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

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 parts per thousand and are not in equilibrium with the alkali basalts which have delta(18)O values between 5.0 and 5.7 parts per thousand. The U-Pb dating of zircons recovered from the basaltic placers provides evidence of two eruptional events: one at similar to 6.5 Ma followed by another one at similar to 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.