دانشگاه اصفهان پترولوژی 2228-5210 14 3 2023 11 22 Metamorphic evolution of Neoproterozoic - Lower Cambrian Schists in NW of Sarve-Jahan (Soltanieh belt NW of Iran) using equilibrium phase diagrams بررسی تحول دگرگونی شیست‌های نئوپروتروزوییک بالایی - کامبرین پایینی شمال‌باختری سروجهان ( نوار سلطانیه، شمال‌باختری ایران) با استفاده از نمودارهای تعادلی فازی 1 24 27830 10.22108/ijp.2023.138284.1304 FA جواد ایزدیار دانشیار، گروه زمین‏‌شناسی، دانشکده علوم، دانشگاه زنجان، زنجان، ایران 0000-0002-1405-542X میرمحمد میری استادیار، گروه زمین‏‌شناسی، دانشکده علوم زمین، دانشگاه شهید چمران اهواز، اهواز، ایران معصومه زارع شولی دکتری، گروه زمین‏‌شناسی، دانشکده علوم زمین، دانشگاه شهید چمران اهواز، اهواز، ایران Journal Article 2023 07 03 Mineral assemblage of metamorphic rocks forms by various factors including temperature, pressure, protolith chemical composition, and fluids. Although, P-T variation is the main factor controlling mineralogical evolution and assemblages, chemical differences of protoliths and metamorphic fluids also play an important role in the formation of final mineral assemblages (e.g. Spear, 1993). Therefore, mineral assemblages can be used in the study of thermodynamic and chemical conditions of metamorphic events. The studied area is located in the northwest of Sarve-Jahan village. For the purpose of the present paper, we investigate the metamorphic evolution of Sarve-Jahan schists (Zanjan province) and the effect of important factors on the formation of their mineral assemblages are through calculating phase diagrams.<br />The study area is located in the northwest of Sarve-Jahan from Soltanieh structural zone and hosts Upper Neoproterozoic - Lower Cambrian schists (Fig. 1). The Soltanieh strip is a narrow and long structural strip trending northwest-southeast with a length of over 150 km and a width of 10-12 km. This belt as an uplifted tectonic bedrock lies in the structural zone of Central Iran. It comprises a complete set of thick clastic, carbonate and pyroclastic sediments (Kahar, Bayandor, Soltanieh, Barut and Lalon Formations) (Stöcklin and Eftekharnezhad, 1969)<strong> </strong>belonging to Precambrian-Paleozoic<strong>.</strong>. In the northwest of Sarve-Jahan and the vicinity of Sarve-Jahan granite, there is a metamorphic complex trending northwest-southeast, composing pelitic schists, metacarbonate (chalk schist) and sandstone (quartz schist and quartzite). The complex belongs to the Kahar Formation (Stöcklin and Eftekhar-nezhad, 1969; Babakhani and Sadeghi, 2004). In the southwest of metamorphic complex, there is a row of slates gradually replaced by phyllites and schists towards the northeast. The exposed Mesozoic deposits in the studied area include the sediments of Shemshak Formation and Lar limestones (Fig. 1) (Babakhani and Sadeghi, 2004, Stöcklin and Eftekhar-nezhad, 1969). Tertiary deposits include a sequence of detrial sediments as well as igneous and volcanic rocks (Stöcklin and Eftekhar-nezhad, 1969).<br /><strong>Materials and Methods</strong><br />Seventy microscopic thin sections were studied by a polarizing microscope with transmitted light. Five samples with the highest number of phases in equilibrium were selected for microprobe analyze. The samples were analyzed in Iranian Mineral Processing Research Center using a CAMECA model SX 100 device with an electron beam acceleration voltage of 15 kv, current intensity of 3 nA and analysis time of 40 seconds for each point. Whole-rock compositions of these samples, also were determined by X-ray Fluorescence (XRF) spectroscopy method using a PHILIPS PW1480 instrument in the Kansaran Binaloud laboratory.<br /><strong>Petrography</strong><br />The studied rocks include garnet and staurolite porphyroblasts lie in a well-oriented matrix composed of biotite + muscovite + chlorite + plagioclase + quartz. Three phases of deformation (D₁, D₂, D₃) and two phases (M₁, M₂) of metamorphism occurred in the metamorphic rocks. The M₁, could be related to the D₁ and characterized by garnet + staurolite + chlorite + biotite + muscovite + plagioclase + quartz assemblage. The M₂, could occur in association with D2 and formed chlorite + biotite₂ + muscovite₂ + quartz. The third deformation phase (D₃) caused crenulation cleavage on D₂ schistosity but with no new mineral phase. Garnet and staurolite porphyroblasts formed during M1. Retrogressive metamorphism replaced these minerals with chlorite and muscovite in their rims.<br /><strong>Whole-rock and mineral chemistry</strong><br />The samples studied contain high SiO₂, Al₂O₃ and K₂O corresponding to a Fe-rich shale protolith (Table 1). The biotites and white-micas have high annite and muscovite end-members, respectively. Garnets and staurolites are also Fe-rich and plagioclase are of albite to oligoclase compositions (Table 2).