Pleistocene basaltic volcanism in the Krông Nô area and vicinity, Dac Nong Province (Vietnam)

Abstract Basaltic and related- pyroclastic products aged 0.67, 0.46 and 0.2 Ma produced by eruption of Chư B'luk (Buon Choah), Phu Son (Pass 52, Quang Phu) volcanoes in the Krong No district, and Ea T’ling and Nam Dong volcanoes in the Cu Jut district covers an area of 120 km 2 with a thickness of a few meters to about 80 m. The volcanoes are aligned along a sub-meridian oriented extensional fault, from Phu Son in the south via Chư B'luk, Ea T’ling in the middle to Nam Dong in the north. The basalts are divided into geochemically alkaline and tholeiitic groups. Ne-normative alkaline basalts include samples from the upper layer of Phu Son volcano, those in the Trinh Nu waterfall area (Ea T’ling volcano) and cored samples conducted near the Dray Sap waterfall. Q-normative tholeiitic basalts include all the samples from Chư B'luk and Nam Dong volcanoes, and massive basalts in the lower layer of Phu Son volcano. Geochemically the tholeiitic basalts are vastly different from the alkaline basalts in that they have higher SiO 2 (50–55 wt% compared to 2 O 3 (>14–16 wt% compared to 2 O ( 1.5–2.5 wt%) and lower TiO 2 ( 2 wt% in alkaline basalt). Melting temperature and pressure calculations based on computing primitive melt compositions showed that the tholeiitic basalts may be produced in a narrow temperature range of 1365–1385 °C, corresponding to pressure of 1.2–1.6 GPa. The alkaline basalts may be produced in a temperature range between 1400 and 1440 °C and pressure varying from 3.1 to 3.35 GPa. Both the tholeiitic and alkaline basalts are free of crustal contamination. They show geochemically similar characteristics to those of continental intraplate basalts although the alkaline basalt show much more enrichment in incompatible elements. The 87 Sr/ 86 Sr and 143 Nd/ 144 Nd ratios of the two basaltic groups are quite similar, varying within narrow ranges of, respectively, 0.7038–0.7041, 0.5129–0.51286 but their 206 Pb/ 204 Pb isotopic ratios vary widely relative to 207 Pb/ 204 Pb and 208 Pb/ 204 Pb suggesting their interaction with different isotopic components and/or isotopic evolution history. Isotopic modeling relative to major mantle isotopic components suggests that the alkaline basalts may be generated by interaction between an N-MORB-like and an enriched mantle type 1 (EM1) at deep levels. The isotopic component is usually known as DUPAL-like regional mantle isotopic anomaly believed to be widespread in SE Asian mantle. The isotopic compositions of tholeiitic basalt may be generated by mixing between the DUPAL-like component with an enriched mantle type 2 (EM2)-like at higher levels in the lithospheric mantle.