Volcanogenic and granitoid rocks from northwest Stewart Island

Abstract

Metamorphosed andesite flows and dacite/rhyolite‐dominated, lithic‐rich and crystal‐rich volcaniclastics of unknown thickness crop out north and east of Little Hellfire Beach on western Stewart Island, New Zealand. Andesite geochemistry is of calc‐alkaline affinity, and an active continental margin eruptive setting is inferred. Petrographic and geochemical similarities to adjacent Paterson Group metavolcanics suggest that the volcanogenic suite should be incorporated in the Paterson Group. The Paterson Group is intruded by three plutons; Ruggedy Granite, Codfish Granite, and Richards Point Porphyry. A fourth, the Mason Bay Pluton, intrudes the Ruggedy Granite. Together these four plutons form the northern part of the Rakeahua Batholith. Ruggedy Granite, Codfish Granite, and Richards Point Porphyry are all evolved bodies with biotite as the dominant mafic phase. The Mason Bay Pluton is a heterogeneous, xenolith‐rich body ranging in composition from quartz monzodiorite to granite with biotite as the dominant mafic phase. I‐type affinities and correlation with either the Separation Point or Rahu Suites of northwest Nelson are inferred. Deformation in all rock units is restricted to narrow high‐strain zones characterised by early ductile deformation followed by locally developed brittle deformation. Greenschist facies metamorphism accompanied and may have predated ductile deformation. Development of lower amphibolite/hornblende homfels facies assemblages in the westernmost Paterson Group outcrops may be the result of contact or regional style metamorphism.