<br /><strong>Discussion </strong><br /><strong>Thermobarometry</strong><br />Various thermobarometry calculations based on garnet-biotite (Bhattacharya et al., 1992), muscovite-chlorite (Vidal and Parra, 2000) and garnet-plagioclase-biotite-quartz (Hoisch, 1990) methods resulted c.a. 600 °C and 5-7 kbar for the M1 and 500°C and 6 kbar for the M2.<br /><strong>Equilibrium phase diagram </strong><br />The equilibrium phase diagrams were calculated using GeoPs software (version 3.3) and Bi(w), Chl(W), Crd(W), Ctd(W), St(W), Gt(W), Mica(W), Opx(W), melt(W) and Fsp(C1) (Holland and Powell, 2003; White et al, 2014) solid solution models for average composition of the studied samples. The P-T pseudosection diagram (Fig. 5) shows that the M1 assemblage garnet + staurolite + biotite + muscovite + chlorite + quartz + plagioclase is stable in 550 – 650 °C and 5 – 7.2 kbar, while the assemblage biotite2 + muscovite2 + chlorite2 + quartz + plagioclase corresponding to M2, is stable in T<550 °C and P<5 kbar. The calculated isopleths for pyrope in garnet and phlogopite in biotite intersect at the stable filed of M1 assemblage (Fig. 6) confirming the thermobarometry results.<br /><strong>Protolith composition</strong><br />The Mg# [Mg# = (MgO/MgO+FeO)] versus T variations phase diagram (Fig. 7) shows that the stability of garnet and staurolite is affected by Mg and Fe contents of the protolith. They leave the system with increasing Mg# at constant T. In other words, the high FeO contents of the Sarve-Jahan schist protolith had a significant role in the formation and the stability of the peak metamorphism mineral assemblage.<br /><strong>Fluid composition</strong><strong> </strong><br />The CO₂ contents of metamorphic fluids is important in the stability of minerals (e.g. Miri et al., 2023). A T-XCO₂ phase diagram was calculated for the sample assuming a H₂O-CO₂ fluid to investigate the role of CO₂ in the metamorphic of the samples (Fig. 8). The diagram indicates that the M1 mineral assemblages was stable at XCO₂ < 0.3, but the M2 was stable only in the presence of pure H2O fluid.<br /><strong>Conclusion</strong><br /><br />Microstructures show that the deformation phases (D1, D2, and D3) and the two metamorphic phases (M1 and M2) occurred in the area. The M1 and M2 were contemporaneous with D1 and D2, respectively, while D2 was not in association with a metamorphic phase.<br />The mineral assemblage garnet + staurolite + chlorite + biotite + muscovite + plagioclase + quartz formed in M1 and peak of metamorphism (amphibolite facies) and biotite2 + muscovite2 + chlorite2 + quartz + plagioclase formed in the retrogressive metamorphism M2 phase (greenschist facies).<br />The traditional (mineral pairs) and modern (phase diagram) thermobarometry calculations show obtain 550 – 650 °C and 5 –7 kbar for M1 and < 500 °C and < 5kbar for M2. On this basis, a clockwise P-T-t path can be considered for metamorphism of the Sarve-Jahan schists. Which could have occurred in a subduction-collision tectonic site.<br />The calculated T-Mg# and T-XCO₂ phase diagrams show that the composition of the protolith and the metamorphic fluid played a significant role in the minerals assemblage event occurring during peak metamorphic conditions. در شمال‏‌باختری سروجهان، از بالاآمدگی سلطانیه در شمال‏‌باختری ایران، شیست‏‌های نئوپروتروزوییک بالایی - کامبرین زیرین در آن رخداد دارند. بررسی‏‌های ریزساختارها نشان می‏‌دهند شیست‏‌های یادشده در سه مرحلة دگرریختی (D₁، D₂ و D₃) و دو مرحلة دگرگونی (M₁ و M₂) پدید آمده‏‌اند. دگرگونی M₁، همزمان با D₁ رخ داده است و همتافت کانیایی گارنت + استارولیت + بیوتیت + مسکویت + کلریت + کوارتز + پلاژیوکلاز را پدید آورده است. دگرگونی M₂ به‏‌صورت پسرونده همزمان با فاز دگرریختی D₂ روی داده است و تبلور کانی‏‌های کلریت، بیوتیت، مسکوویت و کوارتز را به‌دنبال داشته است. فاز D₃ که به‏‌صورت ریزچین روی فابریک قدیمی‌تر (برگوارگی S₂) گسترش یافته، با فاز دگرگونی همراه نبوده است. نمودارهای تعادلی فازی به‌دست‌آمده و نتایج دما-فشارسنجی نشان می‏‌دهند دگرگونی M₁ در بازة دمایی 550 تا 650 درجة سانتیگراد و فشارهای 5 تا 7 کیلوبار و دگرگونی M₂ در دماهای کمتر از 500 درجة سانتیگراد و فشارهای کمتر از 5 کیلوبار رخ داده‏‌اند. الگوی ساعت‌گرد مسیر فشار-دمای به‌دست‌آمده برای این شیست‏‌ها نشان‌دهندة تحولات دگرگونی و زمین‏‌ساختی در محیطی برخوردی است. همچنین، نمودارهای فازی آشکار کردند محتوای بالای آهن سنگ مادر و محتوای کم کربن‏‌دی‏‌اکسید در سیال دگرگونی نقش بسزایی در پایداری همتافت کانی‏‌های گارنت + استارولیت + بیوتیت در این سنگ‏‌ها داشته‏‌اند. https://ijp.ui.ac.ir/article_27830_13e415057d82a28c01484cbc569febfd.pdf