Nb-rich baotite in carbonatites and fenites at Haast River, New Zealand

University of Otago, Geology Department, Dunedin, New Zealand

Baotite occurs as an accessory mineral in carbonatites, fenites and carbothermal veins associated with a lamprophyre dyke swarm in the Haast River area of south Westland, New Zealand. Carbonatites are petrogenetically evolved, with assemblages dominated by ankerite, siderite and Ba-Sr-REE carbonates. Microprobe analysis indicates baotite compositions more Nb-rich than previously recorded, with compositions close to Ba ₄ [Ti ₃ (Nb,Fe) ₅ ]Wi ₄ O ₂₈ Cl. Ti must be partially replaced in both crystallographically-independent octahedral sites. Compositional zoning, and stoichiometric consideration suggest that the dominant octahedral substitution is the same as that described in rutile, namely 3Ti (super 4+) 2Nb (super 5+) +Fe (super 2+) . Contrary to previous suggestions, Fe in the octahedral site should, therefore, be dominated by Fe (super 2+) . The presence of baotite further documents the involvement of halogens in carbonatite magmas. In the New Zealand occurrences it is suggested that the chlorine originates from associated phonolitic magmas and is partitioned into carbonatite during liquid immiscibility.

This record provided courtesy of AGI/GeoRef.