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In order to verify the early Paleozoic accretionary assemblage in the Indochina interior and constrain the Prototethyan tectonic evolution in Southeast Asia, this study presents a set of new U-Pb geochronological, elemental, and Sr-Nd-Pb-Hf-O isotopic data for the fifty-two representative granitoids in South Laos. The granitoids from the Kontum terrane, Tam Ky-Phuoc Son tectonic zone, and southern Truong Son igneous zone in South Laos yield the crystallization ages of 464–485 Ma, 455–471 Ma, and 427–446 Ma, respectively, with a northerly younging trend within the Indochina interior. They are mainly monzogranite with A/CNK = 0.96–1.99 and K2O > Na2O, which are marked by enrichment in large-ion lithophile elements and depletion in high field strength elements with remarkable Nb-Ta, Sr-P, and Ti negative anomalies. Their initial 87Sr/86Sr ratios range from 0.70510 to 0.71559, εNd(t) from −9.5 to −3.0, (206Pb/204Pb)i from 18.65 to 19.72, (207Pb/204Pb)i from 15.66 to 15.80, and (208Pb/ 204Pb)i from 38.84 to 39.79. The corresponding zircon ɛHf(t) and δ18O values are in the range of −10.6 to +1.0 and 6.88‰ to 8.94‰, respectively. In addition, their Sr-Nd-Pb and Hf-O isotopic compositions are generally similar with those of time-equivalent granitoids in South Tibet and SW Yunnan, China, and synchronous mafic-intermediate igneous rocks in South Laos, but distinctive from those of the supracrustal sedimentary-derived South China Paleozoic granite and Lincang-Sukhothai S-type granite. The early Paleozoic granitoids in South Laos might have originated from a mixed source of the wedge-derived juvenile crust coupled with supracrustal materials. All these data synthetically suggest the southward subduction of the Tam Ky-Phuoc Son Ocean and the northerly on-growing Ordo-Silurian accretionary orogenesis within the previously defined “single-ancient” Indochina block. The assemblage of the Indochina block might initiate at ca. 430 Ma in the Silurian and terminate in the Early-Middle Devonian.

In Southeast Asia, the Jinshajiang–Ailaoshan suture in Southwest China and the Song Ma zone in Vietnam were generally accepted as the amalgamation boundary of the South China and Simao/Indochina blocks. However, the tectonic location of the Paleotethyan suture in North Laos and its tectonic affinity remain ambiguous. To address this issue, we present the geochemical and geochronological data of a set of mafic and plagiogranitic rocks along the Song Ma suture zone in North Laos. Three groups of rocks with distinctive petrological, geochronological, and geochemical signatures were identified. The ca. 370 Ma Group 1 plagioclase amphibolite was preserved as xenoliths in the ca. 260 Ma granitoids. It exhibits a normal mid–oceanic–ridge basalt (N–MORB)-like, multi-elemental pattern with highly positive εNd(t) (+3.3 to +10.7) and εHf(t) (+6.68 to +7.41) values and low δ18O values (5.3‰−5.5‰), that are interpreted as products of the Late Devonian − earliest Carboniferous continental rift setting. The ca. 270 Ma Group 2 gabbro/diabase–diorite exhibits arc-like geochemical affinity with the εNd(t), εHf(t), and δ18O values of –5.85 to –3.76, –3.8 to +3.2, and 6.6‰–7.6‰, respectively. It is derived from the metasomatized wedge. The ca. 260 Ma Group 3 plagiogranite shows Nb-Ta and Ti negative anomalies and positive εNd(t) (+3.21 to +4.21), εHf(t) (+8.4 to +13.6), and δ18O (5.9‰−6.8‰) values, which are indicative of its derivation from mafic oceanic crust in a subduction setting. In combination with the published data, these results support the development of the Carboniferous–Permian Paleotethyan branch in North Laos, which represents the tectonic location of the Song Ma suture zone that spatially connects the Jinshajiang–Ailaoshan suture zone in Southwest China and the Chenxing–Bangxi suture zone in Hainan Island, South China.

