GEOCHEMICAL CHARACTERISTICS OF OLIVINES FROM NORTHEASTERN PHU QUY VOLCANIC ISLAND AND THEIR RELATION TO MELT VARIATIONS IN THE MAGMA SOURCE

Le Duc Anh^1,3, Nguyen Hoang^2,3, Phung Van Phach ^1,3,
Malinovskii A,I⁴ , Dinh Quang Sang⁵, Shakirov R,B⁶

¹ Institute of Marine Geology and Geophysics, VAST, Hanoi, Vietnam

² Institute of Geological Sciences, VAST, Hanoi, Vietnam

³ Graduate University of Science and Technology

⁴ Federal State Budgetary Institution of Science Far East Geological Institute, Far Eastern Branch of Russian Academy of Sciences, Vladivostok, Russia

⁵ South Vietnam Geological Mapping Division, Ho Chi Minh, Viet Nam

⁶ V.I. Il’ichev Pacific Oceanological Institute (POI), Far Eastern Branch of Russian Academy of Sciences, Vladivostok, Russia

Corresponding author: leducanh010282@gmail.com

Abstract: Phu Quy volcanic island is formed following at least two major volcanic eruptions at ca. 2.64 Ma and 1.32 Ma. The early episode appeared as fissure eruptions producing a shield-like basement comprising of tholeiitic basalt to sub-alkaline (olivine basalt). The later episode occurred as monogenic volcanic, explosive eruption producing mainly sub-alkaline basalt with a minor amount of (mantle xenolith- bearing) alkaline basalt. Olivine phenocrysts and single crystals were separated from, respectively, basalt and volcanic tuff to analyze for chemical compositions with aim to understand the nature of mantle source region and melt generation and evolution. The chemical compositions of olivine revealed that Phu Quy magmatic melt was generated by mantle peridotite melting at a depth of about 90 km. Regardless of volcanic occurrence at two separate periods the continuity between high – low melting pressure- produced melts may reflect melt mixing via column melting. After generation, melt of the early eruption episode (2.64 Ma) moved up to a depth of about 35 km before erupting. The later melt moved directly to the surface from the segregation source. At a shallow depth (about 40-45 km) the melt partially mixed with melt of the early phase. This happened may be due to the intrusion of asthenospheric flows from beneath, the consequence of the Neo-Tethys ocean closure following the India-Eurasian collision, causing the mantle temperature to rise, making melting easier to form Phu Quy volcanic island.

Keywords: East Vietnam Sea, Phu Quy Island, olivine chemistry, mantle extrusion, column melting.

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