Petrogenesis of Cenozoic basalts from Vietnam: Implication for origins of a 'Diffuse igneous province'

Basalt magmatism occurred throughout east and southeast Asia after the early Tertiary, India-Asia collision. This activity does not conform to the 'Large Igneous Province' model in view of lower eruption and melt production rates: wide dispersal of centres and the appal ent absence of deep mantle upwelling. Age data for Vietnamese plateau basalts reflect spatial-temporal patterns consistent with a rotating stress field rather than supra-hotspot lithosphere migration. For most of the volcanic centres there are two eruptive episodes. an early series formed by high SiO(2), low-FeO* quartz and olivine tholeiites-large melt fractions of refractory (lithosphere-like) mantle-and a later series made up of low-SiO(2) high-FeO* olivine tholeiites, alkali basalts and basanites-smaller melt fractions of more fertile (asthenosphere-like) mantle. Comparison of Mg-15 normalized basalt compositions with parameterized anhydrous and hydrous experimental melt compositions allowed calculation of melt segregation pressures and temperatures. Computed for anhydrous conditions these range from <4 GPa and similar to 1470 degrees C (for alkali basalts) to <0.5 GPa and similar to 1400 degrees C (quartz tholeiites), and for H(2)O-undersaturated conditions, from <3.5 GPa and similar to 1450 degrees C to similar to 1.5 GPa and 1350-1400 degrees C, respectively. Hydrous conditions are more realistic in view of high measured basalt H(2)O(+) contents, pressure estimates consistent with melting below a thinned mechanical boundary layer (MBL) and interpolated mantle adiabats of 2-3 degrees C/km (compared with <1 degrees C/km for anhydrous conditions), consistent with fluid dynamic constraints and a 1440 degrees C potential temperature. After collision-induced 'extrusion' of east and southeast Asia, the lithosphere was probably thinned during heating and transtension; this converted refractory MBL into a low-viscosity thermal boundary layer (TBL), and caused upward penetration and polybaric melting of TBL-asthenosphere columns.