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

Le Duc Anh1,4*, Nguyen Hoang2, Renat B. Shakirov3, Tran Thi Huong2
1 Institute of Marine Geology and Geophysics, VAST, Hanoi, Vietnam
2 Institute of Geological Sciences, VAST, Hanoi, Vietnam
3 V.I. Il’ichev Pacific Oceanological Institute (POI), Far Eastern Branch of Russian Academy of Sciences,Vladivostok, Russia
4 Graduate University of Science and Technology, VAST, Hanoi, Vietnam
Received 21April2016.Accepted 19July2017

ABSTRACT
The chemical compositions of late Miocene - Pleistocene basalts in PhuQuy 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, overliesthe 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 Fo89-90 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 lose 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  emonstrated that the post-opening volcanism was unrelated to an important tectonic phase and that the calculated extension factor () for the regional major extension fault systems is not significant (ca. 1.3) to trigger mantle melting. Therefore, the infiltration of asthenospheric flows resulting from the Neo-Tethys closure following the collision between India and Eurasia in the late Tertiary, may not only raise the mantle temperature leading to the melting but also appear to be the major driving force of marginal sea opening in the western Pacific, including the East Vietnam Sea.
Keywords:Intraplate basalt volcanism, PhuQuy island, East Vietnam Sea, Vietnam late Cenozoic volcanism, mantle source.
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