پتروژنز ماگماتیسم میان چینه ای تریاس شمال شهرضا بر مبنای شیمی کلینوپیروکسن (جنوب اصفهان، پهنه سنندج-‌سیرجان)

کربنات های تریاس پیشین شمال شهرضا، میزبان 4 افق آذرین موازی میان چینه ای بازیک با ترکیب سنگ شناسی الیوین بازالت تا بازالت کوارتزدار هستند. این سنگ ها دارای بافت اینترسرتال تا اینترگرانولار بوده و پلاژیوکلاز، کلینوپیروکسن، الیوین، آمفیبول و کوارتز کانی‌های اصلی آنها را تشکیل می دهند. کلینوپیروکسن ها حاوی مقادیر بالایsio2 و mgo بوده و محتوای al2o3 و na2o آنها پایین است. نسبت های متوسط alvi/aliv کلینوپیروکسن (0.05-‌‌0.01)، نشان‌دهنده تبلور ماگما در شرایط فشار متوسط است. دمای تبلور کلینوپیروکسن‌ها از 1150 تا 1200 درجه سانتی گراد متغیر است. بر اساس ویژگی های ژئوشیمیایی کلینوپیروکسن، ماهیت ماگمای سازنده تولئیتی و فوگاسیته اکسیژن در زمان تبلور نسبتاً پایین بوده است. میان چینه ای بودن با رسوبات کربناته، تولئیتی‌بودن سرشت ماگمایی و ویژگی زمین ساختی پشته های میان ا قیانوسی نشان‌دهنده ارتباط افق های آذرین با یک رژیم زمین ساختی کششی است. افق های بازیک مزبور احتمالاً از یک ماگمای تولئیتی و در یک رژیم زمین‌ساختی کششی کافت‌زایی که به گسترش پوسته اقیانوسی نئوتتیس در زمان تریاس آغازین در جنوب اصفهان انجامیده است، تشکیل شده اند..

Clinopyroxene chemistry based Petrogenesis of Triassic interlayer magmatism in the north of Shahreza (South of Isfahan, Sanandaj–Sirjan zone)

