Fenitization of metabasic rocks by ferrocarbonatites at Haast River, New Zealand

• Characteristics of fenitization of metabasite by ferrocarbonatite are described.
• Fenites are enriched in Na, C, Sr, Ba, Nb, Th, and Zn and depleted in Fe, Ca, V, and Cr.
• REE are mobile, with fenites enriched in MREE and depleted in LREE and HREE.
• Fenitization involves a two-way migration of carbonatitic and metamorphogenic fluids.
• Sodic fenitization of metabasite results in cancrinitization/nephelinitization.

Dolomite–ankerite–siderite ferrocarbonatite dykes of the Alpine Dyke swarm, Westland, New Zealand, intrude greenschist facies metabasites and produce carbonate–albite–muscovite–hematite–Nb–rutile and cancrinite–aegirine–carbonate fenites. The protolith foliation and metamorphic segregations are retained during fenitization despite complete mineralogical reconstitution. Quartz veins in metabasite are reconstituted to albite–aegirine–arfvedsonite/riebeckite assemblages. Geochemically the metabasites have been enriched in Na, C, Sr, Ba, Nb, Th and middle REE, and depleted in Fe, Ca, Mg, V, Cr, Ni, Zr and Hf, light- and heavy-REE. Si and Al show variable behaviour, with Si being consistently depleted in the generation of cancrinite fenite. The process approximates to an equal volume replacement. Field relationships of these dykes suggest that the carbonatites are the sole source of alkaline fenitizing fluids and that ferrocarbonatite magma was, therefore, considerably more sodic than the observed crystalline composition. Sr isotope ratios of fenite and carbonatite minerals indicate they have been partially modified by interaction with carbonatite-derived and schist-derived fluids respectively, and that fenitization, therefore, represents a two-way fluid infiltration. Carbonate minerals in fenite and carbonatite have been subsequently partially recrystallized during oxidation, highlighting the complex emplacement history involving carbonatite crystallization, fenitization and late-stage hydrothermal/carbothermal processes.