Chromitite and ultramafic rock compositional zoning through a paleotransform fault, Poum, New Caledonia

Universite Montpellier, Geofluides-Bassins-Eau, Montpellier, France

The major mylonitic shear zone of the northern part of the ophiolitic peridotite sheet of New Caledonia has been described as a paleotransform fault (Secher, 1981). Located along this structure, the Poum ultramafic massif shows a wide spectrum of peridotite compositions ranging from plagioclase lherzolite to dunite across a 2-km-thick transition zone. The mylonitized plagioclase lherzolite contains abundant pyroxenite and mafic veins. The Poum ultramafic massif includes more than 30 chromitite orebodies. Their composition vanes according to the nature of the host peridotite: Al rich in the plagioclase lherzolite zone (31-43% Al ₂ O ₃ and 25-36% Cr ₂ O ₃ ), Cr rich (39-60% Cr ₂ O ₃ ) in the spinel lherzolite and clinopyroxene harzburgite zones, and still richer in the harzburgite-dunite zone (49-64% Cr ₂ O ₃ ). In peridotites, the accessory Cr spinel also displays a Cr enrichment from lherzolite to dunite. Segregations of Cr-rich chromite (50-60% Cr ₂ O ₃ ) are present in orthopyroxenite veins displaying boninitic affinities. The Poum chromitites display low platinum-group element (PGE) contents and Pd/Ir ratios as do the chromitite segregations of the orthopyroxenite veins, suggesting a common parent magma or/and common fractionation processes. The compositional zoning of the Poum massif can be explained by the contrasting behavior of ultramafic rocks and upwelling mafic melts, away from the axis of the transform fault, using the current ideas about the genesis of boninitic melts and the formation of dunite and harzburgite by magma-rock interactions during melt percolation (Nicolas and Prinzhofer, 1983; Kelemen, 1990; Kelemen et al., 1992; Takazawa et al, 1992). In the lithospheric lherzolite zone that was cooled and mylonitized along the older wall of the transform fault, melts were channeled along fractures leaving Al-rich chromitite pods. By contrast, pervasive percolation along the lithosphere-asthenosphere boundary allowed important melt-rock interactions resulting in the progressive formation of harzburgite and dunite and generating Mg-Cr-rich melts from which precipitated Cr-rich chromitites.

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