Geochemistry of Neogene Basalts in the Nghia Dan district, western Nghe An

Nghia Dan Neogene basalts occur as monogenetic volcanoes and thin lava layers (up to tens of meters thick). They are alkaline basalts and basanites, some containing mantle xenoliths such as spinel lherzolite. Compared with Tay Nguyen (Western Highlands) Cenozoic basalts (for example, Pleiku and DacNong) the Nghia Dan basalts show much lower SiO 2 (45-48.5wt.%) and higher FeO*(up to 9-11 wt.%), TiO 2 (2.5-3 wt.%) and CaO (9-10 wt.%); they are very high in trace element contents especially Ba, Th, Nb (up to 130 ppm), Sr (up to 2000 ppm) and Eu (up to 4 ppm). Their rare earth concentrations are high, much higher as compared to those of Tay Nguyen. Melting parameter modeling shows the Nghia Dan melts generated from about 3 - 4% partial melting of a combined garnet- spinel- lherzolite source between a pressure range of 20 to 25 Kb (about 75 km deep). The parameters are consistent with the low SiO 2 and high trace element, including the rare earth, contents in the Nghia Dan basalts. High FeO*, TiO 2 , CaO and Sr may also be a result of interaction with mafic components in the lithospheric mantle by the mantle-derived melts on the way to the surface.