Abstract

The coexistence of both high-MgO, low-alumina basalt (LAB) and low-MgO, high- alumina basalt (HAB) magmatic inclusions within a single andesitic host lava on Kanaga Island provides the opportunity to rigorously evaluate the presence (or absence) of a fractionation relation between the two basalt-magma types. Traditional mass-balance calculations, Pearce ratio arguments, comparative phenocryst modes and compositions, and incompatible element (Rb, Ba, Zr) abundances all indicate that selective clinopyroxene and orthopyroxene fractionation from the plagioclase-clinopyroxene-orthopyroxene-titanomagnetite-saturated LAB magma yielded the plagioclase-phyric HAB magma. The same data are consistent with a model of low-pressure HAB generation calling upon selective mafic crystal fractionation along the margins of a crystallizing and convecting LAB magma chamber. The ability to demonstrate that nonliquid HAB magma can be produced from LAB through crystal fractionation eliminates many of the arguments used in support of a primary eclogite melting origin for HAB.

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