ND-SR-PB ISOTOPIC, AND MAJOR-ELEMENT AND TRACE-ELEMENT GEOCHEMISTRY OF CENOZOIC LAVAS FROM THE KHORAT PLATEAU, THAILAND - SOURCES AND PETROGENESIS

Basaltic rocks from Khorat Plateau, Thailand, dated at 0.9 Ma, coincid e in time with the lithospheric extension of continental southeast Asi a that began in the mid-Cenozoic. Dominated by alkali-olivine basalt a nd hawaiite compositions, they are generally alkalic and show specific petrologic and geochemical variations. Their trace-element and isotop ic compositions are generally similar to those of ocean island basalts , and define two distinct groups. The group-I rocks have moderately de pleted and relatively homogeneous isotopic ratios with Nd-143/Nd-144 = 0.51287-0.51296, Sr-87/Sr-86 = 0.70354-0.70388 and Pb-isotopic ratios that are fairly nonradiogentic (Pb-206/Pb-204 = 18.23-18.32, Pb-207/P b-204 = 15.47-15.53 and Pb-208/Pb-204 = 38.16-38.27). The group-II roc ks show an enriched isotopic signature with Nd-143/Nd-144 = 0.51266-0. 51281, Sr-87/ Sr-86 = 0.70486-0.70585 and more radiogenic Pb-isotopic ratios (Pb-206/Pb-204 = 18.49-18.65, Pb-207/Pb-204 = 15.54-15.60 and P b-208/Pb-204 = 38.48-38.84). The isotopic data display linear trends o n the Nd-143/Nd-144 and Pb-206/Pb-204 vs. Sr-87/Sr-86 diagrams, with g roup-I rocks clustering near the less depleted end of the field for In dian Ocean MORE and group-II rocks extending toward the EM2 (i.e. enri ched mantle of type ?) end-member Thus two source domains, one with a moderately depleted Indian Ocean MORE-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 as thenospheric mantle, and the latter likely from the lithospheric mantl e. We suggest that the primary melts of group-I rocks formed by decomp ressional polybaric partial melting of asthenospheric materials simila r in their isotopic compositions to the source of less depleted Indian Ocean MORE. This was followed by some fractional crystallization, chi efly of olivine, but with little contamination by the continental lith ospheric materials en route to the surface. However, variations in the incompatible major- and trace-element concentrations of the group-I r ocks are mainly due to the different pressures of melting (i.e. differ ent depths) and different degrees of partial melting of source materia ls in the asthenosphere. The group-II rocks which show an enriched iso topic signature, on the other hand, are interpreted to be products of mixing between materials with highly radiogenic Sr and Pb, and nonradi ogenic Nd, most likely aged frozen melts in the lithospheric mantle an d young asthenospheric melts similar in their isotopic character to th e moderately depleted Indian Ocean MORE. It is likely that this asthen ospheric source is prevalent beneath continental southeast Asia.