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 (εHf(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.