Major, trace element, and isotopic compositions of Vietnamese basalts: Interaction of hydrous EM1-rich asthenosphere with thinned Eurasian lithosphere

Abstract

Intraplate magmatism affected much of Indochina following the mid-Miocene cessation of South China Sea opening. Thick basalt plateaus formed on accreted terrains of varying age as extensional fractures were reactivated following the Indo-Eurasian collision. The basalts are part of a diffuse igneous province affecting much of eastern and southeastern Asia and western Pacific marginal basins. Most Indochina basalt centers comprise two eruptive episodes, an early (lower) series of high-Si0 2, low-FeO * quartz and olivine tholeiites, tapping a relatively-refractory, lithospheric mantle-type source, and a later (upper) series of low-SiO 2, high-FeO * olivine tholeiites, alkali basalts, and basanites, tapping a fertile, asthenospheric source. This pattern is observed elsewhere in the region (e.g., Hainan Island) and resembles several continental flood basalt provinces. While some crustal contamination is suggested, incompatible trace element and strontium, neodymium, and lead isotopic compositions reflect secular changes from the inferred lithospheric to asthenospheric reservoirs. Lower Series basalts reflect hybrids of 206Pb/ 204Pb-rich EM2 and N-MORB reservoirs, with high K 2O/P 2O 5 and low Rb/Sr and Ba/Nb ratios, consistent with the involvement of lithospheric mantle. In contrast, Upper Series basalts show lower K 2O/P 2O 5 and higher Rb/Sr and Ba/Nb ratios and reflect hybrids of 206Pb/ 204Pb-poor EM1 and N-MORB sources. These resemble anomalous (A) -MORB compositions that are typical of eastern/southeastern Asian and western Pacific marginal basin asthenosphere. Despite its resemblance to Indian Ocean (I-) MORB, A-MORB “plum-pudding” asthenosphere may be explained in terms of an endogenous Asian model whereby EM1-rich subcratonic lithosphere was entrained by asthenosphere extruded by the Indo-Eurasian collision. This model is consistent with the restriction of diffuse regional magmatism to the late Cenozoic (i.e., following tectonic extrusion), evidence for shallow, thermally-anomalous mantle, and absence of A-MORB signatures from the pre-extrusion continental mantle.