دانشگاه اصفهان پترولوژی 2228-5210 17 1 2026 03 21 Mafic Granulites in the Crystalline Core of the Neybaz Complex گرانولیت‏‌های مافیک در هستة کریستالین درجه بالای کمپلکس نی‏‌باز 27 52 30219 10.22108/ijp.2026.147351.1374 FA رسول ولیئی دانشجوی دکتری گروه زمین‌شناسی، دانشگاه اصفهان، اصفهان، ایران سید محسن طباطبایی منش استاد، گروه زمین‌شناسی، دانشگاه اصفهان، اصفهان، ایران علیرضا ندیمی دانشیار، گروه زمین‌شناسی، دانشگاه اصفهان، اصفهان، ایران Journal Article 2025 11 12 Introduction Formation and evolution of the continental crust is one of the fundamental subjects in Earth sciences studies, as these processes usually occurred during the Archean and Proterozoic (e.g., Condie, 1980; Taylor and McLennan, 1981). Regions with high-grade metamorphic rocks such as granulites are considered key parts of the lower continental crust (e.g., Smithson and Brown, 1977; Harley, 1989; Bohlen, 1991). Precambrian-Cambrian basement sequences in Iran are exposed in some structural zones such as Central Iran and the Sanandaj-Sirjan zone (e.g., Nadimi, 2007; Hosseini et al., 2015). In these zones, the Chapedony complex is considered the oldest basement complex in Iran, exposed between two major faults: the Chapedony Fault to the west and the Chatak-Neybaz Fault to the east. The Neybaz Crystalline Core, situated in the West Central Iranian Microcontinent, is a high-grade metamorphic zone composed of diverse lithologies ranging from Precambrian to Cenozoic in age. It has undergone multiple intense deformation phases (Valiei et al., 2025). The main goal of this research is to describe the petrography, geochemistry, and metamorphic conditions of the Neybaz Granulite Complex. Based on new data and comparison with other published models, we tried to determine the metamorphic history, tectonic settings, and origin of the old basement protoliths. These findings will help us understand the geodynamic evolution of the Precambrian basement of Central Iran. Analytical Methods Ten samples were analyzed for geochemical composition at the Zarazma Minerals Research Company laboratory (Tehran, Iran) using XRF and ICP-MS. Major oxide contents were determined by the alkaline fusion method (lithium borate fusion). In this method, samples were melted with lithium metaborate, and the resulting product was dissolved in dilute nitric acid. The final solution was then analyzed using an ICP-OES instrument to measure major oxide concentrations. FeO content was determined by the wet chemical method, and loss on ignition (LOI) was measured using the conventional gravimetric method. For trace and minor elements, samples were digested using a multi-acid digestion method with a microwave digestion system, and the resulting solutions were analyzed by ICP-MS. Results and Discussion Petrography The occurrence of granulites in the high-grade crystalline core of the Neybaz area was first reported by Valiei et al. (2025). The studied granulites are dark gray to grayish-black in color. They are in contact with the amphibolite unit and, to a lesser extent, with migmatite gneisses. These granulites are exposed as irregular patches and have medium to coarse grain sizes. Macroscopic studies The high-grade crystalline core of the Neybaz area comprises several rock units, including metacarbonate, calc-silicate, garnet/cordierite schist, amphibole schist, migmatite, mylonitic gneiss, amphibolite, granulite, and anatectic granite. Microscopic studies Microscopic investigations reveal that the main minerals in the Neybaz granulites include orthopyroxene, clinopyroxene, plagioclase, amphibole, garnet, biotite, apatite, and opaque minerals. Gneissic fabrics are widespread at the microscopic scale, characterized by ferromagnesian minerals (pyroxene and amphibole) and stretched plagioclase. Geochemistry The studied rock units exhibit a compositional range, with SiO₂ contents varying between 48.08 and 56.18 wt%, and TiO₂ contents between 0.21 and 1.05 wt%. Al₂O₃ contents show a wide range (14.81–24.35 wt%; mean: 17.60 wt%). The samples are enriched in CaO (9.10–13.97 wt%) and show variations in MgO