Nephrite and metagabbro in the Haast Schist at Muddy Creek, northwest Otago, New Zealand

Abstract

Abstract At Muddy Creek, northwest Otago, a pod of gabbro, probably in a matrix of serpentinite, was tectonically emplaced into quartzofeldspathic sediments before the main metamorphism of the Haast Schist. Metasomatic diffusion during pumpellyite actinolite to greenschist facies metamorphism produced talc‐, tremolite‐, chlorite‐, and muscovite‐rich reaction zones at the margin of the pod. Subsequent shearing disrupted the metasomatic sequence and produced nephrite from the tremolite zone precursor. Relict magmatic hornblende and clinopyroxene from the metagabbro have compositions appropriate to a volcanic arc tectonic setting. In contrast, relict pyroxenes from four metabasaltic knockers in the Greenstone Melange have compositions indicating generation of magmas in both ocean‐floor and ocean‐island environments. Ultramafic and mafic rocks at Muddy Creek and in the Greenstone Melange are thought to represent fragments of the ophiolitic basement to the Haast Schist, the tectonic settings of which help cons...