SUMARRY

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

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

Vũ Đình Tải, Vũ Quang Lân, La Mai Sơn, Ngô Xuân Thành, Trần Mỹ Dũng, Đinh Mạnh Hà

Felsic magma samples were collected from the Tú Lệ, Phu Sa Phìn 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 Tú 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 254.1±0.75 Ma. This age is similar to that of one granite sample from the Phu Sa Phìn complex (zircon U-Pb age = 253.7±1.34 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 253.05±0.96 Ma. The geochronological data from our study indicated that the magmatic rocks in the Tú Lệ Depression were mainly generated during the Permian time, similar to that of the Sông Đà 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 Tú Lệ and Sông Đà 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, Tú Lệ Depression, zircon U-Pb isotope age.