دانشگاه اصفهان پترولوژی 2228-5210 17 2 2026 06 22 Petrogenesis and tectonomagmatic evolution of the Sabzevar ophiolitic complex based on whole rock geochemistry, zircon U–Pb geochronology, and zircon trace element composition in pillow lavas and sheeted dikes of the Sultanabad area بررسی پتروژنز و تحولات تکتونوماگمایی مجموعة افیولیتی سبزوار با استفاده از ژئوشیمی سنگ‌کل، سن سنجی U-Pb و ترکیب عنصرهای کمیاب زیرکن در گدازه‌های بالشی و دایک‌های ورقه‌ای منطقه سلطان‌آباد 95 130 30439 10.22108/ijp.2026.148624.1385 FA سید علی مظهری دانشیار، مرکز پژوهشی علوم جغرافیایی و مطالعات اجتماعی، دانشگاه حکیم سبزواری، سبزوار، ایران دانشیار، هستۀ پژوهشی فلزات سنگین، دانشگاه حکیم سبزواری، سبزوار، ایران فرناندو به آ استاد، گروه زمین شناسی، دانشگاه گرانادا، اسپانیا Journal Article 2026 04 01 <strong>Introduction</strong> Ophiolitic complexes—fragments of oceanic lithosphere tectonically emplaced onto continental margins—serve as crucial archives for reconstructing the formation and evolution of ancient ocean basins. The Sabzevar ophiolite in northeastern Iran represents one of the most significant remnants of Mesozoic Neotethyan oceanic lithosphere and provides a key window into the tectonic development of the northern Neotethys branch. Previous studies have assigned this ophiolite to diverse tectonic settings, including mid‑ocean ridge, supra‑subduction zone, and back‑arc environments (Khalatbari Jafari et al., 2013a, b; Rezaei et al., 2018; Jafari and Ghasemi, 2023). Nevertheless, the co‑occurrence of contrasting magmatic signatures within its volcanic sequences points to a polyphase magmatic history in a complex geodynamic framework (Omrani et al., 2018; Moghadam et al., 2025). This study investigates the pillow lavas and sheeted dikes exposed in the Sultanabad area, in the eastern sector of the Sabzevar ophiolite. Its primary aims are to characterize the geochemistry, constrain the crystallization ages of the volcanic units via zircon U–Pb geochronology, and assess their tectonomagmatic significance for the evolution of the Sabzevar oceanic basin. <strong>Geological Setting</strong> The Sabzevar ophiolite, situated in northeastern Iran, constitutes an integral component of the tectonic structure of the Central Iranian microcontinent (Alavi, 1994). It is widely interpreted as a relict of the northern Neotethyan oceanic realm, which formed within the Central Iranian terrane between Central Iran and the Alborz belt during the Mesozoic (Agard et al., 2005). The ophiolitic succession comprises ultramafic mantle rocks, layered ultramafic–mafic cumulates, sheeted dike complexes, pillow basalts, and pelagic sedimentary cover (e.g., Shojaat et al., 2003; Khalatbari Jafari et al., 2013a, b; Moghadam et al., 2025). The Sultanabad area hosts well‑preserved volcanic units, dominated by pillow basalts intercalated with sheeted dikes and minor massive lava flows. These rocks represent the uppermost levels of the oceanic crust. Field relations indicate that the pillow lavas erupted in a submarine setting and were subsequently affected by tectonic deformation during the Neotethyan closure and obduction of the ophiolitic complex onto the continental margin. <strong>Materials and Methods</strong> Representative samples of pillow basalts and sheeted dikes were collected from the Sultanabad area. Petrographic examination was performed using optical microscopy to characterize mineral assemblages and textural features. Whole‑rock major and trace element concentrations were determined via X‑ray fluorescence (XRF) and inductively coupled plasma mass spectrometry (ICP‑MS). Zircon grains were extracted from selected samples and subjected to U–Pb isotopic dating and trace element analysis using laser ablation‑inductively coupled plasma mass spectrometry (LA‑ICP‑MS). The geochemical data served to classify magma types, constrain magma sources, and discern tectonic affinities. Zircon trace element compositions were additionally examined to offer complementary constraints on magmatic processes and crystallization conditions. <strong>Results</strong> Petrographic observations reveal that the studied volcanic rocks consist predominantly of plagioclase, clinopyroxene, and secondary alteration minerals. The pillow lavas typically exhibit porphyritic to intersertal textures, whereas the sheeted dikes show fine‑grained to subophitic textures. Whole‑rock geochemical data indicate that the studied samples fall into three main magmatic series: alkaline, calc‑alkaline, and tholeiitic varieties. The alkaline basalts are enriched in incompatible elements and display pronounced LREE enrichment relative to HREE. Their trace element patterns are akin to those of ocean island basalts (OIB). The tholeiitic