Ultramafic and associated rocks from southern New Caledonia

Abstract

The New Caledonian ultramafic belt is a peridotite-gabbro-granodiorite, alpine-type complex which crops out over the length of the archipelago (some 500 km) as a series of upstanding, perched massifs. The southern portion of the largest of these massifs consists predominantly of partially serpentinised harzburgite containing a dunite-chromitite-eucrite core. Relict cumulus textures, gradational contacts between major lithologic units, and mineral compositions indicate that this core is a gravity differentiate formed at ~1GNm-2, 1200-1400C and constant oxygen fugacity from a mafic magma derived by partial melting of primitive pyrolite, the refractory residue of which constitutes the marzburgites. Upwelling within the lithosphere of the combined harzburgite residue and cumulate mush, resulted in partial obliteration of igneous textures by recrystallisation and the development of tectonite fabrics and foliation. Emplacement onto the New Caledonian sialic segment was probably by overthrusting from the northeast as suggested by Avias (1967) and Coleman (1971) resulting in cataclasis, mylonitisation and metamorphic differentiation of the basal rock of the complex. This Qverthrust followed eruption of subalkaline, quartz bearing basalts derived from the original magmatic episode. Granodiorite/diorite stocks,. representing the final alkali­rich fractionate, intruded the peridotite cover along faults within the basement.