Zircon U–Pb ages of the pegmatites in the Kontum massif, central Vietnam and their quality evaluation for ceramic industry
Truong Chi CUONG, Pham Trung HIEU, Pham MINH, Kenta KAWAGUCHI, Nong Thi Quynh ANH, Khuong The HUNG
Pegmatites occur in the Kontum massif as NE–SW oriented large dykes ranging from a few meters to several tens of meters in width. They consist of 3 stages of formation based on the difference of petrographic characteristics and ages: the first–stage pegmatites are characterized by quartz (40–45 vol%) and K–feldspar (55–60 vol%), the second–stage pegmatites are composed of K–feldspar (70 vol%), quartz (25 vol%), muscovite (<5 vol%), and biotite (<5 vol%), and the third–stage pegmatites are characterized by K–feldspar (60 vol%) and quartz (25 vol%). K–feldspar has common grain sizes of 0.5–1.0 cm with milky white color and shows perthitic texture. The emplacement ages of pegmatites determined by the zircon LA–ICP–MS U–Pb method yielded weighted mean 206Pb/238U ages of 448.2 ± 6.4 Ma (the first–stage), 269.4 ± 3.2 Ma (the second–stage), and 239 ± 3 Ma (the third–stage). The first–stage pegmatites coincide with the Ordovician–Silurian magmatism probably associated with the closure of an ancient oceanic basin within the Kontum massif. The second– and third–stage pegmatites can be linked with Permian–Triassic collisional magmatic activities. Host granitoids were also collected for petrographic investigations and age dating. Biotite–granite (Ben Giang–Que Son Complex) hosting the first– and second–stage pegmatites showed a magmatic age of 236.9 ± 2.9 Ma. Granites of the Van Canh Complex hosting the third–stage pegmatites yielded a similar age range between 244.2 ± 3.2 and 241.6 ± 1.7 Ma indicating that the third–stage pegmatite intruded shortly after the formation of the host granite. Based on quality assessments, samples from the second–stage pegmatites (~ 269 Ma) in the study area have the most applicable quality for ceramic manufacture and the technological samples fulfill Vietnam’s ceramic standards (VN 6598:2000 standards).