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• The Nan and Luang Prabang mafic rocks were dated at ~265–294 Ma. • They derived from a wedge source recently modified by slab-related components. • They generated in the continental back-arc basin in the Indochina block. • The Nan-Luang Prabang back-arc basin spatially linked with Banpo and Sa Kaeo BABs. • The Nan-Luang Prabang back-arc basin finally closed at the early-middle Triassic. Southeast Asia was mainly created by the late Paleozoic assemblage of the Eastern Cimmeria with other continental fragments. However, the tectonic nature and geodynamic setting of the key tectonic zones in the Indochina block are still in dispute, hindering the understanding of the Eastern Paleotethyan evolution of Southeast Asia. This study presents a set of new geochronological, elemental and Sr-Nd isotopic data for the late Paleozoic mafic rocks in the Nan and Luang Prabang tectonic zones. The mafic rocks from the Nan and Luang Prabang zones give zircon U-Pb ages of 279–294 Ma and 265–294 Ma, respectively, with xenocryst ages of 302–2524 Ma. They have SiO 2 = 45.99–57.47 wt%, Al 2 O 3 = 12.74–21.06 wt%, FeOt = 8.34–15.03 wt% and MgO = 2.79–14.74 wt% with mg-number of 34–74, and fall into the fields of basalt/basaltic andesite and its equivalents. These rocks are characterized by enrichment in LILEs and LREEs, depletion in HFSEs, low initial 87Sr/86Sr ratios (0.7032–0.7057) and high ε Nd (t) values (+4.8 to +7.4). They might be derived from a mantle wedge source modified by slab-derived components, and formed in a continental back-arc basin setting. Our analysis indicates the development of the Carboniferous-Permian mafic igneous event in the Nan and Luang Prabang tectonic zones, and that the Nan tectonic zone linked northerly with the Luang Prabang zone during the late Paleozoic, tectonically comparable to the Banpo-Nanlinshan back-arc basin in SW Yunnan, SW China. The back-arc basin finally closed in the early-middle Triassic and the subsequent assemblage continued till the latest Triassic in response to the eastward subduction of the Paleotethyan slab. [ABSTRACT FROM AUTHOR]