| Abstract:The Neoproterozoic magmatism widely developed in the Micangshan and Hannan blocks of the northwestern Yangtze Craton is of great significance for investigating the tectonic-magmatic evolution of the Yangtze Craton during the Neoproterozoic and its role in the evolution of the Rodinia supercontinent. This study conducts detailed petrographic, zircon U-Pb geochronological, Lu-Hf isotopic and geochemical analyses on the porphyritic granite and granitic gneisses in the Micangshan block to reveal their formation mechanisms and dynamic setting. Petrographic observations indicate that these rocks underwent intense regional metamorphism and deformation after their formation. LA-ICP-MS zircon U-Pb dating reveals that the porphyritic granites formed at 822 ± 2 Ma, while the protoliths of the granitic gneisses formed between 838 and 852 Ma. Geochemical analyses show that the porphyritic granites exhibit high Sr, low Y and Yb, and high Sr/Y and La/Yb ratios, resembling typical adakitic rock. Their low MgO, Cr, and Ni contents, coupled with positive εHf(t) values, suggest derivation from partial melting of thickened juvenile basaltic crust. In contrast, the granitic gneisses display low Mg#, Ni, and Zr concentrations, classifying them as I-type granites. Their primitive mantle-normalized trace element patterns are enriched in large-ion lithophile elements (Rb, Ba, U, K) but depleted in high-field-strength elements (Nb, Ta, Ti), indicative of arc-related magmatism. Combined with their positive zircon Hf isotopic compositions, these features imply formation via partial melting of juvenile crust. Extensive studies suggest that the northwestern Yangtze Craton was situated in a continental margin arc setting between 870 and 830 Ma due to oceanic subduction, generating arc-type magmatic rocks such as the Zhengyuan gabbro, Shantan diorite, and Zhengyuan I-type granitic gneisses in the Micangshan region. By ~820 Ma, slab rollback led to the formation of adakitic granite in Micangshan and A-type granites in Xihe. Subsequently, between 770 and 707 Ma, the collision and amalgamation of the Hannan arc with adjacent continental blocks, followed by orogenic collapse, resulted in the widespread emplacement of adakites and A-type granites. In conclusion, the Micangshan and Hannan blocks likely represent a prolonged Neoproterozoic continental arc accretionary environment along the northwestern Yangtze Craton, evolving from the Micangshan arc to the Hannan arc during the assembly of the Rodinia supercontinent. |
