Laterization of the ultramafic-Gabbro Association at North Cape, Northernmost New Zealand

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Abstract
Laterites occur at North Cape, above an ultramafic-mafic plutonic association, the Kerr Pluton, composed predominantly of seperntinites by layered gabbros. Laterite profiles average 32 m over gabbros, but only 6 m over serpentinites. Iron pan, formed from a layer of near-surface residual boulders, is abundant over sepentinite-derived laterites. Pain d'épice facies-is more common over gabbros. Initial decomposition of serpentinites yields hisingerite which later crystallizes to smectite + chlorite. At the top of the profile limonite, goethite and minor smectite occur. Gabbros initially decompose to form kaolinite, halloysite, smectites and pyrophyllite which are susbsequently replaced by limonite-goethite, kaolinite and gibbsite. During laterite formation, elements are released from tlte serpentinites in the following general order: MgO, CaO, TiO2, SiO2, Fe2O2, Al2O3, NiO, MnO. Gahbros hehave similarly except that iron is always enriched relative to alumina in the soils. It is believed that under low pH conditions, alumina and other elements were dissolved from gabbroic laterites and redeposited in underlying serpentinite laterites where pH was higher. Differential weathering of compositionally layered parent gabbro results in the formation of layers of residual boulders in the profiles which, ultimately, lead to formation of cyclic profiles. Laterite formation was initiated under tropical conditions in the Miocene and is continuing today under sub-tropical conditions.