The character of the subcontinental mantle in southeast Asia: Evidence from isotopic and elemental compositions of extension-related cenozoic basalts in Thailand

In Thailand, S.E. Asia, the Central Valley and its flanks have been sites of basaltic volcanism due to extensional deformation and accompanying decompressional melting since at least 10 Ma. Major-element concentrations show that volcanic flows at Bo Phloi, Chanthaburi-Trat and Denchai, three of the several localities in Thailand situated on different tectonic blocks, are dominated by basanite, ne-hawaiite, hawaiite and mugearite rock compositions, generally sub-alkaline to alkaline. The trace element patterns of the rocks are similar to those of ocean island basalts (OIB), typically with enrichments in light rare earth elements (LREE), other large-ion lithophile elements, and high field strength elements, compared to N-MORB. ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd values are depleted relative to bulk silicate earth (BSE) and are more homogeneous compared to the contemporaneous basalts in eastern China and the South China Sea Basin. The samples define a flat trend in ¹⁴⁷Sm/¹⁴⁴Nd versus ¹⁴³Nd/¹⁴⁴Nd space, with the two parameters having been decoupled from each other, which suggests that crustal contamination was not an important process as magmas rose to the surface. Pb-isotopic compositions fall above the Northern Hemisphere Reference Line (NHRL) on the ²⁰⁷Pb/²⁰⁴Pb versus ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb versus ²⁰⁶P/²⁰⁴Pb diagrams, and require the source materials to have evolved with a time integrated record of high Th/U. While these Pb-isotopic ratios fall within the realm of the Dupal isotopic anomaly, their corresponding Sr-isotopic compositions do not. This has become a widely recognized feature of S.E. Asian basalts, and we conclude that it was generated in situ via local subduction and incubation of pelagic and hemipelagic sediments. The Pb data define trends that lie between the chemical geodynamic end member known as enriched mantle of type-II (EMII) and another similar to the Indian Ocean MORB-type source. Inasmuch as there are no significant differences in isotopic and trace element compositions for basaltic rocks extruded through the different Thai crustal blocks, we conclude that crustal contamination was minimal, and that the EMIT signature was introduced directly into the source by subduction of marine sediments, either during amalgamation of the crustal blocks in the early Mesozoic or during convergence along the eastern Eurasian margin in the mid Mesozoic.