شیمی کانی و زمین دما-‌فشارسنجی متابازیت های مجموعه آذرین‌دگرگونی ماجراد (جنوب شرق شاهرود)

مجموعه دگرگونی‌آذرین ماجراد به سن نئوپروتروزوئیک پسین، در جنوب شرق شاهرود و شمال پهنه ساختاری ایران مرکزی رخنمون دارد که طیف وسیعی از سنگ های آذرین و دگرگونی را شامل می شود. متابازیت های این مجموعه متشکل از آکتینولیت شیست، آمفیبولیت و گارنت آمفیبولیت هستند. آمفیبول، پلاژیوکلاز و گارنت کانی های اصلی این سنگ ها هستند. آمفیبول های متابازیت های این مجموعه به گروه کلسیم دار تعلق دارند. ترکیب آمفیبول در آکتینولیت شیست ها آکتینولیت و منیزیوهورنبلند و در آمفیبولیت ها منیزیو‌هورنبلند و چرماکیت است. پلاژیوکلازها اغلب دارای ترکیب الیگوکلاز، لابرادوریت تا بیتونیت هستند. با استفاده از دما‌‌فشارسنجی زوج هورنبلند‌‌پلاژیوکلاز، دمای تعادلی 450 تا 690 درجه سانتی گراد و فشار 4 تا 11 کیلوبار برای آمفیبولیت های مجموعه آذرین‌‌دگرگونی ماجراد برآورد شده است. این شرایط دما‌‌فشار با شرایط رخساره آمفیبولیت در رژیم دگرگونی نوع بارروین متعارف مطابقت می کند.

Mineral chemistry and geothermobarometry of metabasites of the Majerad igneousmetamorphic complex (SE of Shahrood)

Introduction;Thermobarometric models based on the chemical equilibrium among coexisting mineralmineral or mineralmelts pairs are useful tools widely used to estimate the PT path and chemical evolution during igneous processes. The high sensitivity of amphibole to physicochemical changes makes it a good tracer for thermobarometric models. Majerad IgneousMetamorphic Complex with NESW trend, 40 kilometer length, and 10 kilometer width is located in the southeast of Shahrood in the northern margin of the Central Iran structural zone. Late Neoproterozoic sequence of Majerad metamorphic complex includes a wide range of metamorphic rocks with extensive compositional variety of metacarbonate, metapsammite, metapelite, metabasite and metarhyolite. Metabasites of the Majerad metamorphic complex consist of a greenschist to garnet amphibolite. Late Iranian Neoproterozoic complexes have been studied by   numerous researchers, and a lot of papers have been published related to them (Rahmati Ilkhchi et al., 2011; Balaghi Einalou et al., 2014; Faramarzi et al., 2015; Hosseini et al., 2015; MalekpourAlamdari et al., 2017). These complexes have cropped out in the different parts of Iran, except the Kopeh Dagh, Makran and the East Iran Flysch structural zones.;  ;Analytical methods;The wholerock major element compositions were determined by Xray fluorescence using fused glass disks at the Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China. Trace elements were determined by ICPMS (Agilent 7500a) at IGGCAS after more than 5day acid digestion of samples in Teflon bombs. Compositional mineral analyses were performed at the State Key Laboratory of Continental Dynamics, Northwest University, Xian China, using a Cameca JXA8230 instrument at an acceleration voltage of 15 KV, and beam current of 10 nA.;  ;Results;In the metamorphic environment, aluminous hornblendebearing assemblages are stable over a wide PT field that extends from amphibolite to granulite, and highT eclogitefacies conditions. At lower temperatures, the hornblendic amphibole is replaced by sodiccalcic amphibole at relatively highP and by actinolite at lowerpressure greenschistfacies conditions (Spear, 1993; Ernst and Liu, 1998; Molina et al., 2015).;Amphibole formulas were calculated with the AmpExcels spreadsheet using the 13 cations method (Leake et al., 1997). Amphiboles of metabasites are calcic, and Amphiboles of actinoliteschists are in the range of actinolite to magnesiohornblende, and in amphibolites, they are plotted in the range of magnesiohornblende to tschermakite. Plagioclase are usually oligoclase to bytownite.;Temperatures range of metamorphism events of amphibolites of the Majerad complex have been estimated by using the hornblendeplagioclase thermometer. This thermometer is based on the Ca and Na equilibrium exchange between plagioclase and amphibole (Holland and Blundy, 1994). The hornblendeplagioclase pair thermobarometer estimates temperatures of 450 to 690ºC and pressures of 4 to 11 Kb for the formation of the Majerad amphibolites. These temperaturepressure ranges correlate with PT conditions of the greenschist and amphibolite facies in the typical Barrovian type metamorphism.;  ;References;Balaghi Einalou, M., Sadeghian, M., Ghasemi, H., Zhai, M.G., and Mohajjel, M., 2014. Zircon U–Pb ages, Hf isotopes and geochemistry of the schists, gneisses and granites in Delbar MetamorphicIgneous Complex, SE of Shahrood (Iran): implications for Neoproterozoic geodynamic evolutions of central Iran. 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Chemie der Erde, 75(2): 207–218.;Leake, B.E., Woolley, A.R., Arps, C.E.S., Birch, W.D., Gilbert, M.C., Grice, J.D., Hawthorne, F.C., Kato, A., Kisch, H.J., Krivovichev, V.G., Linthout, K., Laird, J., Mandarino, J., Maresch, W.V., Nickel, E.H., Schumaker, J.C., Smith, D.C., Stephenson, N.C.N., Ungaretti, L., Whittaker, E.J.W. and Youzhi, G., 1997. Nomenclature of amphiboles: report of the subcommittee on amphiboles of the International Mineralogical Association Commission on New Minerals and Mineral Names. The Canadian Mineralogist, 35‌(1): 219–246.;MalekpourAlamdari, A., Axen, G., Heizler, M. and Hassanzadeh, J., 2017. Largemagnitude continental extension in the northeastern Iranian Plateau: Insight from Kfeldspar 40Ar/39Ar thermochronology from the Shotor Kuh–Biarjmand metamorphic core complex. Geosphere, 13(4): 1207–1233.;Molina, J.F., Moreno, J.A., Castro, A., Rodriguez, C. and Fershtater, G.B., 2015. 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