ژئوشيمي و سنگ‌زايي سنگ‌هاي آذرين در شمال‌شرقي زفره (استان اصفهان، بخش مركزي پهنه ماگمايي اروميه-دختر)

Geochemistry an‎d petrogenesis of igneous rocks in the northeast of Zefreh (Isfahan Province, central part of the Urumieh-Dokhtar magmatic zone)

زمين شناسي

Abstract This research investigates subduction-related plutonic rocks from the Oligocene-Miocene interval in the central part of the Urumieh-Dokhtar Magmatic Arc (located within the Alpine-Himalayan orogenic belt). The composition of these rocks varies from gabbro/diorite (~34–32 Ma) to tonalite (~20 Ma), an‎d their geochemical characteristics align with calc-alkaline, meta-aluminous Type I granitoids. Mineralogical studies indicate that plagioclases range from bytownite to an‎desine an‎d labradorite. The presence of zoned plagioclases with sieve textures an‎d dusty rims confirms magmatic disequilibrium an‎d magma mixing. Additionally, amphiboles in the region fall within the calcic amphibole range (tremolite-actinolite to magnesio-hornblende), while clinopyroxenes are augite. The identified chlorites are of the clinochlore type. Geochemical data reveal enrichment in large-ion lithophile elements (LILE) an‎d light rare earth elements (LREE) compared to high-field-strength elements (HFSE) an‎d heavy rare earth elements (HREE) (LaN/YbN = 3.05–4.45). Based on hornblende thermobarometry, the estimated formation conditions of these minerals range from 3.0 to 3.6 kbar an‎d 685 to 732°C. These compositional variations, along with mineralogical evidence, suggest lower crustal melting, progressive crustal contamination, an‎d magma chamber evolution over time. The chemical evolution of these plutonic rocks has been primarily influenced by two key processes: - Increased contribution of Th-rich crustal sediments, indicating progressive lower crustal melting, compared to the subordinate role of Ba-rich aqueous fluids released from the subducted oceanic slab into the subcontinental mantle. - Gradual injection of fertile mantle melts, which acted as an effective agent in modifying magma chamber compositions, leading to systematic changes in the rocksʹ geochemical signatures. These processes not only explain the compositional differences between Oligocene an‎d Miocene rocks but also influence their mineralization potential. Oligocene plutons are mostly barren in terms of copper mineralization, whereas Miocene intrusions, influenced more by mantle-derived slab fluids, show a probable potential for copper mineralization. The proposed geodynamic model in this study suggests that the magmatic evolution in this region results from a complex interplay between slab dynamics, crustal thickening, an‎d extensional processes during the closure of the eastern Neotethys. These processes have not only controlled the composition of plutonic rocks but also played a decisive role in the evolution of the Urumieh-Dokhtar Magmatic Arc within the framework of the Alpine-Himalayan orogeny. Keywords: Petrology, Geochemistry, Intrusive Rocks, Northeast Zefreh, Urumieh-Dokhtar Magmatic Zone

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