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The early Paleozoic tectonic nature and subsequent orogenic process remain poorly known in the Indochina interior. This study introduces the general geology of South Laos and presents a set of new zircon U Pb and Hf O isotopic analyses, and whole-rock elemental and Sr-Nd-Pb isotopic data for the mafic-intermediate igneous rocks in South Laos. The representative samples from the previously-mapped Ordo-Silurian strata in the southern Truong Son zone give the formation ages of 434–438 Ma. Five samples from the Tam Ky-Phuoc Son tectonic zone yield zircon U Pb ages of 447–485 Ma. The corresponding zircon ε Hf (t), δ18O and T DM values are in range of +1.6~ −6.5, 6.1–8.6‰, and 1.02–1.79 Ga, respectively. Forty-five mafic-intermediate igneous rocks in South Laos with mg-number = 42–69 have SiO 2 = 53.04–61.66 wt%, Al 2 O 3 = 12.13–20.92 wt% and MgO = 2.73–9.55 wt%, and are characterized by enrichment in LILEs and depletion in HFSEs with Nb Ta negative anomalies. Their (87Sr/86Sr) i ratios range from 0.7044 to 0.7109 and ε Nd (t) values from −6.2 to −2.6, with (206Pb/204Pb) i = 18.08–18.68, (207Pb/204Pb) i = 15.50–15.77, and (208Pb/204Pb) i = 38.04–38.97. Ten gabbroic samples, with mg-number = 77–79, show similar incompatible elemental ratios and Sr-Nd-Pb-Hf-O isotopic compositions with the other mafic-intermediate igneous rocks, but have lower SiO 2 and higher MgO, Cr, Ni and V contents, reflective of a primary mafic magma. Such characteristics suggest a newly-modified wedge source by recycled sediment-derived components. In combination with other geological observations in Central Vietnam, we propose a northerly-younging early Paleozoic igneous event in response to the on-growing accretionary orogenesis in the Indochina interior. The southward consumption of the early Paleozoic Tam Ky-Phuoc Son Ocean in the Indochina interior continued until at least ~430 Ma and the assemblage of the Northern Indochina with the Kontum terrane likely terminated at the early Devonian. • The Ordo-Silurian mafic-intermediate igneous rocks are identified in South Laos. • The lower Paleozoic sequence in the Truong Son zone is mainly of Silurian origin. • They were derived from the recycled sediment-modified wedge source. • An oceanic consumption might continue until ~430 Ma in the Indochina interior. [ABSTRACT FROM AUTHOR]

• Mafic-felsic volcanic rocks in NW Laos were emplaced at the Late Tiassic. • Mafic-intermediate rocks were derived from a metasomatized lithospheric mantle. • Rhyolites were derived from partial melting of juvenile mafic rocks. • These volcanic rocks were formed by the asthenospheric upwelling in a post-collisional setting. Early Mesozoic volcanic rocks in northwestern Laos are rarely reported and important for investigating the Paleotethyan tectonic evolution and regional correlation in SE Asia. In this study, New zircon ages and whole-rock geochemical data are presented for the newly identified volcanic rocks in northwestern Laos. Twelve volcanic rocks yield zircon U–Pb ages of 228–234 Ma. The volcanic rocks in northwestern Laos contain basalt, basaltic andesite, andesite, and rhyolite and can be geochemically subdivided into three groups. Both Group 1 and 2 samples belong to the calc-alkaline mafic-intermediate volcanic rocks. Group 1 has ε Nd (t) values of −0.52 to +1.20, ε Hf (t) values of +0.9 to +3.6 and δ18O values of 6.3–7.0‰. In contrast, Group 2 has lower ε Nd (t) values of −2.82 to −5.89 and ε Hf (t) values of −4.0 to +1.4 and higher δ18O values of 6.6–8.1‰ than Group 1. Group 3 samples are high-K calc-alkaline rhyolites with high ε Nd (t) (−0.08 to +0.74) and positive ε Hf (t) (+4.6 to +13.2) values. Geochemical characteristics suggest that the Group 1 and 2 samples were derived from a metasomatized lithospheric mantle modified by sediment-derived components, and Group 3 rhyolites were derived from partial melting of juvenile mafic rocks with a crustal component. Our results, along with the geological observations and reported ages suggest that the Late Triassic volcanic rocks in northwestern Laos were formed in a post-collisional setting, and provide the key studies for the connection between the Lancangjiang and Chiang Khong-Lampang-Tak zones. [ABSTRACT FROM AUTHOR]