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ویژگی های بازالت های کواترنری منطقه بیجار در امتداد گسل زاگرس
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نویسنده
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ملکی سحر ,ملکی سحر ,امامی محمدهاشم ,خیرخواه منیره ,سعیدی عبدالله
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منبع
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كواترنري ايران - 1394 - دوره : 1 - شماره : 1 - صفحه:29 -43
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چکیده
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منطقه مورد بررسی یکی از جوانترین جریان های آتشفشانی واقع در زون ساختاری سنندج- سیرجان و در امتداد خطی شمال باختری- جنوب خاوری در راستای گسل زاگرس است. ترکیب کلی سنگ های منطقه بازیک و شامل اولیوین بازالت، پیروکسن بازالت، تراکی بازالت بیشتر با ماهیت آلکالن است. وجود گزنولیت های گنایسی و گزنوکریست های کوارتز نشانگر آلایش پوسته ای این سنگ ها است. نمودار تغییرات عناصر کمیاب در برابر di نشان می دهند که در تشکیل سنگ های منطقه فرایند تفریق موثر بوده است و پراکندگی های موجود در نمودارها می تواند وابسته به پدیده آلایش ماگما با پوسته باشد. غنی شدگی این سنگ ها از lil نشان می دهد که افزون بر پدیده تفریق ماگمایی فرایندهای دیگری شامل اختلاط ماگمایی و آلودگی پوسته ای نیز در پتروژنز سنگ های منطقه دخالت داشته اند. شیب منفی نمودار عناصر ree، تفاوت الگوی سنگ های منطقه با الگوی بازالت های مورب، شباهت الگوی عناصر کم تحرک مثل y,yb,sr با الگوی بازالت های آلکالن oib و همچنین ترکیب آلکالن سنگ های منطقه، گویای این است که ماگمای سازنده بازالت ها از ذوب بخشی درجه پایین یک گوشته به نسبت غنی شده سر چشمه گرفته است.
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کلیدواژه
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آلودگی ,هضم ,گنیس ,آلکالن ,آتشفشانی
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آدرس
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پژوهشکده علوم زمین, ایران. پژوهش های کاربردی سازمان زمین شناسی و اکتشافات معدنی کشور, ایران, پژوهشکده علوم زمین, ایران. پژوهش های کاربردی سازمان زمین شناسی و اکتشافات معدنی کشور, ایران, دانشگاه آزاد اسلامی واحد اسلامشهر, ایران, پژوهشکده علوم زمین سازمان زمین شناسی و اکتشافات معدنی کشور, ایران, پژوهشکده علوم زمین سازمان زمین شناسی و اکتشافات معدنی کشور, ایران
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پست الکترونیکی
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Authors
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Maleki Sahar ,Maleki Sahar ,Emami Mohammad Hashem ,Kheirkhah Monireh ,Saeedi Abdollah
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Abstract
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1 IntruductionThe Research specifically examines the Quaternary magmatism of Kurdistan ( Bijar) and its relationship with the region's tectonics. Due to parallelism with Zagros main fault and continental collision zone, It seems that such a magmatism indicates continental collision zone as well. But the reason of this magmatism is not clear… However, there are exles in the world that can represent magmatism in this region continued even with a 15 million years interval. Considering the breadth of the magma composition, thickness of the lithosphere can be an important parameter in understanding of magma petrogenesis. Reported thickness of the lithosphere in the study area is about 150 200 km. The thickness of lithosphere causes changes in the composition and chemistry of magma in the region. One of the points in this research is phase delay that has been active after the closure of Neotethys Ocean till Quaternary magmatism. Allen et al. (2013) provided a model for melting involving subduction and dewatering of sediment of Arabian passive margin beneath the Eurasian plate as a result of melting of the Eurasian lithosphere. In this study, it was necessary to consider more precisely these three reservoirs of magma in terms of petrography and chemistry and then rising of these magmas and its relation with Area’s tectonic. Observations show three sources of magma in different combinations and sometimes different chemistry. So magmatism of Bijar can provide a good view of the processes that control the composition of the continental lithosphere.2 Material and method:Field study and systematic sling was performed in vertical and horizontal orientation from the volcanic flows. 70 sles were taken in order to thin section preparation and petrography. 