Petrology of the calcalkaline lavas of the Permian Takitimu Group, southern New Zealand

Abstract

Abstract The 14 km sequence of primary volcanic and marine volcaniclastic strata of the Takitimu Group in the central Takitimu Mountains forms an important reference section for the Permian Brook Street terrane of southern New Zealand. The group comprises a calcalkaline association in which porphyritic basalt-basaltic andesite (46–56 wt% SiO2) predominates strongly over more evolved compositions. Evolution is from purely basaltic volcanism, to bimodal basalt and rhyodacite (66–71 wt% SiO2), back to unimodal basalt, and finally to a continuous compositional range of 46–62 wt% SiO2. The Takitimu lavas are typical of island arc series being high in CaO, A12O3, and LREE and low in MgO, TiO2, Ni, and Cr relative to MORB. The dominant plagioclase-phyric basalts lack negative europium anomalies and have simple mineralogy (plagioclase — augite ± olivine ± magnetite) of relatively constant composition suggesting slow equilibrium crystallisation. Subordinate aphyric basalts are chemically distinct (CaO, A12O3, and MgO poor; FeO, Na2O rich) and possess slightly more fractionated REE patterns. Andesites contain phenocrysts of plagioclase, augite, hypersthene, and magnetite, with pigeonite in the groundmass. The onset of hypersthene and magnetite crystallisation is marked by sharp depletion in Cr, Ni, V, and Cu. REE patterns are subparallel to those of the porphyritic basalts but possess a marked negative europium anomaly. Features of disequilibrium crystallisation are shown by both andesitic plagioclase (complex fine oscillatory zoning, resorption, abundant glass inclusions) and pyroxenes in the aphyric basalts (sector zoning). The Takitimu rhyodacites possess high K/Rb ratios and higher abundances of all large ion lithophiles than the andesites. Internal compositional trends for the rhyodacites on the Harker variation diagram are not an extension of the basalt-andesite trend lines. Values of U and Th are far lower, and Zr much higher, than typical upper crustal values. The model for Takitimu Group magmatism proposes that a body or bodies of high-alumina, relatively fractionated basaltic magma rose to shallow levels and underwent further plagioclase-dominated fractionation. Some portions of this magma were discharged at the surface, while others supplied heat to induce melting of pre-existing mafic crust to generate rhyodacite. Fractionation of the basalt was towards andesite, by removal of plagioclase - Ca-rich pyroxene — magnetite ± olivine, and eventually portions of the more differentiated magmas were erupted along with parent basalt. Towards the close of volcanism, portions of the basaltic magma intruded the sequence and differentiated to a gabbro — granophyre — aplite assemblage paralleling the porphyritic basalt-andesite trend.