Closure of the Early Palaeozoic Proto-Tethys ocean in the Changning–Menglian suture zone, SW China: evidence from the geochronology and geochemistry of amphibolites in the Nantinghe area

Numerous Palaeozoic ophiolites occur in the Changning – Menglian suture zone in southwestern Yunnan Province, China, which are important for reconstructing the evolution of the Proto- and Paleo-Tethys oceans. In this study, zircon U-Pb age and whole-rock geochemical data were acquired for amphibolites in the Nantinghe ophiolites within this suture zone, to constrain the timing of closure of the Proto-Tethys Ocean. The amphibolites can be classified into epidote amphibolites and migmatitic garnet-bearing amphibolites, with the latter containing leucocratic and melanocratic domains. Zircon U-Pb dating yielded 206Pb/238U ages for zircon cores of 466–456 and 480.8 ± 5.1 Ma (weighted-mean), which represent the protolith age. Zircon rims have ages of 445–436 Ma, which date metamorphism or anatexis. The epidote amphibolites have relatively low SiO2 and high Al2O3, MgO, and TFe2O3 contents, are light rare earth element (REE) enriched and heavy REE depleted, and have negative Rb, Th, Nb, Ta, Zr, and Hf anomalies in trace element diagrams. These geochemical affinities suggest a protolith origin from arc-related magmatism. Synthesizing documented records of Early Palaeozoic calc‑alkaline igneous rocks, these rock suites demonstrate that subduction of the Paleo‑Tethys Oceanic lithosphere had initiated in the Early Ordovician with its peak activity occurring in the Middle Ordovician (466–456 Ma).The metamorphic ages (445–436 Ma) of the garnet-bearing amphibolites are comparable to those of garnet amphibolites in Taoxinghu and high-pressure mafic granulites in Xiangtaohu in the Longmucuo – Shuanghu area. They record the timing of continental collision accompanying closure of the Proto-Tethys Ocean. The Longmucuo – Shuanghu – Changning – Menglian suture zone had a similar Early Palaeozoic tectonic evolution to that of the Proto-Tethys Ocean in areas such as North Qilian, East Kunlun, and northern Qaidam.