Podiform chromitites in lherzolitic mantle rocks (Andaman ophiolite, India): the role of magma/rock interaction and parental melt composition

摘要

In this study, Alpine-type (ophiolite hosted) podiform chromitites from the lherzolite dominant mantle sequence of the Andaman ophiolite, India are described. The mantle host rocks for the chromitite pods show features of melt-mantle interaction. The chromites of the podiform chromitites have high-Cr# (= Cr/[Cr + Al] atomic ratio) ranging between 0.69 and 0.75 and high-Mg# (= Mg/[Mg + Fe~(2+)] atomic ratio) ranging between 0.58 and 0.72. Geochemically, the chromites of these high-Cr chromitites have an arc-spinel trend and suggest a supra-subduction zone setting. Contrarily, the low-Cr chromitites (reported elsewhere) show MORB-spinel trend. Parent melt composition is assessed on the basis of liquidus chromite compositions using various methods. The deduced composition shows that the melt parental to the high-Cr chromitite of the Andaman ophiolite had 12-13 wt.% Al_2O_3 and 0.2-0.5 wt.% TiO_2 whereas the same for the low-Cr chromitites had ~ 16 wt.% Al_2O_3 and 0.5-0.9 wt.% TiO_2. A correlation of the deduced parent melt compositions with the volcanic rocks of the Andaman ophiolite suggests a close affinity with the East Coast volcanics (south-Andaman) for the high-Cr chromitites. Contrarily, a similar correlation for the low-Cr chromitites is not so certain.