French Creek Granite and Hohonu Dyke Swarm, South Island, New Zealand: Late Cretaceous alkaline magmatism and the opening of the Tasman Sea

The Hohonu Dyke Swarm and French Creek Granite represent contemporaneous an d cogenetic alkaline magmatism generated during crustal extension in the We stern Province of New Zealand. The age of 82 Ma for French Creek Granite co incides with the oldest oceanic crust in the Tasman Sea and suggests emplac ement during the separation of New Zealand and Australia. The French Creek Granite is a composite A-type granitoid, dominated by a subsolvus biotite s yenogranite with high silica, low CaO, MgO, Cr, Ni, V and Sr and elevated h igh-field-strength elements (Zr, Nb, Ga, Y). Subordinate varieties of Frenc h Creek Granite include a hypersolvus alkali amphibole monzogranite and a q uartz-alkali feldspar syenite. Spatially associated rhyolitic dykes are con sidered to represent hypabyssal equivalents of French Creek Granite. The Ho honu Dyke Swarm represents mafic magmatism which preceded, overlapped with, and followed emplacement of French Creek Granite. Lamprophyric and dolerit ic varieties dominate the swarm, with rare phonolite dykes also present. Ge ochemical compositions of French Creek Granite indicate it is an A(1)-subty pe granitoid and suggest derivation by fractionation of a mantle-derived me lt with oceanic island basalt - like characteristics. The hypothesis that t he French Creek Granite represents fractionation of a Hohonu Dyke Swarm com position, or a mantle melt derived from the same source, is tested. Major- and trace-element data are compatible with derivation of the French Creek G ranite by fractionation of amphibole, clinopyroxene ene and plagioclase fro m mafic magmas, followed by fractionation of alkali and plagioclase feldspa r at more felsic compositions. Although some variants of the French Creek G ranite have Sr and Nd isotopic compositions overlapping those of the Hohonu Dyke Swarm, most of the French Creek Granite is more radiogenic than the H ohonu Dyke Swarm, indicating the involvement of a radiogenic crustal compon ent. Assimilation-fractional crystallisation modelling suggests isotopic co mpositions of French Creek Granite, are consistent with extreme fractionati on of Hohonu Dyke Swarm magmas with minor assimilation of the Greenland Gro up metasediments.