MAJOR, TRACE-ELEMENT, AND ISOTOPIC COMPOSITIONS OF VIETNAMESE BASALTS- INTERACTION OF HYDROUS EM1-RICH ASTHENOSPHERE WITH THINNED EURASIANLITHOSPHERE

Intraplate magmatism affected much of Indochina following the mid-Mioc ene cessation of South China Sea opening. Thick basalt plateaus formed on accreted terrains of varying age as extensional fractures were rea ctivated following the Indo-Eurasian collision. The basalts are part o f a diffuse igneous province affecting much of eastern and southeaster n Asia and western Pacific marginal basins. Most Indochina basalt cent ers comprise two eruptive episodes, an early (lower) series of high-Si O2, low-FeO quartz and olivine tholeiites, tapping a relatively-refra ctory, Lithospheric mantle-type source, and a later (upper) series of low-SiO2 high-FeO olivine tholeiites, alkali basalts, and basanites, tapping a fertile, asthenospheric source. This pattern is observed els ewhere in the region (e.g., Hainan Island) and resembles several conti nental flood basalt provinces. While some crustal contamination is sug gested, incompatible trace element and strontium, neodymium, and lead isotopic compositions reflect secular changes from the inferred lithos pheric to asthenospheric reservoirs. Lower Series basalts reflect hybr ids of Pb-206/Pb-204-rich EM2 and N-MORB reservoirs, with high K2O/P2O 5 and low Rb/Sr and Ba/Nb ratios, consistent with the involvement of l ithospheric mantle. In contrast, Upper Series basalts show lower K2O/P 2O5 and higher Rb/Sr and Ba/Nb ratios and reflect hybrids of (206)pb/P b-204-poor EM1 and N-MORE sources. These resemble anomalous (A)-MORB c ompositions that are typical of eastern/south-eastern Asian and wester n Pacific marginal basin asthenosphere. Despite its resemblance to Ind ian Ocean (I-) MORE, A-MORE ''plum-pudding'' asthenosphere may be expl ained in terms of an endogenous Asian model whereby EM1-rich subcraton ic lithosphere was entrained by asthenosphere extruded by the Indo-Eur asian collision. This model is consistent with the restriction of diff use regional magmatism to the late Cenozoic (i.e., following tectonic extrusion), evidence for shallow, thermally-anomalous mantle, and abse nce of A-MORE signatures from the pre-extrusion continental mantle.