Rb-Sr Isochron and K-Ar ages of igneous rocks from the Samnua Depression Zone in Northern Vietnam

摘要

The Samnua Depresion Zone (SNDZ) extends NW-SE for more than 400 km from Northern Vietnam to Southwestern China. It represents the northern edge of the Indochina block and is separated from the South China block by the Songma fault. The zone is occupied mainly by Paleozoic sedimentary rocks and Mesozoic volcano-sedimentary successions that are intruded by Mesozoic intrusives and extrusives and by Cenozoic extrusives. Representative igneous rocks of the SNDZ were geochronologically investigated by the Rb-Sr whole rock and K-Ar biotite methods. The Rb-Sr ages obtained are 222 Ma for the Nuichua gabbros, 213 Ma for the Songma granitoid (SMG), 203 Ma for the Piabioc granitoid (PBG), 138 Ma for the Banmuong subvolca-nic granite, 27 Ma for the Banchieng granitoid (BCG), 218 Ma for the Dongtrau felsic volcanics, 186 Ma for the Muonghinh felsic volcanics and 5.8 Ma for the Cenozoic basaltic rocks (CBR). The K-Ar methods give 252 Ma for the SMG, 237 Ma for the PBG and 24 Ma for the BCG. The groundmass fraction of the CBR yields an age of 1.9 Ma. Geochronological comparison and using fossil records suggest that the Rb-Sr ages of Mesozoic igneous rocks provide apparent young age, a pseudo isochron age. The major igneous activity of the SNDZ took place during Late Permian-Triassic time to form granitic and felsic volcanic rocks, which were related to the continental volcanic arc magmatism. Minor intraplate igneous activity occurred in the Paleogene. This suggests that the tectonic environment of the SNDZ changed from a convergent plate margin to a matured continental margin in the time span between the Triassic and Paleogene.