Detrital Apatite Records of Neoproterozoic Subduction–Accretion evolution along the Northwestern Margin of the Yangtze Block

Key words: 
• Detrital apatite /
• Geochronology /
• Neoproterozoic /
• Yangtze Block /
• Subduction-accretion

Abstract: [Objective] Forearc basins along active continental margins can effectively preserve important sedimentary records of the tectonic, magmatic, and metamorphic evolution of adjacent magmatic arcs. The Hengdan Group exposed in the Bikou area at the northwestern margin of the Yangtze Block, serves as a well-preserved Neoproterozoic forearc-basin succession. However, its response to arc-related tectono-magmatic processes in adjacent areas remains poorly constrained. [Methods] In situ U–Pb dating and trace-element analyses were conducted on detrital apatite from sedimentary rocks of the lower Baiyang Formation and middle-upper Yangtianba Formation of the Hengdan Group. These data were combined with genetic-type discrimination of apatite to reveal changes in provenance composition and their response to the Neoproterozoic subduction–accretion evolution along the northwestern margin of the Yangtze Block. [Results] Detrital apatite ages from the Hengdan Group are dominated by Neoproterozoic populations, with major age peaks at approximately 810 Ma and 770 Ma for the Baiyang and Yangtianba formations, respectively. The Baiyang Formation contains relatively more Paleoproterozoic to Mesoproterozoic detrital components, indicating a greater contribution from ancient basement materials during its deposition. Based on apatite trace-element characteristics and SVM classification results, detrital apatite from the Hengdan Group is dominated by I-type granitoids and mafic igneous rocks (IM), with ages mainly concentrated between ca. 740 and 860 Ma, corresponding to regional Neoproterozoic arc magmatism. High-grade metamorphic apatite (HM) shows age peaks concentrated at 800–900 Ma, which may record the exhumation and input of regional high-grade metamorphic basement during arc–continent convergence. Low- to medium-grade metamorphic and metasomatic apatite (LM) displays two age groups of 800–1000 Ma and 600–700 Ma. The former may be related to low- to medium-grade metamorphism under a prolonged early Neoproterozoic subduction setting, whereas the latter may record the influence of late Neoproterozoic intraplate magmatic thermal events. The age peak of IM-type detrital apatite youngs from ca. 810 Ma in the Baiyang Formation to ca. 770 Ma in the Yangtianba Formation. Together with the age-related variations in Sr/Mn and Sr/Y ratios, this trend indicates that the provenance of the basin shifted from relatively diverse sources to a dominance of young arc-derived magmatic materials during deposition of the Hengdan Group, and that the magmatic arc itself evolved from a juvenile arc system into a more mature continental arc system. [Conclusion] Integrated detrital apatite U-Pb geochronology and trace-element characteristics suggest that the main deposition of the Hengdan Group most likely occurred during the late stage of active continental-margin forearc-basin development under a long-lived Neoproterozoic subduction–accretionary setting along the northwestern margin of the Yangtze Block, or during the early stage of transition to post-orogenic extension environment. The maximum depositional age of the Hengdan Group should be close to 720 Ma, and the succession was subsequently overprinted by late Neoproterozoic regional thermal events.