SUMARRY
Microstructural
feature and zircon U-Pb isotope age of the Tân Hương granite
in Lào
Cai area: implication for the timing of left-lateral shearing of the Red River Fault
Ngô Xuân Thành, Vũ Đình Tải, Vũ
Quang Lân, 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 Dãy Núi 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 Dãy Núi 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 Núi 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 Tân
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
25.42±0.08 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, Tân Hương
granite, zircon U-Pb isotope.