دانشگاه اصفهان پترولوژی 2228-5210 17 2 2026 06 22 Origin, Nature, and Crystallization Conditions of the Dacitic Magma Hosting the Koushk Zn-Pb Deposit, NE Bafq: Based on zircon crystal morphology, mineralogical evidence, and whole-rock geochemical خاستگاه، سرشت و شرایط تبلور ماگمای داسیتی میزبان کانسار روی-سرب کوشک، شمال‏‌خاوری بافق: بر پایة ریخت‌شناسی بلورهای زیرکن، شواهد کانی‌شناسی و زمین‌شیمی سنگ کل 1 44 30457 10.22108/ijp.2026.148209.1380 FA کیامرث حسینی دانشجوی دکتری زمین‌شناسی اقتصادی، دانشکده منابع معدنی و آب‌های زیرزمینی، دانشگاه شهید بهشتی، تهران، ایران محمد یزدی استاد، گروه زمین‌شناسی، گروه منابع معدنی و آب‌های زیرزمینی، دانشگاه شهید بهشتی، تهران، ایران حسین عزیزی استاد، گروه معدن، دانشکده مهندسی، دانشگاه کردستان، سنندج، ایران Journal Article 2026 01 31 Introduction The Kushk Pb–Zn deposit in northeastern Bafq, Central Iran, is associated with Neoproterozoic–Early Cambrian magmatism related to the Proto-Tethyan continental arc system. These magmatic processes, accompanied by crustal extension and interaction between mantle-derived magmas and continental crust, played an important role in regional metallogeny. Therefore, investigating the origin and crystallization conditions of the host dacitic magma is crucial for understanding magma evolution and ore-forming processes in the Bafq district. In this study, whole-rock geochemistry together with zircon morphology and internal textures are used to evaluate the magma source, tectonic setting, and magmatic evolution. Because zircon preserves geochemical and crystallization features under different geological conditions, it is widely applied in petrogenetic studies. Regional Geology The Koushk Zn–Pb deposit is situated within the upper part of the Lower Cambrian volcano‑sedimentary sequences, in the central part of the Zarigan–Chahmir Basin (Figure 2). Two major stratigraphic sequences have been recognized in the area (Figure 3): (1) a mineralized sequence comprising the upper interval of the Lower Cambrian volcano‑sedimentary sequences at the base, and (2) an overlying volcano‑sedimentary sequence that includes Lower Paleozoic shales, argillaceous limestone, dolomitic units which host Fe-mineralization, as well as the rhyolitic and tuffaceous rocks (Gibbs, 1976). Rhyolitic and dacitic domes are present in the southeastern part of the deposit, adjacent to the mineralized sequence, as well as to the north and northwest of the deposit. Materials and Methods In this study, 65 samples were collected from the dacitic units hosting the Koushk Zn–Pb deposit. Petrographic and mineralogical investigations were carried out on 47 thin sections using an optical microscope at Shahid Beheshti University, Iran. To evaluate whole‑rock geochemistry, 27 samples of the rhyolites and rhyodacites hosting the Koushk Zn–Pb deposit were analyzed by XRF using a Philips PW 2404 instrument at Tarbiat Modares University, and a separate suite of 9 samples was processed for trace elements by ICP‑MS using a Perkin Elmer NexION 300 instrument at Zarazma Company Laboratory. Zircons were separated using a Wilfley shaking table, a Frantz magnetic separator, and heavy liquids, followed by handpicking under a binocular microscope at the Geological Survey of Iran. Zircon grains selected for CL and BSE‑SEM were mounted in epoxy resin and polished to expose their internal structures. CL images were obtained using a JEOL JXA 8900RL electron microprobe, and BSE‑SEM images were acquired using a Hitachi S3400N scanning electron microscope at Nagoya University, Japan, and Aria Electron Optics Co., Ltd. Discussion Examination of the external and internal morphology of zircon can aid in identifying the origin of magmatic rocks, the degree of aluminum and alkali saturation (Pupin, 1980), and the temperature of the melt (Pupin and Turco, 1972). Temperature and Zr saturation are the main factors governing the relative growth of different prismatic zircon morphologies. Zircons crystallizing from alkali, water-poor, and tholeiitic melts fall within the (101) and (100) fields; those derived from peraluminous melts plot within the (101) field; and in the presence of high-water content in the melt, they occur within the (101) and (110) domains. Based on the above considerations, the examination of zircon crystals associated with the Koushk dacites indicates that they predominantly display prismatic faces (110) and (110) >> (100), along with pyramidal faces (101) and (211) >> (101). The development of prismatic forms (110) and (100) reflects a high-temperature crystallization index, whereas the dominance of the (101) pyramids