SUMARRY

Microstructural feature and zircon U-Pb isotope age of the Tn Hương granite

in Lo Cai area: implication for the timing of left-lateral shearing of the Red River Fault

Ng Xun Thnh, Vũ Đnh Tải, Vũ Quang Ln, Trần Mỹ Dũng,

La Mai Sơn, Trần Trung Hiếu

 

The Red River-Ailao Shan shear zone (RASZ) underwent a sinistral strike-slip shearing which is characterized by ductile deformation along the Dy Ni Con Voi range. The timing and duration of left-lateral shearing along the RASZ are the most importances in constraining the nature and regional significance of the shear zone. In this paper, granitic samples from the Dy Ni Con Voi, along RASZ were collected for studying microstructure and age dating. Field observation indicated that the granite bodies intruded into metamorphic rocks of the Ni Con Voi complex, but were cross-cut and displaced by the activity of the RASZ. Microstructural features of the granite showing that the magmatism produced the Tn Hương granite was coeval with the left-lateral shearing activity of the RASZ. The emplacement time of granite is determined by LA-ICP-MS zircon U-Pb isotope yielded an average 206Pb/238U age of 25.420.08 Ma. All Th/U ratios in zircons vary from 0.29 to 0.54, in combination with their internal structure indicated magmatic origin of zircon. The age of ca.25 Ma, together with microstructural feature of the granites suggested that the activity of left-lateral shearing of the RASZ had initiated earlier than ca. 25.45 Ma and weakly acted during time period of ca. 25-20 Ma ago. Furthermore, the strongly left-lateral shearing of the RASZ was not accompanied with main rifting of the Vietnam East Sea (ca. 23-16.5 Ma).

Keywords: Red River Shear Zone, Tn Hương granite, zircon U-Pb isotope.