TWO ORIGINS OF DUNITES IN THE NEW CALEDONIA OPHIOLITES: MELT-ROCK REACTION VERSUS FRACTIONAL CRYSTALLIZATION

Abstract

Ophiolitic dunites offer critical insights into mantle and crustal processes within oceanic lithosphere. This study presents an integrated petrological and geochemical investigation of dunites from the New Caledonia ophiolite (Peridotite Nappe), a forearc lithospheric remnant. Two distinct dunite types with contrasting origins are identified. Type I dunites are crustal cumulates formed by fractional crystallization of primitive magmas. They occur at the base of the crustal ultramafic sequence with sharp contacts against pyroxenite layers, display a medium- to coarse-grained granular texture characterized by olivine grains forming triple junctions (~120°), and contain olivine with low Fo and spinel with low Cr#. Their mineral compositions follow fractional crystallization trends. Type II dunites are mantle-derived replacive bodies formed by melt-rock reaction between silica-undersaturated melts and harzburgite. They occur as veins
or lenses within harzburgites with irregular contacts, exhibit granular to porphyroclastic textures with spinel forming “holly-leaf” or vermicular shapes commonly intergrown with orthopyroxene, and are characterized by extremely high olivine Fo and spinel Cr#. Geochemical data indicate that the two types likely formed during separate magmatic events. Furthermore, postcumulus infiltration of boninite-like melts into Type I dunites is evidenced by clinopyroxene with higher Mg# than coexisting olivine. The coexistence of these two dunite types underscores multiple magmatic processes in the formation of the New Caledonia ophiolite, involving both melt migration and crystal accumulation from depleted mantle-derived magmas.