and FeOT contents (MgO: 0.54–10.42 wt%; FeOT: 1.90–10.18 wt%). The contents of K₂O, Na₂O, and P₂O₅ range from 0.15 to 0.83, 1.71 to 5.66, and 0.06 to 0.29 wt%, respectively. Loss on ignition (LOI) in the studied granulites ranges from 0.63 to 1.76 wt%. The Mg number in these rocks ranges from 33.70 to 73.61. Trace element concentrations also show wide variations, including Rb (14–35 ppm), Ba (70–162 ppm), Sr (166.8–1064.6 ppm), Zr (9–36 ppm), V (6–227 ppm), Nb (<1–47.4 ppm), and Y (15.7–89 ppm). Conclusion The Neybaz high-grade metamorphic complex is located in the western domain of the Central Iranian microcontinent, situated between the Chatak-Neybaz and Chapedony basement faults. Within this complex, metamorphosed mafic rocks occur as small masses in contact with, and within, gneisses that exhibit granulite facies metamorphic grade. The high-grade metamorphism in these rocks led to the development of a mineral paragenesis primarily composed of clinopyroxene, orthopyroxene, euhedral garnets, amphibole, and plagioclase. Field, petrological, and geochemical studies on the metamorphic mafic rocks in the Neybaz crystalline core indicate that the garnet/pyroxene granulite has a basaltic to andesitic protolith and is metaluminous in composition. Geochemical investigations of these granulites indicate that their protoliths range from basalt to andesite and basaltic andesite, showing calc-alkaline and tholeiitic affinities. Lithogeochemical variation diagrams suggest that these rocks formed in a plate margin setting. Furthermore, the close association of the mafic granulites with gneisses—some of which have a metagraywacke protolith—points to their origin from an active plate margin. Primitive mantle- and chondrite-normalized spidergrams show enrichment in light rare earth elements (LREEs) relative to heavy rare earth elements (HREEs), along with enrichment of LILEs and depletion of HFSEs, confirming the involvement of both mantle and crustal sources in the formation of the protoliths. Based on the geochemical data and structural evidence, this study suggests that the formation of the mafic melts in the Neybaz complex was related to cratonization processes in an active margin environment during the Paleo- to Neoproterozoic era. The geochemical characteristics of these granulites, including negative Nb and Ti anomalies in the spidergrams, link their origin to arc magmatism (e.g., island arc tholeiites or calc-alkaline basalts). کمپلکس دگرگونی درجه بالای نی‏‌باز در قلمروی باختری خردقاره شرق-ایران مرکزی و در میان دو گسل پی‌سنگی چاتک-نی‏‌باز و چاپدونی جای دارد. در این کمپلکس، سنگ‌های مافیک دگرگون‌شده به‏‌صورت توده‌های کوچکی در همبری و در درون گنیس‌های میلونیتی/میگماتیتی با درجه دگرگونی رخساره گرانولیت یافت می‌شوند. دگرگونی درجه بالا در این سنگ‏‌ها به پدیدارشدن کانی‌های همایند شامل کلینوپیروکسن و اورتوپیروکسن، گارنت‏‌های یوهدرال، آمفیبول و پلاژیوکلاز به‌عنوان فازهای اصلی انجامیده است. بررسی زمین‌شیمیایی این گرانولیت‌ها نشان می‌دهد بازة ترکیبیِ سنگ مادر آنها از بازالت تا آندزیت و آندزیت بازالتی با ماهیت کالک‌آلکالن/آلکالن و توله‌ایتی بوده است. نمودارهای تغییرات سنگ‌زمین‌شیمیایی گویای آن است که این سنگ‌ها در جایگاه حاشیة صفحه پدید آمده‌‌اند. همچنین، ارتباط نزدیک گرانولیت‏‌های مافیک با گنیس‌هایی که بخشی از آنها سنگ مادر متاگری‌وکی دارند، نشان‌دهندة خاستگاه آنها از یک مرز فعال صفحه‌هاست. نمودارهای عنکبوتی بهنجارشده به ترکیب گوشتة اولیه و کندریت، غنی‌شدگی در عنصرهای کمیاب خاکی سبک (LREE) نسبت به عنصرهای کمیاب خاکی سنگین (HREE)، غنی‌شدگی عنصرهای LILE و تهی‌شدگی عنصرهایHFSE را نشان می‌دهند که گواهی بر دخالت هر دو خاستگاه گوشته‌ای و پوسته‌ای در پیدایش سنگ مادر این سنگ‌هاست. بر پایة داده‌های زمین‌شیمیایی و شواهد ساختاری، این پژوهش نشان می‌دهد پیدایش مذاب‌های مافیک در کمپلکس نی‏‌باز، با فرایندهای کراتونی‌شدن در یک محیط حاشیة فعال در دوران پالئو- تا نئوپروتروزوییک مرتبط بوده است. ویژگی‏‌های زمین‌شیمیایی این گرانولیت‌ها، شامل ناهنجاری‏‌‌های منفی Nb و Ti در نمودارهای عنکبوتی، خاستگاه آنها را به ماگماتیسم کمانی (مانند توله‌ایت‌های جزیره‌های کمانی یا بازالت‌های کالک‌آلکالن) مرتبط می‌کند.

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