Apatite U-Pb Dating and Geochemistry Reveal Pan-African Thermal Overprint in the Vestfold Hills, East Antarctica

【Objective】The Vestfold Hills represent an Archean cratonic nucleus situated on the eastern coast of Prydz Bay, East Antarctica. Due to its distinct lithological assemblage and metamorphic evolution compared to adjacent regions, it has been considered an "exotic" terrane relative to East Antarctica. Although Pan-African orogenesis has been extensively documented throughout most of the Prydz Belt, its extent and intensity in the Vestfold Hills to the east remain poorly constrained. 【Methods】In this study, apatite grains from a noritic intrusion in the northern Vestfold Hills were systematically investigated through detailed petrographic observation, combined with LA-ICP-MS U-Pb dating and in-situ trace element analysis, electron probe microanalysis of major elements, and SHRIMP oxygen isotope analysis. These results were further compared with zircon U-Pb-O isotopic data and whole-rock alteration indices from the same norite. 【Results】The apatite U-Pb analyses yield a lower intercept age of 538 ± 29 Ma on the Tera-Wasserburg diagram, significantly younger than our previously determined zircon U-Pb age (2479 ± 3.7 Ma). The apatite grains exhibit relatively homogeneous major and trace element compositions, enriched in F, Fe, Mn, Sr, and rare earth elements (REE), with middle REE-enriched “cap-shaped” chondrite-normalized REE patterns and moderate negative Eu anomalies, and their δ¹⁸O values range from 5.87 to 7.04‰, consistent with those of coeval zircon (4.7-7.0‰), which indicate a magmatic origin as fluorapatite. 【Conclusion】These features demonstrate that the apatite U-Pb age records resetting during the Pan-African thermal event, whereas its major-trace element and O isotopic compositions preserve primary magmatic signatures, implying that the Vestfold Hills likely occupied a marginal position relative to the Prydz Belt, where Pan-African modification was dominated by distal thermal influence, with temperatures between the closure temperatures of the apatite and zircon U-Pb systems. This thermal input was sufficient to reset the apatite U-Pb system but failed to modify elemental and O isotopic compositions or to promote the growth of new metamorphic minerals. This study reveals that under anhydrous conditions, the apatite U-Pb system can be thermally reset while elemental and O isotopic systems remain inert; however, when fluids are involved, all systems may be simultaneously modified. Consequently, caution should be exercised when applying apatite U-Pb dating for geochronological constraints and provenance tracing, with due attention to the differential responses of various isotopic and geochemical systems to subsequent thermal events.