studies were carried out. Then 30 samples were selected and analyzed by X-ray fluorescence (XRF) method for 32 main and minor elements. Four samples from the basalts of the study area were selected and at the University of Carleton، Canada، radiogenic isotopic analysis was performed for Sr-Nd isotopes، then the lithological and tectonomagmatic diagrams were drawn. Results and discussion: Most of the basalts are black in color and a little red in the dark brown color، indicating their oxidation. The samples are aphanithic and no minerals can be detected. Microscopically، the texture of these rocks are porphyry with microlithic and sometimes microgranular and some samples also have a porphyric texture with a glass microlithic mesostar. phenocrysts are olivine، clinopyroxene augite and plagioclase with a labradorite compound. . Some samples do not have phenocryst. Phenocrysts vary from about 5 to 20 percent. Magmatic series of these rocks are often alkaline and some of them are sub alkaline. The young basaltic rocks of the Tabas region are derived from an alkaline magma، and this magma is contaminated by crust material on its ascent with different intensities، and where this contamination has been intensified، magma has found the characteristics of sub alkaline. In the tectonic environment، the basalts of the region are within the continental plate. Moho depth along the Nayband fault has the lowest value in Iran's plateau. The decrease in pressure along the fault، especially in areas with new sub-divisions، causes the formation and rise of magma، probably repeated injections of new and hot melt materials have been done in the magmatic chamber. The magma began to differentiate and crystallize in the magmatic chamber and then climb upward along the faults and minor fractures of the Nayband great and has been impregnated with silica stones along its path and its chemical composition has changed a bit ،after passing through the crust، magma erupted on the surface of the earth and became cold and crystallized. The tectonic studies of the area indicate the presence of the Nayband fault during the quaternary period، there are several springs along the Nayband fault، some of which are hot springs، and the most important is the hot spring of Dige Rostam، which itself indicates the depth of this fault. The Nayband fault zone is a straight-right fault whose displacement is estimated to be 50 to 100 kilometers. Signs of activity of Nayband fault during Quaternary، presence of spa springs along the fault zone، dikes of volcanic materials in alluvial deposits، Presence of fault precipitations in alluvial deposits، Channel deviation along fault stretch، The earthquake of 1979، Tabas، which occurred due to the activity of one of the branches of this fault. At the same time as the tension is applied to the northeastern direction، Nayband fault begins to move with the right movement. The continuation of this movement in the fault leads to the branching of the flap in the lateral sections of this fault to be split into the Parvade block، the branches and move dislocation slip on the fault Nayband to amend. The movement of the Tabas block to the west or southwest has created an expansion zone in the south-east of the Tabas plain and the quaternary basaltic eruption. Conclusion: The south-east bases of Tabas to Quaternary، located in the Lut zone، were part of the eastern volcanism of Iran، which erupted in the vicinity of the large and active fault of Nayband . According to geochemical studies، these basalts are part of the alkaline sodic series، some of which show sub alkaline properties. The origin of the primary magma from the fertile layer of the upper mantle، which was followed by the process of separation and normal crystallization، with the presence of phenocrysts of the various indicating the severity of separation of magma before the eruption، which was accompanied by repeatedly hot and fresh injections into the magmatic chamber. Basaltic magma has been impregnated with siliceous rocks during the ascent to the surface of the earth and erupted، and its chemical composition has changed slightly، resulting in sub alkaline properties. Regarding field studies and tectonomagmatic diagrams، Tabas basalts are part of the continental basalts that erupted due to the activity and of the major fault of Nayband and its lateral branches in the early quaternary، indicating the activation of this fault and branches during Quaternary.