NdSrPb isotopic, and major- and trace-element geochemistry of Cenozoic lavas from the Khorat Plateau, Thailand: sources and petrogenesis

Abstract

Basaltic rocks from Khorat Plateau, Thailand, dated at 0.9 Ma, coincide in time with the lithospheric extension of continental southeast Asia that began in the mid-Cenozoic. Dominated by alkali-olivine basalt and hawaiite compositions, they are generally alkalic and show specific petrologic and geochemical variations. Their trace-element and isotopic compositions are generally similar to those of ocean island basalts, and define two distinct groups. The group-I rocks have moderately depleted and relatively homogeneous isotopic ratios with 143 Nd 144 Nd = 0.51287−0.51296 87Sr 86Sr = 0.70354−0.70388 and Pb-isotopic ratios that are fairly nonradiogentic ( 206Pb 204Pb = 18.23−18.32 , 207Pb 204Pb = 15.47−15.53 and 208Pb 204Pb = 38.16−38.27 ). The group-II rocks show an enriched isotopic signature with 143Nd 144Nd = 0.51266−0.51281 , 87Sr 86Sr = 0.70486−0.70585 and more radiogenic Pb-isotopic ratios ( 206Pb 204Pb = 1849−1865 , 207Pb 204Pb = 15.54−15.60 and 208Pb 204Pb = 38.48−38.84 ). These isotopic data display linear trends on the 143Nd 144Nd and 206Pb 204Pb vs. 87Sr 86Sr diagrams, with group-I rocks clustering near the less depleted end of the field for Indian Ocean MORB and group-II rocks extending toward the EM2 (i.e. enriched mantle of type 2) end-member. Thus two source domains, one with a moderately depleted Indian Ocean MORB-like isotopic signature and the other with a EM2-like character, are thought to have been involved in the generation of these lavas, with the former originating from the asthenospheric mantle, and the latter likely from the lithospheric mantle. We suggest that the primary melts of group-I rocks formed by decompressional polybaric partial melting of asthenospheric materials similar in their isotopic compositions to the source of less depleted Indian Ocean MORB. This was followed by some fractional crystallization, chiefly of olivine, but with lottle contamination by the continental lithospheric materials en route to the surface. However, variations in the incompatible major- and trace-element concentrations of the group-I rocks are mainly due to the different pressures of melting (i.e. different depths) and different degrees of partial melting of source materials in the asthenosphere. The group-II rocks which show an enriched isotopic signature, on the other hand, are interpreted to be products of mixing between materials with highly radiogenic Sr and Pb, and nonradiogenic Nd, most likely aged frozen melts in the lithospheric mantle and young asthenospheric melts similar in their isotopic character to the moderately depleted Indian Ocean MORB. It is likely that this asthenospheric source is prevalent beneath continental southeast Asia.