Concentrically zoned ultramafic pods from the Haast schist zone, South Island, New Zealand

Abstract

Ultramafic pods enclosed within greenschist are described from the Haast River area, South Westland. The largest body, 60 ½ 800 m in outcrop area, is exposed in Douglas Creek and consists of a central core of serpentinite rimmed successively by concentric, essentially monomineralic or bimineralic shells of talc-magnesite, talc, tremolite, chlorite, biotite, and albite, the last interlaminated with country-rock greenschist. The zones are interpreted as having formed by metasomatic exchange between two masses of dissimilar composition during garnet zone metamorphism—the albite, biotite, and chlorite zones were derived from greenschist, and tremolite, talc, and talc-magnesite rocks from serpentinite. The serpentinite contains a highly magnesian olivine (FØ₉₈) interpreted as a metamorphic phase crystallising under conditions of high P₀₂. Relict chromite in the serpentinite suggest a pendotitic parent lithology. Rare harzburgite cobbles occur in float, downstream of the pod. Ultramafic schist occurences elsewhere in western Otago are restricted to linear and arcuate belts coincident with green schist-rich zones. The serpentinite-bearing greenschist zones are defined magnetically by narrow bands of low amplitude positive anomalies. In the Haast area the distribution of ultramafites coincides with the western boundary of a terrain dominated by macroscopic, isoclinal F₂ folds. To the west of the ultramafite occurrences F₂ folds are rare. It is suggested that the ultramafites and possibly some of the associated greenschists represent a slice or slices of the simatic basement of the Haast Schist Zone, tectonicly emplaced during thrusting of the synmetamorphic F₂ deformational phase.