Trace element geochemistry and U-Pb dating of zircon inclusions in sapphire from Southern Vietnam: Indicator of basalt-related sapphire formation

Trace element geochemistry of zircon inclusion in sapphire from Southern Vietnam shows a large variation in Hf (1.1 to 2.7 wt%), Y (779 - 4220 ppm), Th (329 - 5199 ppm), U (224 - 3587 ppm), and n-ary sumation REE (706 - 2710 ppm) abundances with clearly positive Ce and mildly negative Eu anomalies observed in chondrite-normalized REE patterns. These characteristics suggest that the majority of the host sapphires were crystallized from evolved felsic melt at the lower crustal levels. The U-Pb zircon dating yields the ages ranging between 35.5 +/- 1.6 Ma and 14.7 +/- 0.29 Ma for Krong Nang sapphire, 5.9 +/- 0.13 Ma for Binh Thuan sapphire, 5.6 +/- 0.07 Ma for Dak Nong sapphire, and 5.5 +/- 0.08 Ma for Di Linh sapphire. Except the Krong Nang sapphire, these ages are coeval with the alkali basalt eruptions in Southern Vietnam. This implies the crystallization of sapphire during early stages of the basaltic magmatism. Sapphire and associated minerals were extensively transported from different depths to the surface during the later stages of basalt magmatism (<= 5 Ma).