Carbonatites and Fenitization Associated with a Lamprophyric Dike-Swarm Intrusive into Schists of the New Zealand Geosyncline

Abstract

At Haast River, South Westland, a lamprophyric dike swarm with carbonatitic affinities intrudes schists of the New Zealand Geosyncline. The dike swarm comprises a series of consanguineous members ranging in composition from ultramafic hornblende-mica peridotite, through camptonitic lamprophyre and mafic sodalite-nepheline microsyenite, to sodalite-tinguaite, potassic trachyte, and carbonatite. One carbonatite dike, 1.25 m thick, consists predominantly of the Ba-Mg carbonate, norsethite (Ba 0.96 Mg 0.79 Fe 0.14 Sr 0.02 Mn 0.03 Ca 0.01 (CO 3 ) 2.02 ) with subordinate magnesian siderite, aegirine, albite, pyrite, sphalerite, galena, and monazite. Other carbonatites contain siderite, norsethite-ankerite, and calcite-dolomite-barite assemblages. Metasomatic alteration of the country-rock schist, likened to fenitization, is most pronounced at the margins of alkalic and carbonatite dikes. It involves the introduction of Na 2 O, CO 2 , S, Nb, and Th into the country rock, with possible depletion in K 2 O. A progressive fenitization sequence from quartzofeldspathic schist to aegirine-crossite-albite schist is petrographically described and illustrated. An analysisis presented of crossite, typical of the strongly pleochroic blue amphibole produced during fenitization. The Haast River dike swarm intruded a geosynclinal terrain in immediately postorogenic times, postdating widespread metamorphism and intensive deformation of the Rangitata orogeny by at most about 10 m.y. Similarities between the Haast River occurrence and other carbonatite complexes from geosynclinal areas are briefly discussed.