Journal of Petrology | Volume 39 | Number 3 | Pages 369-395 | 1998
© Oxford University Press 1998
Petrogenesis of Cenozoic Basalts from Vietnam: Implication for Origins of a ‘Diffuse Igneous Province’
Department of Geological Sciences, University of Illinois at Chicago (M/C 186) 845 W. Taylor Street, Chicago, IL 60607-7059, USA
Received July 8, 1996; Revised typescript accepted August 28, 1997
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Abstract |
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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 apparent 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-SiO2, low-FeO* quartz and olivine tholeiites—large melt fractions of refractory (lithosphere-like) mantle—and a later series made up of low-SiO2, 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 
1470°C (for alkali basalts) to <0.5 GPa and 1400°C (quartz tholeiites), and for H2O-undersaturated conditions, from <3.5 GPa and 1450°C to 1.5 GPa and 1350–1400°C, respectively. Hydrous conditions are more realistic in view of high measured basalt H2O+ contents, pressure estimates consistent with melting below a thinned mechanical boundary layer (MBL) and interpolated mantle adiabats of 2–3°C/km (compared with <1°C/km for anhydrous conditions), consistent with fluid dynamic constraints and a 1440°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.
KEY WORDS: Vietnam; basalt; Cenozoic; geochemistry
Corresponding author. Extended data set can be found at: http://www.oup.co.uk/jnls/list/petroj
* Present address: University of Tokyo, Ocean Research Institute, 1-15-1 Minamidai, Nakano, Tokyo 164, Japan.
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