Felsic igneous rocks in northern Laos are key elements of the Truong Son and Song Ma tectonic zones and are important for constraining the Paleotethyan evolution in southeastern Asia. This paper presents new geochronological, geochemical and Sr–Nd–Hf–O isotopic data from granitoids and rhyolites in northern Laos. Geochronological data show that these samples have zircon U-Pb ages ranging from ca. 281 Ma to 221 Ma. These samples can be subdivided into four groups based on their lithology and geochemical composition. Group 1 samples are granodiorite and granite with I-type geochemical characteristics. They have lower 87Sr/86Sr (i) ratios (0.7036–0.7059) and higher eNd (t) values (+0.7 − +0.8) than Group 2 biotite granite samples (87Sr/86Sr (i) = 0.7087–0.7129 and eNd (t) = −5.3 to −9.2) and Group 3 rhyolite samples (87Sr/86Sr (i) = 0.7092–0.7103 and eNd (t) = −5.9 to −8.9). Zircon grains from Group 1 samples yield positive eHf (t) values of +5.1 − +13.8, TDM ages of 0.37–0.71 Ga and δ18O values of 5.6–6.3‰. In contrast, zircons from Group 2 and Group 3 samples have mainly negative eHf (t) values (−9.6 to +1.6 and −4.7 to −14.7), older TDM ages (1.08–1.70 Ga and 1.42–1.97 Ga), and higher δ18O values (5.7–10.3‰ and 6.7–8.0‰), respectively. Our geochemical data suggest that the Group 1 granite samples are the products of the partial melting of newly underplated mafic rocks, whereas the Group 2 and 3 samples originated from a mixed source of “ancient” metaigneous and metagreywacke components. Group 4 samples include Late Triassic I-type granodiorite and granite and have eNd (t) = −1.9 to −3.9, 87Sr/86Sr (i) = 0.7079–0.7084, zircon eHf (t) = 0.0 − +7.5, and δ18O = 5.1–6.9‰, indicating a derivation from a metagreywacke source with a component of juvenile mafic crust. Our data suggest that the Permian felsic igneous rocks in northern Laos formed in an arc setting related to the Song Ma Paleotethyan branch/BAB subduction, whereas the Late Triassic I-type granitoids formed in a post-collisional setting in response to the gravitational collapse of thickened crust during the collision of the South China and Indochina blocks. The temporal-spatial pattern of igneous rocks along the Truong Son zone is related to the Paleotethyan evolution and resembles that along the Ailaoshan-Song Ma zone and its eastward extension in northern Hainan.

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 eNd (t) values of −0.52 to +1.20, eHf (t) values of +0.9 to +3.6 and δ18O values of 6.3–7.0‰. In contrast, Group 2 has lower eNd (t) values of −2.82 to −5.89 and eHf (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 eNd (t) (−0.08 to +0.74) and positive eHf (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.

The Indochina block is one of a number of continental fragments that lay along or outboard of the East Gondwana northern margin. Its early Paleozoic tectonic nature and the relationship with East Gondwana remain poorly-known. This paper shows the general geology of northeastern Indochina and presents a set of new geochronological and geochemical data with respect to its early Paleozoic tectonics in East Laos. These results reveal early Paleozoic (~485-410 Ma) igneous activity in the northeastern Indochina, which are divisible into four geochemical groups. The ~450-434 Ma Group 1 and ~485-434 Ma Group 2 mafic-intermediate igneous rocks were derived from MORB-like and supra-subduction zone wedge source modified by the recycled-sedimentary components, respectively. The Group 3 rhyolites at ~424 Ma represent the melting products of juvenile mafic crust. The Group 4 granitoids and equivalent volcanics formed at ~485-407 Ma and were derived from crustal materials with variable input of juvenile crust. The detrital-zircon age-spectra are marked by peaks at ~560 Ma, ~960 Ma, ~1600 Ma, ~1850 Ma and ~2450 Ma for the pre-Silurian paragneiss in the NE Indochina, reflective its geographical proximity to the India-Antarctica segment of East Gondwana until the earliest Paleozoic. The age-peaks of ~447 Ma and ~780 Ma is additionally given for the detrital-zircon from the Silurian sandstones in the Truong Son zone. An accretionary assemblage model is proposed for the early Paleozoic tectonic pattern of the SW Indochina with Yangtze fragments. Such an orogenesis formed the part of the North Indo-Australie Orogen related to subduction of the Prototethyan Ocean and the subsequent accretion of Asian micro-continental fragments along the Peripheral East Gondwana until earliest Devonian.

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 eHf(t), δ18O and TDM 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 SiO2 = 53.04–61.66 wt%, Al2O3 = 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 eNd(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 SiO2 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.

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 SiO2 = 45.99–57.47 wt%, Al2O3 = 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 eNd(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.