Detrital zircon geochronology and its geological implications of the Nanduan formation in the Changning-Menglian tectonic belt, southwestern Yunnan

This study reports LA-ICP-MS U–Pb ages and Lu–Hf isotope data of detrital zircons from sedimentary rocks of the Nanduan Formation in the Changning–Menglian tectonic belt. The U–Pb age spectra display three dominant populations: Pan-African (530–680 Ma), Late Grenvillian (900–980 Ma), and Early Grenvillian (1100–1300 Ma), with subordinate peaks at ∼350 Ma and 1550–1700 Ma. Among the eight analyzed samples, four contain 1%–8% Carboniferous zircons, which constrain the maximum depositional age of the formation to the Carboniferous—consistent with previously reported Carboniferous ammonoid fossils. The age spectra combined with Hf isotopic compositions ( ε Hf ( t ) ranging from −49.42 to +10.23) indicate that the detritus was predominantly sourced from the Gondwana-affiliated Lancang Group. Notably, zircons of ∼410–440 Ma exhibit ancient crustal isotopic signatures, implying an additional contribution from the Simao Block (Yangtze affinity). The mixing of detrital materials derived from both sides of the Proto-Tethyan Ocean indicates its closure during the early Paleozoic. Together with regional stratigraphic correlations and published early Paleozoic magmatic and high-to ultrahigh-pressure metamorphic records, these results provide new sedimentary evidence for the closure of the Proto-Tethys Ocean, which resulted from widespread collisions between Gondwana-derived blocks and the northern margin of East Gondwana.