Abstract
A late Oligocene - early Miocene alkali lamprophyre dyke swarm was emplaced in western South Island, New Zealand, during transtensional development of the Alpine Fault plate boundary. Intrusive rocks comprise ultramafic and feldspathic lamprophyres, tinguaites, trachytes and carbonatites, the evolved members of which cause intense local fenitization of the country rock, producing aegirine-riebeckite schists. Carbonatites show a wide range of compositions from calcite- through dolomite-ankerite- to siderite- and norsethite-rich varieties. Carbonates and phosphocarbonates of Sr, Ca-Sr, Ca-Ba and REE occur in accessory amounts. Ratios of Sr, Nd, and Pb isotopes and nodule petrology indicate magma derivation from a previously metasomatized source in a part of the mantle dominated by the FOZO component. Very restricted ranges in radiogenic and stable isotope values indicate that all members of the swarm are cogenetic. Efficient fractional crystallization of a camptonite-producing magma yields a phonolitic or tinguaitic residuum that then separates immiscibly to form carbonatitic magma. Not all carbonatites are derived from a single lineage, so immiscibility may operate repeatedly during late fractionation. The final stages of carbonatite evolution (< 400 degrees C) are dominated by carbothermal processes, during which the carbonatites and associated silicate rocks develop a LREE-depleted, MREE-enriched pattern.