Whole rock Rb-Sr and U-Pb zircon age of granitoids in basement
of Tiger and Dragon oil fields of the
Cửu Long Basin,
Việt Nam and its geological implications

Vũ Như Hng, Thi Quang, Lưu Thế Long, Nguyễn Thị Bch Thủy

Granitoid in the Rồng, Bạch Hổ oilfield basement of the Cửu Long Basin, south Việt Nam consists of diorite, granodiorite and less granite. The rocks are fractural, cataclastic and dark grey in colour. Biotite and horblende are main colour mineral in the rocks. These granitoids were previously assigned to the T3-J1 Hn Khoai or C3-P1 Bến Giằng- Quế Sơn complexes based on the K-Ar age data. In order to elucidate the emplacement time of the granitoids in the Cửu Long Basin and their geological implications, zircon separated from granitods collected in the Rồng, Bạch Hồ oilfield basement for U-Pb isotope analyses and yielded a mean 206Pb/238U age of 94.2 Ma and 115.5 Ma, respectively. Regression of 3 and 9 data points defined whole rock Rb-Sr isochrons that gave ages of 79.5+5.6 Ma with MSWD = 1.09 for the Rồng granite and 130+21 Ma for the Bạch Hổ granitoids with MSWD = 385, respectively. The age data from Bạch Hổ granitoids yielded an errisochron and due to mobility of Rb, therefore, the U-Pb zircon ages of 94.2 and 115.5 Ma, corresponding to the early Cretaceous, were considered as the best emplacement ages of the granitoids. These ages are similar to those of Đo Cả and Định Qun granitoids in the Đ Lạt zone and indicate magmatism took place during Cretaceous time. This enables US to extend the search areas for oil and gas along the coast of Vietnam where Early Cretaceous granitoids are present.

Người bin tập: TS. Phạm Trung Hiếu.