Journal of GEOLOGY, Series B, No.47-48/2018.

Unit 8: Granite of the Bản Ngậm complex: SHRIMP zircon U-Pb isotope age and its geological implication

Bùi Thế Anh¹, Nguyễn Thị Bích Thủy², Nguyễn Thị Xuân¹,
Hồ Thị Thư², Trần Bá Duy¹, Đặng Văn Long³

¹Vietnam Institute of Geosciences and Mineral Resources; ²General Department of Geology and Minerals; ³University of Natural Sciences of Ho Chi Minh city

Corresponding author: theanhvgmr@gmail.com

Received:05/8/2018
Accepted: 20/10/2018

Abstract: Ban Ngam granite is distributed mainly in Northwest Vietnam. The sample in this study is taken from the granite massif at Ban Ngam waterfall, Ban Ngam stream, Lien Hoa commune, Moc Chau district, Son La province. This granite massif is covered with bottom pebbles of the Sinh Vinh Formation. Granites are medium to coarse grain, containing pink K-feldspar and exhibit gneiss texture. Formation age of the Ban Ngam complex is previously considered as Neoproterozoic-early Cambrian based on geological relationship. The Ban Ngam complex is the first time dated by the SHRIMP U-Pb isotope zircon in this study. Zircon grains separated from the Ban Ngam granite is brown, dark brown, prismatic and large in size, ranging from 120 to 250 µ with length/width ratio ranging from 1.2 : 1.0 to 2.5: 1.0. The internal structure of the zircons exhibit fine zoning in the central part, typical for the magmatic zircon. The rim of zircon is unzoned and has a sponge shape, this implies that the Ban Ngam granite underwent a metamorphic process after crystallization. A total of 10 analytical points on 9 zircon grains yielded ²⁰⁷Pb/²⁰⁶Pb age values of 1146-2294 Ma and showed lead loss. Regression of all data points gave upper and lower intercepts at 2325 and 346 Ma, respectively. In combination with internal structure of analyzed zircon grains, the value of 2325 Ma is thought to be the crystalline age of the granite. This age value indicates an existence of a magmatic activity during Paleoproterozoic time.