Models of corundum origin from alkali basaltic terrains: a reappraisal

Corundums from basalt fields, particularly in Australia and Asia, include a dominant blue-green-yellow zoned "magmatic" suite (BGY suite) and subsidia ry vari-coloured "metamorphic" suites. The BGY corundums have distinctive t race element contents (up to 0.04 wt% Ga2O3 and low Cr/Ga and Ti/Ga ratios < 1). Different melt origins for BGY corundums are considered here from the ir inclusion and intergrowth mineralogy, petrologic associations and tecton ic setting. Analysed primary inclusion minerals (over 100 inclusions) cover typical feldspars, zircon and Nb-Ta oxides and also include hercynite-magn etite, gahnospinel, rutile-ilmenite solid solution, calcic plagioclase, Ni- rich pyrrhotite, thorite and low-Si and Fe-rich glassy inclusions. This wid ens a previous inclusion survey; New England, East Australia corundums cont ain the most diverse inclusion suite known from basalt fields (20 phases). Zircon inclusion. intergrowth and megacryst rare earth element data show si milar patterns, except for Eu which shows variable depletion. Temperature e stimates from magnetite exsolution, feldspar compositions and fluid inclusi on homogenization suggest that some corundums crystallized between 685-900 degrees C, Overlap of inclusion Nb, Ta oxide compositions with new comparat ive data from niobium-yttrium-fluorine enriched granitic pegmatites favour a silicate melt origin for the corundums. The feasibility of crystallizing corundum from low-volume initial melting of amphibole-bearing mantle assemb lages was tested using the MELTS program on amphibole-pyroxenite xenolith c hemistry from basalts. Corundum appears in the calculations at 720-880 degr ees C and 0.7-1.1 GPa with residual feldspathic assemblages that match mine ral compositions found in corundums and their related xenoliths. A model th at generates melts from amphibole-bearing lithospheric mantle during magmat ic plume activity is proposed for BGY corundum formation.