A Comparison of the Geology of the Papuan and New Caledonian Ultramafic Belts

Abstract

The Papuan and New Caledonian Ultramafic Belts lie 1,770 km apart on the southern branch of Glassener's inner Melanesian arc; a site of a former Mesozoic geosyncline. Both belts show strong affinities with the alpine-type ultramafic association. In both instances a dominant, noncumulate harzburgite forms a substrate on which cumulates of peridotite and gabbros appear to have formed. Overthrusting from the northeast of the complexes onto the sialic crust of the islands was preceded by eruption of voluminous tholeiitic basalts in the Cretaceous or Eocene. Intrusion of felsic stocks into both basement and overthrust sheets succeeded emplacement. The belts differ in their tectonic relationship to Tertiary blueschist metainorphism and to the tholeiitic basalts; in the greater variety and abundance of cumulate rocks in Papua; in the widespread occurrence of chromitites in New Caledonia; and in the relative abundance of clino-pyroxenites in the non-cumulate substrate. An upper mantle origin is indicated for the non-cumulates which perhaps represent depleted pyrolite following removal of the basaltic fraction.