Vol 10, No 2 (2016)

New petrological and isotope–geochemical data are obtained for rocks from the South China Sea shelf zone (Thu, Cu-Lao Re, Hong Jo islands and Katuik–Ile des Cendres island group). These data are correlated with the available published data on the volcanic rocks of the terrestrial part of Vietnam and with data on the basaltoids of the South China Sea, Thailand, and the northern part of Hainan island. Despite the fact that the studied volcanic rocks belong to different structural zones—continental margin, shelf zone, marginal sea—their formation is related to the same type of rift volcanism. Owing to this, the different compositions of the primary magmatic melts indicate, first of all, the heterogeneity of the mantle sources. The uniformity of manifestations of volcanism over the entire studied territory within the age boundaries excludes any zoning at the transition from one structure to another. The leading role of mantle diapirism in the evolution of volcanism throughout the entire Indochina region is shown. It is proposed that the formation of alkaline series and “alkaline” trends of changes in their compositions are consequences of fractionation of melts which originated from an asthenosphere plume chamber. Meanwhile, the tholeiitic magma series are of reactionary origin and “tholeiitic” trends record the processes of mixing of melts generated during melting of the lithospheric mantle with those of a plume chamber. Individual samples show insignificant contamination of melts by crustal material.

The paper presents the results of U–Pb geochronological and geochemical studies of the rocks of the monzodiorite–granodiorite association in the northeastern flank of the South Mongolia–Khingan orogenic belt, which composes a tectonic block among the provisionally Lower Paleozoic volcanosedimentary complexes of the Nora–Sukhotinsky Terrane. It is shown that the studied rocks have similar petrographic features (with the presence of transitional varieties) and form common trends in the petrographic diagrams. This suggests that they are members of a single magmatic association. The geochemical features of the monzodiorites, quartz monzodiorites, and granodiorites, in particular their enrichment in large ion lithophile elements (LILE) and depletion in some HFSE, indicate their similarity with island-arc magmatic rocks. The presence of monzonites and quartz monzonites in the studied monzodiorite–granodiorite association along with high K, Rb, Th, and Pb concentrations gives reasons to believe that it formed in active continental margin or ensialic island-arc environments. The granodiorites of the monzodiorite–granodiorite associations of the Nora–Sukhotinsky Terrane are dated at 440 ± 10 Ma and may be considered as a fragment of the early Silurian active continental margin or ensialic mature island arc in the structure of the South Mongolia–Khingan orogenic belt.

Analysis of the distribution and fractionation of rare earth elements in the groundwater of the Sikhote Alin fold region reveals that their concentrations, geochemistry, and fractionation ability vary in different groundwater types depending on many factors, including the pH–Eh parameters and mineralization of the solutions and the composition of the host rocks. The results of monitoring of changes in the REE concentrations in the groundwater of the region over ten years provides the opportunity to establish the range of their variations. Inorganic forms of REE migration are rated for all geochemical types of water in the meteoric–surface–ground water system.