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Mineralogical Magazine; February 2005; v. 69; no. 1; p. 21-38; DOI: 10.1180/0026461056910233
© 2005 Mineralogical Society of Great Britain and Ireland
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Basalt petrology, zircon ages and sapphire genesis from Dak Nong, southern Vietnam

V. Garnier1, D. Ohnenstetter1, G. Giuliani2,1, A. E. Fallick3, T. Phan Trong4, V. Hoàng Quang4, L. Pham Van5 and D. Schwarz6

1 CRPG/CNRS, UPR 2300, BP 20, 15 rue Notre-Dame des Pauvres, 54501 Vandooevre-lès-Nancy Cedex, France
2 IRD, UR154, LMTG, 14 avenue Edouard Belin, 34100 Toulouse, France
3 Scottish Universities Environmental Research Centre, Scottish Enterprise Technology Park, Rankine Avenue, East Kilbride G75 0QF, Scotland
4 Institute of Geological Sciences, CNST, Nghia Dô, Câu Giây, Hanoi, Vietnam
5 Vietnam National Gem and Gold Corporation, 91 Dinh Tien Hoang Street, Hanoi, Vietnam
6 Gübelin Gemmological Laboratory, 102 Maihofstrasse, CH-6000 Lucerne 9, Switzerland

* E-mail: virginie_garnier{at}inrs-ete.uquebec.ca

Basalts associated with sapphire deposits are widespread in Europe, Africa, Asia and Australia. In Vietnam, blue, green and yellow sapphires are recovered from eluvial and alluvial placers hosted in basaltic fields of the Dak Lak Province. Two distinct basalt suites are recognized in this field: a tholeiitic suite without any xenocrysts and an alkaline suite with mantle and lower crustal xenocrysts. The sapphires are enriched in Fe (0.43 to 1.26 wt.%), with moderate contents of Cr (33–1582 ppm), Ti (35–1080 ppm), Ga (149–308 ppm) and V (28–438 ppm) and they are poor in Zn and Mg. Their O-isotope compositions range from 6.0 to 6.9{per thousand} and are not in equilibrium with the alkali basalts which have {delta}18O values between 5.0 and 5.7{per thousand}. The U-Pb dating of zircons recovered from the basaltic placers provides evidence of two eruptional events: one at ~6.5 Ma followed by another one at ~1 Ma. The petrography of the basalts and the oxygen isotopic composition of the sapphires suggest that the sapphires are xenocrysts and that they crystallized in a deep magma chamber, at the lower continental lithosphere and the upper mantle boundary, in evolved melts issued from the fractionation of alkali basaltic magmas contaminated with lower crustal fluids.

KEYWORDS: sapphire, basalt, petrography, geochemistry, zircon, U-Pb dating by SIMS, oxygen isotopes, Vietnam




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