Geochemistry, mineralogy, and metamorphic history of kyanite-orthoamphibole-bearing Alpine Fault mylonite, South Westland, New Zealand

Abstract

Mylonite outcropping in the Alpine Fault Zone at Makawhio River, South Westland, shows mineralogy unlike rocks found elsewhere in the Haast Schist. The Makawhio mylonites comprise kyanite-orthoamphibole hornblendite, garnet amphibolite, chlorite-hornblende schist, kyanite-plagioclase schist, and talc-tremolite rocks, are characterised by ultrabasic to intermediate chemistry, show extreme enrichment in Cr and Ni, and collectively include kyanite, margarite, fuchsite, scapolite, and the first reported occurrence of gedrite and anthophyllite from the Eastern Province. These unusual mylonites form a pod-like body enveloped by chlorite-hornblende schist within Torlesse-derived metasediments. The geochemistry and mineralogy of the Makawhio rocks suggests an origin as a veined ophiolitic rock metasomatised before or during upper amphibolite facies Alpine Schist metamorphism and mylonitisation, resulting in a localised occurrence of rocks atypical of the Haast Schist overall. These kyanite-bearing, metasomatised mylonites are juxtaposed with quartzofeldspathic and calc-silicate mylonites, and with amphibolites derived from ocean-floor basalt, within the Alpine Fault Zone. The Makawhio River kyanite-bearing mylonites and associated rocks probably represent a tectonised sliver of the Pounamu Ultramafic Belt ophiolite, which is exposed to the north and south as part of the imbricated basement to the Torlesse Terrane along the western margin of the Southern Alps. The range of lithologies and unusual nature of the mylonites at Makawhio River may also represent an extension of the Torlesse Caples Terrane boundary incorporated into the plate boundary zone during oblique convergence and dextral shear on the Alpine Fault. © 2003 Taylor & Francis Group, LLC.