Petrology of the Sams Creek peralkaline granite dike, Takaka, New Zealand

Abstract The Sams Creek peralkaline granite is a discontinuous dike, 7 km long and up to 60 m thick, with an “outlier” a further 12 km along strike. It cuts lower Paleozoic metasedimentary rocks of the Takaka Terrane. Isotopic data suggest a Late Triassic age (226 ±1.1 Ma), which in turn suggests only minor displacement on the Devil River Thrust since this time, and a Mesozoic age for at least some of the regional deformation/metamorphism which produced north‐south‐trending fold axes in Nelson. Where fresh, the fine‐grained (av. 0.2 mm) dike comprises quartz (32%), mesoperthitic alkali feldspar (50%), aegirine (3%), arfvedsonite andriebeckite (13%), and traces of zincianilmenite, zircon, and fluorite. The bulk of the dike has been altered by late and postmagmatic F‐rich fluids to assemblages rich in albite, quartz ± sericite‐rutile‐biotite (annite?)‐magnetite‐fluorite‐pyrite‐arsenopyrite. The altered rocks host significant Au mineralisation. Very low AI 2 O 3 renders the dike distinctly peralkaline. This feature, together with high values for the high field strength elements Zr, Nb, Ga and Y, suggests, by analogy with similar rocks elsewhere, emplacement into an extensional tectonic environment. The east‐west orientation of the dike is consistent with localised foreland extension parallel to the Gondwana margin during Triassic subduction/collision of the Rakaia Terrane to the east. Only one other peralkaline granite has been reported from the Western Province of New Zealand and both are shown to be similar to classic peralkaline granites of Nigeria and a rare occurrence in the Lachlan Fold Belt of Australia.