suggests a high alkalinity index an interpretation that is further supported by the geochemical characteristics of the samples. Moreover, the morphological analysis reveals that some zircon crystals exhibit noticeable variations in their length-to-width ratios, indicating differences in growth dynamics during crystallization. This may be attributed to crystallization rate (Bussy and Cadoppi, 1996) or the possible presence of two zircon generations in these samples. However, based on the detrital zircon dating from the Koushk area, (Mahmoudi, 2022; Vickers-Rich et al., 2017), the presence of a single zircon generation in these samples is confirmed. Therefore, crystallization rate is likely the main factor responsible for the increased length-to-width ratio observed in these zircons. Conclusion These crystals predominantly fall within the P2, P5, S5, and S25 fields, with fewer crystals plotting in the AB5, D, L5, P3, P4, R3, S10 and S20 domains. The minimum zircon crystallization temperatures in the studied dacites, based on zircon morphology and the temperature (I.T) and alkalinity (I.A) indices, are 740°C and 706.6°C, respectively. The temperature of the rhyolitic melt, based on zircon saturation thermometry, ranges from 703.49 to 830.74 °C (Watson and Harrison, 1983) and from 702.94 to 877.49 °C (Boehnke et al., 2013). Whole‑rock geochemical data yield a temperature range of 700 to 780 °C. The data show that rhyolitic magma was supersaturated with respect to Zr from the earliest stages of crystallization making zircon one of the first minerals to form-a temperature interval of 700 to 780 °C is inferred for zircon crystallization in the rhyolitic magma. The presence of zoning within zircon crystals likely reflects a decrease in HREE and an enrichment in LREE, U, Th, and Y in these zones. The presence of narrow and closely spaced zoning in both elongated needle‑like zircons and shorter crystals indicates that the magma remained supersaturated with respect to zircon from the earliest to the latest stages of crystallization. Internal structures of some zircon crystals reveal rounded cores with zoning patterns distinct from their rims, suggesting that these cores acted as non‑reactive mineral relics during partial melting. The zircons within the dacites also exhibit evidence of partial resorption, implying episodic intervals of zircon undersaturation in the melt. Structural and compositional characteristics, crystal growth conditions, zircon age and provenance and metamictization processes, can all contribute to reduced lattice order and, consequently, diminished or absent cathodoluminescence (CL) in zircon crystals. Based on geochemical studies, these rocks originate from crust‑derived melts, and their magmas correspond to A‑type granites, specifically the A2 subtype within the calc‑alkaline series. According to tectonic discrimination diagrams, the dacites are derived from volcanic arc granites. Acknowledgements The authors appreciate Shahid Beheshti University Research Council that supported this work. در این بررسی خاستگاه، سرشت و شرایط تبلور داسیت‌های پورفیری میزبان کانسار روی و سرب کوشک در شمال‏‌خاوری بافق بررسی شدند. برای این منظور ریخت‌شناسی خارجی و ساختار درونی بلورهای زیرکن با استفاده از روش کلاسیک پوپین بررسی شد. بر پایة روش کلاسیک پوپین زیرکن‌ها بیشتر در گسترة P2، P5، S5 و S25 جای دارند و شمار کمتری در بخش‌های AB5،D ، L5،P3 ،P4 ، R3، S10 و S20 جانمایی می‌شوند. همچنین، مقدارهای شاخص دما (I.T) و شاخص آلکالن (I.A) به‏‌ترتیب برابر 740 و 6/706 است. بررسی ساختار درونی زیرکن‌ها نشان‏‌دهندة وجود پهنه‌بندی نوسانی ماگمایی هستند. تصویرهای CL دانه‌های زیرکن، ویژگی‌های رشد ثانویه و وجود مقدارهای ناچیز میانبارهای مذاب را نشان می‌دهند. رخداد انحلال ناهمگن چه‌بسا بازتابی از بازجذب بلور زیرکن به‌علت تحت‌اشباع‌بودن مذاب از زیرکنیم است. ویژگی‌های ساختار بلوری، شرایط فیزیکوشیمیایی، سن و خاستگاه زیرکن و فرایندهای متامیکتی را می‌توان از عوامل کاهش نظم بلوری و به‌دنبال آن، کاهش یا حذف CL در بلورهای زیرکن دانست. افزون‌بر این، بررسی‌های زمین‌شیمیایی نیز تهی‌شدگی از عنصرهای Ta، Nb و Ti و غنی‌شدگی ترکیب سنگ کل در عنصرهای Pb، Th، U، Rb، Cs و Ba را نشان می‌دهند که گویای نقش فرورانش و آلایش پوسته‌ای در پیدایش این سنگ‌ها هستند. همچنین، بر پایـة نمودارهـای متمایزکننـدة رژیم‌های زمین‌ساختی، خاستگاه داسیت‌ها همانند گرانیت‌های کمـان آتشفشـانی (VAG) است. این ویژگی‌ها همانند گونه‌شناسی زیرکن، با ویژگی‌های گرانیت نوع A و زیرگروه A2 وابسته به سری کالک‌آلکالن همخوانی دارند. ناهنجاری منفی Eu همراه با غنی‌شدگی LREE در برابر HREE و نسبت بالای (La/Lu)n نشان‌دهندة اهمیت نقش جدایش بلورین در فرایند تکامل این سنگ‌هاست.

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