Zircon U-Pb isotope age of the Na Khoun granite complex in the Sầm Nưa regionand its geological implications

Lưu Công Trí, La Mai Sơn, Nguyễn Thị Thanh Thảo, Nagi Takahashi

The study area is an extension of the Phu Hoat metamorphic terrane from the Northwestern Nghệ An, Vietnam. The Na Khoun complex is made up of coarse-grained biotite granite and porphyritic biotite granite, distributing in the South of Sầm Nưa town, Hủa Phăn province, Laos PDR. Zircon crystals separated from the porphyritic biotite granite sample L.013-1 were analyzed for their U-Pb isotope composition by LA-ICP-MS method. A total of 23 zircon grains were selected and analyzed, which yielded 206Pb/238U ages varying from 24,5 to 26,3 Ma with an average 206Pb/238U value of 25,22 ± 0,19 Ma, similar to previously reported ages for the porphyritic granite of the Bản Chiềng complex, Northwestern Nghe An province of Vietnam. The U-Pb isotope age result from our study indicated a magmatic activity took place during the Oligocene - Miocene time along the Phu Hoạt high metamorphic terrane including the extended part in the territory of Laos. This new zircon U-Pb isotope age is the basis for emplacement age correction of the Na Khoun granite complex expressed on the geological and mineral maps of Sầm Nưa region.

Keywords: Phu Hoạt metamophic terrane, Na Khoun granite, zircon U-Pb isotope.