Geochemical and Sr¡VNd isotopic characteristics of granitic rocks from northern Vietnam

Five major felsic igneous suites from northern Vietnam, with ages from mid-Proterozoic to early Cenozoic, were studied. Representative granitic rocks from the Posen Complex (mid-Proterozoic) and the Dienbien Complex (late Permian to early Triassic) show geochemical characteristics similar to those of calc-alkaline to high-K calc-alkaline I-type granites. However, the former, located in the South China block, has significantly higher initial Nd isotopic ratios [ε(Nd)(T) = +0.7 to +1.5] and older Nd isotopic model ages (T(DM) ~ 1.7 Ga) than the latter [ε(Nd)(T) = -4.7 to -9.7; T(DM) ~ 1.3-1.5 Ga] which were emplaced south of the Song Ma Suture and thus in the Indochina block. The generation of both complexes may be attributed to subduction-related processes that occurred in two distinct crustal provenances with different degrees of mantle inputs. On the other hand, Jurassic to Cretaceous granitic rocks from the Phusaphin Complex, contemporaneous rhyolites from the Tule Basin, and late Paleogene granitic rocks from the Yeyensun Complex, all exposed in the South China block between the Ailao Shan-Red River shear zone and the Song Ma Suture, display geochemical features similar to those of A-type granites with intermediate ε(Nd)(T) values (+0.6 to -2.8) and younger T(DM) ages (0.6-1.1 Ga). These magmas are suggested to have been generated as a consequence of intraplate extension in the western part of the South China block (Yunnan), and to have been transported to their present position by mid-Tertiary continental extrusion along the Ailao Shan-Red River shear zone related to the India-Asia collision. Overall, the isotopic and model age data, reported in this study indicate that in northern Vietnam, the most important crust formation episode took place in the Proterozoic. Likewise, repeated mantle inputs have played a role in the petrogenesis of Phanerozoic granitic rocks. (C) 2000 Elsevier Science Ltd. All rights reserved.