30 sles were selected for chemical analysis and investigation of major and trace elements. Sles from new units and nonmethamorphic area were analyzed by the methods like XRF (S4) (for determination of major elements), ICP (for determination of trace elements) and XRD (in order to mineralogy of powder sles) in geological survey of Iran.3 Results: Bomb fragments, Lapilli, ash and scories can be seen around the crater of the volcano. The Gneissic xenoliths in Quaternary basalts can be seen. It created Porphyiric texture that contains Fine grained or sometime glassy context with big crystals in it. Although in some thin sections, Microlite and intergranular texture also were observed. The main Minerals are Olivine, Pyroxene and Plagioclase. Subsidiary minerals are visible as apatite and Opak minerals. According to geochemical investigations, rocks in this area are located in Sodic Potasic alkaline or potasic zone. In the process of Some of the elements such as MgO, SiO2, TiO2, Sr, Rb, Zr, Y, Th, Pb, Zr / Y, Rb / Th increasing and in some of the elements, such as CaO, Co, Ga and reduction can be seen. In some cases on the variation diagrams, sles are sporadic and do not show a clear linear trend. The slope of diagram of REE Distribution pattern chondrite (Nakamura, 1974) in the rocks of the area is negative and LREE elements are more enriched than HREE elements. The rocks do not show negative Eu anomaly. In the Distribution pattern diagram of trace elements of the area rocks versus mantle rocks (Wood et al., 1979), Titan depletion is observed. Niobium and tantalum also show a slight depletion. Given that the basalts of the area were alkaline, they got compared to alkaline basalts of the Afar and Kenya Rift (Eastern African Rift). Tectono magmatic diagrams are also used somewhat to determine tectonical environment associated with eruption of these magmas. 4 Discussion and conclusion Microscopic studies show that the texture of rocks in this area are more hyaloporphyry, microlitic porphyry, hyalomicrolitic porphyry, and sometimes intergranular. Phenocrysts of clinopyroxene, olivine and plagioclase in Microlitic context are made of the same minerals and glass. Petrographic studies show that, differentiation, digestion and pollution have contributed to the formation of rocks in this area. The evidence of differentiation in these rocks is presence of minerals like Glomeroporphyritic and Glomerocryst and also the presence of olivine, clinopyroxene, Plagioclase with Labradorit bentonite composition in basaltic rocks. Evidence for digestion and pollution are presence of quartzic Gezenocrysts with reactive margin of pyroxen and presence of gneiss zenolite.According to the major elements Diagrams, The rocks belong to the series of alkaline basaltic. In terms of amount of potassium, they located in the rang of potassium from moderate to high. According to the major elements changes diagram versus DI, crustal contamination and subtraction has been effective in the process of rocks formation.considering the trace elements changes diagram versus DI, it can be also concluded that differentiation process has been effective in the formation of rocks and according to the distribution that is sometimes seen in these diagrams, The high Average of crustal elements abundance (Elements that have a high concentration in the crust, such as Rb) and vertical trends, It can be said that in addition to differentiation, the phenomenon of magma contamination with crust is also effective in rocks formation. selective Diagrams of trace elements also indicates that In addition to differentiation, other processes such as magma contamination have been involved in the petrogenesis of intermediate magmatic rocks. It can concluded from The negative slope of REE diagram namely the high LREE versus HREE, the Difference between pattern of rocks in this area with diagonal basalt pattern, similarity of the sedentary element Pattern Like Y, Yb, Sr to the OIB alkaline basalts pattern (resulting of low grade partial melting of the enriched asthenospheric mantle) and also the alkaline composition of basalts in this area wich the origin of basaltic magma was not from a depleted mantle source but rather Originated from an enriched mantle source and is a result of relatively low degree partial melting. The high LREE is not necessarily the reason for the low component melting and generally factors such as magma origin (asthenosphere or lithosphere).Mineralogy and composition of minerals entered during melting, the degree of partial melting, fractional crystallization, magma subtraction and the volatiles can cause this condition. Considering Distinctive tectonical diagrams and evidences in continental crust, tectonical environment is a continental inter plate environment. These diagrams reinforce hypothesis of rocks formation in a tensile environment. Especially that these lavas belong to Quaternary and there is not any active subduction system in Iran right now.
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Keywords
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Pollution ,Digestion ,Gneiss ,Alkaline ,Volcanism
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