Linking Hainan to Indochina through tectono-thermal records in metasedimentary rocks

Abstract

Establishing accurate depositional ages and deciphering the tectono-thermal evolution of sedimentary sequences are essential for constraining their affinity and potential involvement in supercontinent cycles. This study presents an integrated U-Pb dating and trace element analysis on zircon, monazite, and rutile from the Shilu Group in Hainan, South China linking its Neoproterozoic depositional and metamorphic record to Indochina during the transition from Rodinia to Pangea. Based on the mineral assemblages, internal textures, and geochemical features of the accessory minerals from the sixth layer of the Shilu Group, magmatic zircon, metamorphic rutile, and both magmatic and metamorphic monazite are identified. Detrital zircons and monazites yielded the youngest weighted mean ²⁰⁶Pb/²³⁸U ages of 977 ± 13 Ma (MSWD = 1.2, n = 7) and 959 ± 7 Ma (MSWD = 1.8, n = 8), respectively. Metamorphic monazite and rutile grains produced age clusters at 948–816 Ma, 453–329 Ma, and 291–235 Ma, with the three oldest monazite grains yielding a weighted mean age of 946 ± 7 Ma (MSWD = 0.14). These geochronological data collectively suggest that the sixth layer of the Shilu Group was deposited around 960–950 Ma and experienced metamorphism during the intervals of 950–820 Ma, 450 Ma, 350 Ma, and 250 Ma. The documented metamorphic events closely parallel those observed within the Kontum Massif. This concordance, along with published data, suggests a geological connection between Hainan and Kontum from the early Neoproterozoic to the late Paleozoic.