Backarc mafic-ultramafic magmatism in Northeastern Vietnam and its regional tectonic significance

The geology of Northern Vietnam offers critical clues on the convergence history between the South China and Indochina blocks. We constrain the tectonic evolution of the South China and Indochina blocks using geochemical, mineral chemical and geochronological data collected from mafic-ultramafic rocks exposed in the Cao Bang area, Northeastern Vietnam. These rocks show significant enrichment in large ionic lithophile elements (LILEs) such as Cs, Rb, Ba, Th, U, and Pb and depletion in high field strength elements (HFSEs) such as Nb, Ta, Zr, and Ti showing [Nb/La]_N between 0.28-0.41, [La/Yb]_N=3.94-10.00 and Zr/Y=2.0-4.4. These geochemical features as well as the petrology and mineral chemistry of the Cao Bang mafic-ultramafic magmas are comparable to those of magmatic complexes formed in a back-arc environment. The basalts yield Rb-Sr whole rock ages of 263±15Ma, that are consistent with the zircon U-Pb and K-Ar ages reported in previous studies from the same area. The spatial and temporal distribution of the arc magmas within the Indochina block and along the southern margin of the South China block suggest that the Permo-Triassic mafic-ultramafic magmas formed during a tectonic event that is different from the subduction and collision event between the Indochina and South China blocks.