Late Cambrian Magmatic Events in SW Yunnan and Implications for the Tectonic Reconstruction of Northern Gondwana

In this study, we investigated Early Paleozoic magmatic rocks of the Manlai Formation exposed along the eastern margin of the Lancang terrane to better understand the tectonic history of the Proto-Tethys. We present petrological, geochemical and whole-rock Sr-Nd and zircon Hf isotopic data for basalts and gabbros sampled from the Qianmai mélange. Zircon grains from six basaltic and gabbroic samples yielded U-Pb ages of 495–482 Ma. These rocks are characterized by tholeiitic and Nb-enriched compositions, with Nb/La ratios in the range of 0.38–1.38, similar to the typical Nb-enriched basalts. All the mafic rocks show slightly negative to positive ε_Nd(t) (−1.67 to + 4.32) and zircon ε_Hf(t) values (−7.3 to +3.8). Elemental and isotopic data suggest that the Qianmai Nb-enriched mafic rocks were mainly derived from the mixing of an OIB-like source with a subduction-modified mantle wedge source. Together with magmatic and sedimentary records of similar ages in the Lancang terrane and the Baoshan Block, our results reveal Early Paleozoic magmatic and sedimentary sequences along an active margin of the Proto-Tethys. Taking into account the recently identified Early Paleozoic ophiolitic mélange in the Yunxian-Menghai belt, we consider the Qianmai magmatic rocks to represent the products of early-stage subduction-related magmatism within a primitive island arc or fore-arc setting associated with the southward subduction of the Proto-Tethys. We infer that prolonged south-dipping subduction on the northern margin of Gondwana persisted from the Cambrian to the Late Ordovician.