Models of corundum origin from alkali basaltic terrains: a reappraisal
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Corundums from basalt fields, particularly in Australia and Asia, include a dominant blue-green-yellow zoned “magmatic” suite (BGY suite) and subsidiary vari-coloured “metamorphic” suites. The BGY corundums have distinctive trace 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 their inclusion and intergrowth mineralogy, petrologic associations and tectonic setting. Analysed primary inclusion minerals (over 100 inclusions) cover typical feldspars, zircon and Nb-Ta oxides and also include hercynite-magnetite, gahnospinel, rutile-ilmenite solid solution, calcic plagioclase, Ni-rich pyrrhotite, thorite and low-Si and Fe-rich glassy inclusions. This widens a previous inclusion survey; New England, East Australia corundums contain the most diverse inclusion suite known from basalt fields (20 phases). Zircon inclusion, intergrowth and megacryst rare earth element data show similar patterns, except for Eu which shows variable depletion. Temperature estimates from magnetite exsolution, feldspar compositions and fluid inclusion homogenization suggest that some corundums crystallized between 685–900 °C. Overlap of inclusion Nb, Ta oxide compositions with new comparative 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 assemblages was tested using the MELTS program on amphibole-pyroxenite xenolith chemistry from basalts. Corundum appears in the calculations at 720–880 °C and 0.7–1.1 GPa with residual feldspathic assemblages that match mineral compositions found in corundums and their related xenoliths. A model that generates melts from amphibole-bearing lithospheric mantle during magmatic plume activity is proposed for BGY corundum formation.
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