Geochemistry of late miocene-pleistocene basalts in the Phu Quy island area (East Vietnam Sea): Implication for mantle source feature and melt generation

Abstract

The chemical compositions of late Miocene - Pleistocene basalts in Phu Quy island defines two major geochemical groups that reflect the formation and development of the island. The early low alkaline, TiO₂ and P₂O₅, and high SiO₂ group, comprising olivine and tholeiitic basalts, forms the base of the island. The later high alkaline, TiO₂, and P₂O₅, and low SiO₂ group, produced by central-type volcanic eruptions consisting of alkaline olivine and olivine basalts, overlies the early eruptive group. Crustal contamination may be expressed by the positive correlation between Ba/Nb and SiO₂, which are higher in early eruptive basalts, possibly reflecting the involvement of crustal material, either in the source region or interaction of the melt on the way to the surface. However, negative relationship between Ba and SiO₂, and positive correlation between Nb/Y and Zr/Y observed for two basaltic series may reflect the effect of melting pressures and degrees of partial melting. Methods of calculating the primitive basaltic melts based on the principle of olivine incremental additions to the basalt until the composition is equilibrated with the residual olivine at Fo_89-90 may be used. The computed results show that the early basalts were generated under pressures of about 18-20 Kb (ca. 55-60 km) and the later basalts were formed in the pressure range of 20 to 25 Kb (corresponding to the depths about 60 to 75 km). The close range of melting pressures suggests decompression polybaric melting of a mantle source, which allows for mixing of various melt portions, resulting in the formation of geochemically linear relationship. It has been demonstrated that the post-opening volcanism was unrelated to an important tectonic phase and that the calculated extension factor (b) for the regional major extension fault systems is not significant (ca. 1.3) to trigger mantle melting. 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