Abstract

Evidence for variable amounts of melt-induced metasomatism in upper-mantle peridotites has been observed in the ultramafic section of the Zambales Range ophiolite, Luzon, Republic of the Philippines. These peridotites are locally cut by mafic dikes that in some places visibly ooze into the host peridotite. The mafic silicate magma invaded the peridotite along grain boundaries and crystallized as poikilitic plagioclase and clinopyroxene.

Compositional data indicate that the mafic material reacted with the peridotite. Mineral compositions in the mafic dikes are distinct from those in the surrounding peridotite. The compositions of residual olivine and orthopyroxene in the peridotite change with the amount of mafic magma within the peridotite. Fe-Mg exchange predominates; the residual olivine and orthopyroxene in the peridotite are enriched in Fe. Large relative enrichments in other magmaphilic elements (Na, Ti, AT) are also evident. Mafic liquid in transit through the upper mantle may act as a metasomatic agent, locally enriching the peridotite through which it passes and producing small-scale compositional differences in rocks that otherwise have remarkably uniform compositions.

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