PARK VOLCANICS, MURIHIKU TERRANE, NEW-ZEALAND - PETROLOGY, PETROCHEMISTRY, AND TECTONIC SIGNIFICANCE

The Late Triassic to Early Jurassic Park Volcanics Group comprises min or shallow intrusive and extrusive bodies emplaced during mainly marin e sedimentation of the Murihiku Terrane, southern New Zealand. Gowan A ndesite in western Southland and Glenham Porphyry andesites in eastern Southland are high-K andesites. Glassy examples have commonly lost K during alteration. Orthoclase contents of Or(3.6-3.7) in plagioclase p henocrysts at An(50) confirm the high-K nature of the melts at the tim e of phenocryst crystallisation. The Gowan andesites have higher Fe/Mg than the Glenham and related differences in minor element chemistry s uggesting lower fO(2) during fractionation of the parent magma. Pinney Volcanics in western Southland are mostly high-K trachydacites but, l ike Glenham Porphyry, include minor rhyolite. Barnicoat Andesite in th e Nelson area is medium-K olivine andesite, marginally tholeiitic in t erms of its FeO/MgO versus SiO2 behaviour, but otherwise is typically calc-alkaline, as are the Gowan, Glenham, and Pinney. Analyses of pyr oxenes (augites, orthopyroxenes, reaction rim and groundmass pigeonite s) reveal xenocrysts recording an early stage of magma fractionation, slight iron enrichment in the andesite stage, and lowered Fe/Mg and in creased Ca contents in augites of the most felsic rocks. Titanian tsch ermakite and titanian magnesio-tschermakite of deep-seated origin part icipated in fractionation leading to the Pinney Volcanics, and magnesi o-hornblende, edenite, and biotite crystallised as minor late stage mi nerals following high-level emplacement of Gowan Andesite and siliceou s Glenham Porphyry members. Low Sr-87/Sr-86 ratios (c. 0.703-0.7037), REE and multi-element distribution patterns, and the mineralogical fea tures collectively suggest fractionation of the andesites from parenta l basalt originating in an enriched mantle wedge above a subduction zo ne, with minimal contamination by continental crust. High-K andesites appear to be unknown in clearly established forearc basins whereas the y are characteristic of back-are sites. At the time of emplacement of the Park Volcanics, the southern Murihiku sedimentary basin is therefo re unlikely to have occupied a forearc setting. The volcanic are that provided detritus and ash deposits to the basin at that time was proba bly sited on a strip of largely Proterozoic continental crust detached from the Gondwana mainland by a marginal sea with a subduction zone d ipping away from that marginal sea under the volcanic are, with the Mu rihiku sedimentary basin towards the rear on the proto-Pacific side. D rumduan Group, with a low-temperature, high-pressure metamorphic overp rint, and other largely volcaniclastic terrane fragments in the Median Tectonic Zone of southern New Zealand, may be are-front remnants of t he same are system.