basalts and sheeted dikes exhibit relatively flat REE patterns and are marked by negative Nb and Ta anomalies, characteristic of magmas generated in supra‑subduction zone settings. Calc‑alkaline basalts show intermediate geochemical signatures between these two groups. Zircon U–Pb geochronology reveals that magmatic activity in the study area took place during the Cretaceous, yielding ages between ~113 and 90 Ma. The alkaline basalts record the oldest ages (~112–110 Ma), while the tholeiitic and calc‑alkaline rocks give slightly younger but overlapping ages of ~113–91 Ma. Trace element compositions of zircon grains show HREE enrichment and LREE depletion, consistent with a magmatic origin. Variations in elemental ratios such as Th/U and Eu/Eu* point to differences in magma composition and crystallization conditions across the studied rock types. <strong>Discussion</strong> The geochemical signatures of the studied rocks point to the involvement of multiple magma sources in generating the Sultanabad volcanic sequence. The alkaline basalts most likely originated from low‑degree partial melting of a garnet‑bearing enriched mantle source, consistent with an intraplate or seamount‑related affinity. By contrast, the tholeiitic basalts and sheeted dikes appear to have been derived from higher‑degree partial melting of a depleted mantle source metasomatized by slab‑derived fluids. The calc‑alkaline basalts exhibit geochemical features characteristic of subduction‑related magmatism and probably represent melts generated in a mantle wedge modified by slab‑derived fluids. The co‑occurrence of alkaline and supra‑subduction zone magmas within the same volcanic sequence implies that the Sabzevar oceanic basin underwent a complex tectonomagmatic history, encompassing both intraplate and subduction‑related processes. The zircon U–Pb ages obtained in this study show that magmatic activity persisted over a protracted interval during the Middle to Late Cretaceous. This prolonged magmatism likely reflects the progressive evolution of a supra‑subduction zone system linked to the initiation and advancement of subduction within the northern Neotethys. <strong>Conclusions</strong> Combined whole‑rock geochemistry and zircon U–Pb geochronology provide new constraints on the petrogenesis and tectonic evolution of the volcanic units in the eastern Sabzevar ophiolite. The investigated rocks fall into alkaline, calc‑alkaline, and tholeiitic suites, reflecting the contribution of multiple mantle sources and magmatic pathways. Zircon U–Pb dating restricts magmatism to ca. 113–90 Ma, i.e., the Middle–Late Cretaceous. Geochemical fingerprints suggest that the tholeiitic basalts and sheeted dikes formed in a supra‑subduction zone setting, whereas the alkaline basalts represent within‑plate‑related magmatism subsequently incorporated into the ophiolitic succession. Collectively, the results favor a model whereby the Sabzevar ophiolite developed within a complex supra‑subduction zone system during the evolution and final closure of the northern Neotethys Ocean. در این پژوهش، گدازه‌های بالشی و دایک‌های ورقه‌ای منطقۀ سلطان‌آباد در بخش خاوری مجموعۀ افیولیتی سبزوار با به‌کارگیری داده‌های زمین‌شیمیایی سنگ‌کل، زمین‌زمان‌سنجی U–Pb زیرکن و بررسی عنصرهای کمیاب زیرکن بررسی شدند تا سرشت ماگمایی و چارچوب تکتونوماگمایی پیدایش آنها روشن شود. داده‌های زمین‌شیمیایی نشان می‌دهند سنگ‌های بررسی‌شدة این منطقه دربردارندة سه سری ماگمایی متمایز یعنی آلکالن، کالک‌آلکالن و توله‌ایتی هستند. بازالت‌های آلکالن با غنی‌شدگی در عنصرهای ناسازگار و نسبت‌های بالای عنصرهای خاکی نادر سبک، ویژگی‌هایی همانند بازالت‌های درون‌صفحه‌ای نشان می‌دهند؛ اما نمونه‌های کالک‌آلکالن و توله‌ایتی با تهی‌شدگی از برخی عنصرهای با شدت میدان بالا و الگوهای عنصرهای خاکی نادر کمابیش تخت، با ماگماهای مرتبط با محیط‌های فرورانشی همانندی دارند. زمین‌زمان‌سنجی زیرکن‌ها بازۀ زمانی نزدیک به ۱۱۳ تا ۹۰ میلیون سال پیش را برای تبلور این واحدهای ماگمایی نشان می‌دهد که نشان‌دهندة تداوم فعالیت ماگمایی در کرتاسة میانی تا پسین است. ترکیب عنصرهای کمیاب زیرکن‌ها نیز خاستگاه ماگمایی بلورها را روشن می‌کند و تفاوت در سرشت ماگمای اولیه در میان گروه‌های سنگی گوناگون را آشکارا نشان می‌دهد. تفسیر یکپارچة داده‌های زمین‌شیمی و زمین‌زمان‌سنجی گویای آن است که گدازه‌های توله‌ایتی و دایک‌های ورقه‌ای چه‌بسا در پوستة اقیانوسی وابسته به سامانۀ پیش‌کمان یک کمان آتشفشانی پدید آمده‌اند؛ اما بازالت‌های آلکالن چه‌بسا نمایانگر فعالیت ماگمایی درون‌صفحه‌ای در پهنه‌ای اقیانوسی هستند که بعدها در پی فرایندهای همگرایی و بسته‌شدن نئوتتیس به مجموعۀ افیولیتی افزوده شده‌اند. این یافته‌ها دیدگاه تازه‌ای دربارة پیچیدگی ماگماتیسم و تکامل زمین‌ساختی افیولیت سبزوار در چارچوب ژئودینامیکی نئوتتیس ارائه می‌دهد. https://ijp.ui.ac.ir/article_30439_30efa2384e5f9207c4cb9df8924f8279.pdf