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Basalt petrology, zircon ages and sapphire genesis from Dak Nong, southern Vietnam

Published online by Cambridge University Press:  05 July 2018

V. Garnier*
Affiliation:
CRPG/CNRS, UPR 2300, BP 20, 15 rue Notre-Dame des Pauvres, 54501 Vandoœuvre-lès-Nancy Cedex, France
D. Ohnenstetter
Affiliation:
CRPG/CNRS, UPR 2300, BP 20, 15 rue Notre-Dame des Pauvres, 54501 Vandoœuvre-lès-Nancy Cedex, France
G. Giuliani
Affiliation:
CRPG/CNRS, UPR 2300, BP 20, 15 rue Notre-Dame des Pauvres, 54501 Vandoœuvre-lès-Nancy Cedex, France IRD, UR154, LMTG, 14 avenue Edouard Belin, 34100 Toulouse, France
A. E. Fallick
Affiliation:
Scottish Universities Environmental Research Centre, Scottish Enterprise Technology Park, Rankine Avenue, East Kilbride G75 0QF, Scotland
T. Phan Trong
Affiliation:
Institute of Geological Sciences, CNST, Nghia Dô, Câu Giây, Hanoi, Vietnam
V. Hoàng Quang
Affiliation:
Institute of Geological Sciences, CNST, Nghia Dô, Câu Giây, Hanoi, Vietnam
L. Pham Van
Affiliation:
Vietnam National Gem and Gold Corporation, 91 Dinh Tien Hoang Street, Hanoi, Vietnam
D. Schwarz
Affiliation:
Gübelin Gemmological Laboratory, 102 Maihofstrasse, CH-6000 Lucerne 9, Switzerland
*

Abstract

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% and are not in equilibrium with the alkali basalts which have δ18O values between 5.0 and 5.7%. 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.

Type
Research Article
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2005

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Footnotes

Present address: INRS-ETE, 490 rue de la Couronne, Québec, Canada G1K 9A9

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