Summary

Geochemical characteristics and LA-ICP-MS U-Pb zircon age of intermediate-acid extrusive rocks in the Bãi Tiên area, Nha Trang

Nguyen Thi Truong Giang¹, Pham Trung Hieu¹, Pham Minh¹, Luu The Long²

Intermediate-acid extrusive rocks in the Bai Tien – Nha Trang area exposed along the high way DT6571 and cover an area of about 5 km². The extrusive rocks are rhyolite, rhyodacite and dacite in composition and have high contents of SiO₂ (64.65-75.22%) and total alkaline (Na₂O + K₂O= 6.89-8.88%). They belonging to the high potassium and calc - alkaline series. The crystallization temperature of rocks is about 750^oC - 850^oC, it is lower than the crystallized temperature of A-granite and similar to the crystallized temperature of I-granite. Emplacement age of the rocks were obtained by LA-ICP-MS zircon U-Pb isotopes giving ²⁰⁶Pb/²³⁸U age of 103 Ma, corresponding to the early Cretaceous time. The extrusive rocks in the Bai Tien - Nha Trang area were formed at active continental margin during subduction of ocean crust beneath the continental crust. These extrusive igneous rocks closely related to the granitoid rocks of Dinh Quan and Deo Ca complexes.

Key words: Rhyolite, Nha Trang, geochemistry, zircon U-Pb age.