Permian–triassic felsic tuffs in South Island, New Zealand: Significance for oceanic and active continental margin subduction

Abstract

Felsic tuffs play an important role in the Permian–Triassic geology of the Eastern Province in South Island. In the Brook Street Terrane, primary felsic tuff is minor in the south (e.g. Takitimu Mountains) but abundant in the north (Grampian Formation, Nelson area). Felsic fallout tuff dominates one interval of the Maitai Group (Early Triassic Kiwi Burn Formation), south of the Alpine Fault, but is otherwise mainly redeposited by gravity flows. The Murihiku Terrane is characterized by two main intervals of felsic fallout tuff, the Middle Triassic Gavenwood Tuffs and the Late Triassic Bare Hill Tuff Zone, south of the Alpine Fault (e.g. Hokonui Hills and south Otago coast). Counterparts north of the Alpine Fault (Richmond Group) are mainly reworked, with terrigenous admixtures. Tuffaceous sediments are also abundant in the late Middle– early Late Triassic Willsher Group (south Otago coast). Based on combined field, petrographical, semi-quantitative X-ray diffraction (XRD) and chemical evidence, the felsic tuffs of the Brook Street Terrane in the south are interpreted as small-scale eruptions of fractionated oceanic-arc-type magmas. In contrast, the Triassic felsic tuffs of the Murihiku Terrane, Willsher Group and Maitai Group erupted violently and episodically in proximal to distal segments of the SE Gondwana continental margin