Some new petrography and U-Pb zircon ages of magmatic rocks

in the T Lệ depression (NW Vietnam) and implication for regional geology

Vũ Đnh Tải, Vũ Quang Ln, La Mai Sơn, Ng Xun Thnh, Trần Mỹ Dũng, Đinh Mạnh H


Felsic magma samples were collected from the T Lệ, Phu Sa Phn and gabro from the Nậm Chiến complexes for study on petrography and their emplacement time by zircon U-Pb isotope. The results show that the petrographic composition of the T Lệ complex is very complicated, including tuff, volcanic clastics, and volcanic rock. One sample from the volcanic rock was dated by zircon U-Pb isotope yielding an average 206Pb/238U age of 254.10.75 Ma. This age is similar to that of one granite sample from the Phu Sa Phn complex (zircon U-Pb age = 253.71.34 Ma. The Nậm Chiến complex is mainly consisted of gabbro, diabase gabbro, which are highly altered and amphibolized. One meta-gabbro sample collected and analyzed giving an average 206Pb/238U age of 253.050.96 Ma. The geochronological data from our study indicated that the magmatic rocks in the T Lệ Depression were mainly generated during the Permian time, similar to that of the Sng Đ volcanic rocks, as well as to plume magma in the Emeishan region (NW China). The new age data from this study, in combination with previously reported data suggest that both T Lệ and Sng Đ Depressions were mainly formed in the Permian time, perhaps related to the Emeishan plume magmatism took place during the Middle-Late Permian.

Keyword: magmatic rocks, T Lệ Depression, zircon U-Pb isotope age.