IntroductionThe ShahzadeAli Akbar area located at 68 Km south of Isfahan is located in the southern zone of the Cimmerian SanandajSirjan block as a part of the northern shelf of the NeoTethyan Ocean (Stampli and Borel, 2002). This area lies in the ShahrezaAbadehHambast belt and is wellknown for its classic PermianTriassic outcrops. The Lower Triassic carbonate rocks have hosted several interlayer igneous horizons. The composition of these rocks varies from olivine basalt to quartz basalt and their hypabyssal equivalents. Plagioclase (labradorite), clinopyroxene (augite), olivine, amphibole and quartz are major and ilmenite and titanomagnetite are minor minerals. The main objective of this research study is to investigate the geological, geochemical and petrogenesis of igneous rocks using mineral chemistry. Materials and methodsMore than 50 samples representing whole units were selected in order to identify the geological setting of the igneous horizon, and their thin sections were prepared. Minerals and textures of rocks were studied by using polarizing microscope (Olympus BH2). Then, 6 samples were selected for mineral chemistry and their mineral compositions were determined by electron microprobe at the Naruto University, Japan. The EPMA (Jeol JXA8800R) was used at operating conditions of 15 kV, 20 nA. Minpet software and spread sheet have been used for mineral chemistry studies and mineral formula calculations. Discussion Based on the field observations, igneous units that are 120 centimeters to 10 meters thick have basaltic composition and are interlayered with Lower Triassic carbonate rocks. Microscopic study showed that these rocks are composed of plagioclase, clinopyroxene, olivine, amphibole, quartz and opaque minerals (ilmenite and titanomagnetite) and have porphyritic, ophitic, intersertal to intergranular textures. These rocks have undergone alterations and secondary minerals are widespread. EPMA analyses show andesine to labradorite composition, clinopyroxene (augite) and amphibole (edenite). In the QJ diagram (Morimoto et al., 1988), all clinopyroxenes are located in the MgFeCa (Quad) field. In the WoEnFs diagram (Beccaluva et al., 1989), clinopyroxens show augitic with lessor amounts of diopside composition. According to clinopyroxene chemistry diagrams such as Si O2 vs. Al2O3 and Ti vs. Al (Le Bas, 1962), the samples belong to subalkaline series. Discriminate diagrams such as Ti O2 vs. Al2O3 (Le Bas, 1962), Ti vs. Ca+Na and Ti vs. Al (Leterrier et al., 198a2) are used for identification of magma affinity. These diagrams show that the studied rocks are tholeiitic. The rocks under study demonstrate the MORB feature on tectonic discrimination diagrams (TiO2SiO2/100Na2O, Beccaluva et al., 1989)In the 2Ti+Cr+AlVI vs. Na+AlIV diagram (Morimoto et al., 1988) all clinopyroxenes are located below the Fe3+=0 line that indicates low oxygen fugacity during crystallization (Schweitzer et al., 1979). In Helz (1973) diagrams, the pressure and percentage of magma water estimated to be 2 to 10 Kbar pressure and about 10% water content. In YPT vs. XPT diagrams (Soesoo, 1997) the temperatures and the pressure of clinopyroxene crystallization are about 11501200 ◦C and 210 Kbar respectively. ResultsThe studied area had been a part of the Cimmeride microcontinent (Horacek et al., 2007) which had begun separating from the northern margin of Gondwana during Triassic time (Şengör, 1984), and traversed north to the southern Eurasian border (Stampli and Borel, 2002).In this area, several interlayer igneous rocks with basaltic composition are seen with Lower Triassic carbonate rocks.Based on the chemical composition of pyroxenes, the magma has subalkaline and tholeiitic affinity. The crystallization of ilmenitetitanomgenetite and diagram of clinopyroxenes crystallization conditions illustrate the low level of oxygen fugacity in the formation of the rocks under discussion. The pressure of magma crystallization is estimated to be between 2 and 10 kb, and the magmatic water content is about 10%. The studied rocks show MORB characteristics. Interlayering with lower Triassic sediments, subalkaline nature of the magma, low level oxygen fugacity during crystallization and geotectonic environment, suggest that the rocks have been formed in the early stages of the opening of the oceanic crust. A process that has led to the formation of the NeoTethys Ocean in the later stages. ReferencesBeccaluva, L., Macciotta, G., Piccardo, G.‌B. and Zeda, O., 1989. Clinopyroxene composition of ophiolite basalts as petrogenetic indicator. Chemical Geology, 77(3–4): 165–182.Helz, R.‌T., 1973. Phase relations of basalts in their melting range at PH2O= 5 kb as a function of oxygen fugacity. Journal of Petrology, 17(2): 139–193.Horacek, M., Richoz, S., Brander, R., Krystyn, L. and Spötl, Ch., 2007. Evidence for recurrent changes in Lower Triassic oceanic circulation of the Tethys: The δ13C record from marine sections in Iran. Palaeogeography, Palaeoclimatology, Palaeoecology, 252(1–2): 355–369.Le Bas, N.J., 1962. The role of aluminous in igneous clinopyroxenes with relation to their parentage. American Journal of Science, 260(4): 267–88.Leterrier, J., Maury, R.C., Thonon, P., Girard, D. and Marchal, M., 1982. Clinopyroxene composition as a method of identification of the magmatic affinities of paleovolcanic series. Earth and Planetary Science Letters, 59(1): 139–154.Morimoto, N., Fabries, J., Ferguson, A.K., Ginzburg, I.V., Ross, M., Seifert, F.A., Zussman, J., Aoki, K. and Gottardi, D., 1988. Nomenclature of pyroxenes. American Mineralogist, 73(9–10): 1123–1133.Schweitzer, E.L., Papike, J.J. and Bence, A.E., 1979. Statitical analysis of clinopyroxenes from deep sea basalts. American Mineralogist, 64(2): 501–513.Şengör, A.M.C., 1984. The Cimmeride orogenic system and the tectonics of Eurasia. Geological Society of America, Special paper, America, 82 pp.Soesoo, A., 1997. A multivariate statistical analysis of clinopyroxene composition: empirical coordinates for the crystallization PT estimations. Geological Society of Sweden (Geologiska Föreningen), 119(1): 55–60.Stampli, G.M. and Borel, G.D., 2002. A plate tectonic model for the Paleozoic and Mesozoic constrained by dynamic plate boundaries and restored synthetic oceanic isochrones. Earth and Planetary Science Letters, 196(1–2): 17–33.