Bulletin of the Geological Society of Malaysia, Volume 33, Nov. 1993, pp. 211-222
1Department of Geological Sciences, University of Illinois at Chicago, P.O. Box 4348, Chicago, Illinois 60680, U.S.A
2Institute of Geology, National Center for Scientific Research, Hanoi, S.A. Vietnam
3Geological Survey Department No.6, Ho Chi Minh City, S.A. Vietnam
4Department of Geophysics, Texas A&M University, College Station, TX77843
Abstract: Cenozoic magmatism in southeast Asia, initiated at ca. 15 Ma and peaking between ca. 5 and 0.5 Ma, post-dates opening of the South China Sea Basin and is associated with lithospheric extension in Indochina, Thailand, and southern China. Geochemical data allow modelling of the relationship between melting and lithosphere extension in the region and documenting enrichment history of the continental lithosphere mantle. In Vietnam, geological relationships suggest that larger (tholeiite) melt fractions were generated within pull-apart basins along extensional N50°E- and N160°E-trending faults, while lower (alkali basalt and basanite) melt fractions were associated with conjugate strike-slip faults. Petrogenetic conditions interpolated from experimental data suggest quartz tholeiites were generated at ca. 1250°C and <10 kbar pressure (plagloclase/spinel lherzolite), olivine tholeiites at ca. 1300°C. 10-15 kbar (spinel lherzolite), and alkali basalts and basanite at ca. 1350°C, <3O kbar (spinel/gamet lherzolite), close to, or below, the thermal boundary layer of lithospheric mantle. Assuming uniform (pure shear) lithosphere extension, stretching factors needed to produce such melts at normal asthenospheric potential temperatures appear to exceed those believed to characterise recent extension in Indochina. This may suggest an elevated potential temperature resulting from (incipient) mantle plum activity.