Crustal contamination of Late Neogene basalts in the Dien Bien Phu Basin,NW Vietnam: Some insights from petrological and geochronological studies |
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| Institution: | 1. Institute of Geology, China Earthquake Administration, Beijing 100029, China;2. Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibet Plateau Research, Chinese Academy of Sciences, Beijing 10085, China;3. Graduate University of Chinese Academy of Sciences, Beijing 100049, China;4. Department of Geology and Geography, Auburn University, Auburn, AL 36849, USA;1. Department of Earth Sciences, National Taiwan Normal University, Taipei 106, Taiwan;2. Institute of Earth Sciences, Academia Sinica, Taipei 115, Taiwan;3. South Vietnam Geological Mapping Division, Department of Geology and Minerals of Vietnam, Ho Chi Minh City, Vietnam;4. Department of Earth Sciences, National Cheng Kung University, Tainan 107, Taiwan;5. Department of Earth and Environment, Franklin and Marshall College, Lancaster, PA 17604-3003, USA;6. Department of Geosciences, National Taiwan University, Taipei 106, Taiwan |
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| Abstract: | Early Pliocene (Zanclean) basalts in the Dien Bien Phu pull-apart basin in NW Vietnam, associated with the presently sinistral Dien Bien Phu Fault Zone, have been dated by the K–Ar method at 4.4–4.9 and 5.4–5.2 Ma. Rapid migration of basaltic magma to the surface in the Dien Bien Phu Fault Zone may be due to Pliocene transtension of the crust in this region, resulting from asthenospheric upwelling induced by lateral displacement of the mantle. The basalts are moderately phyric ( < 10%) and consist of olivine (hyalosiderite), plagioclase (bytownite–labradorite) and orthopyroxene (bytownite–labradorite) phenocrysts, and a fine-grained crystalline matrix (olivine–hortonolite, plagioclase–labradorite, clinopyroxene–pigeonite and augite, K-feldspar). The presence of Fe-rich olivine and orthopyroxene phenocrysts indicates that the basalts are SiO2-saturated/oversaturated olivine tholeiites which formed under water-undersaturated conditions. The Dien Bien Phu basalts contain both mantle-derived (pyroxenites, dunites, gabbros) and crustal (sillimanite/mullite + Mg–Fe spinel), wallrock xenoliths, indicative of crustal contamination during the ascent of the basaltic magma. The basalts show selective enrichment in some mobile elements (K, Rb, Sr and Th), a feature considered to be a result of metasomatism. These rocks, classified on the basis of their normative composition as quartz tholeiites, could represent primary olivine tholeiites/basalts, in which the geochemical signatures were modified by the processes of contamination. |
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