Abstract
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.
