New Zealand Journal of Geology and Geophysics abstracts

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Park Volcanics, Murihiku Terrane, New Zealand: petrology, petrochemistry, and tectonic significance

D. S. COOMBS
N. D. J. COOK*
Y. KAWACHI
R. D. JOHNSTONE+

Geology Department
University of Otago
P.O. Box 56
Dunedin, New Zealand

I. L. GIBSON

Department of Earth Sciences
University of Waterloo
Ontario, Canada

*Present address: Pasminco Exploration Pty Ltd, P.O. Box 703, Broken Hill 2880, Australia.

+Present address: Geothermal Energy New Zealand Ltd, P.O. Box 9717, Newmarket, Auckland, New Zealand.

Abstract  The Late Triassic to Early Jurassic Park Volcanics Group comprises minor shallow intrusive and extrusive bodies emplaced during mainly marine sedimentation of the Murihiku Terrane, southern New Zealand. Gowan Andesite 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 Or3.6-3.7 in plagioclase phenocrysts at An50 confirm the high-K nature of the melts at the time of phenocryst crystallisation. The Gowan andesites have higher Fe/Mg than the Glenham and related differences in minor element chemistry suggesting lower fO2 during fractionation of the parent magma. Pinney Volcanics in western Southland are mostly high-K trachydacites but, like Glenham Porphyry, include minor rhyolite. Barnicoat Andesite in the Nelson area is medium-K olivine andesite, marginally tholeiitic in terms of its FeO*/MgO versus SiO2 behaviour, but otherwise is typically calc-alkaline, as are the Gowan, Glenham, and Pinney. Analyses of pyroxenes (augites, orthopyroxenes, reaction rim and groundmass pigeonites) reveal xenocrysts recording an early stage of magma fractionation, slight iron enrichment in the andesite stage, and lowered Fe/Mg and increased Ca contents in augites of the most felsic rocks. Titanian tschermakite and titanian magnesio-tschermakite of deep-seated origin participated in fractionation leading to the Pinney Volcanics, and magnesio-hornblende, edenite, and biotite crystallised as minor late stage minerals following high-level emplacement of Gowan Andesite and siliceous Glenham Porphyry members. Low ⁸⁷Sr/⁸⁶Sr ratios (c. 0.7034-0.7037), REE and multi-element distribution patterns, and the mineralogical features collectively suggest fractionation of the andesites from parental basalt originating in an enriched mantle wedge above a subduction zone, with minimal contamination by continental crust.

High-K andesites appear to be unknown in clearly established forearc basins whereas they are characteristic of back-arc sites. At the time of emplacement of the Park Volcanics, the southern Murihiku sedimentary basin is therefore unlikely to have occupied a forearc setting. The volcanic arc that provided detritus and ash deposits to the basin at that time was probably sited on a strip of largely Proterozoic continental crust detached from the Gondwana mainland by a marginal sea with a subduction zone dipping away from that marginal sea under the volcanic arc, with the Murihiku sedimentary basin towards the rear on the proto-Pacific side. Drumduan Group, with a low-temperature, high-pressure metamorphic overprint, and other largely volcaniclastic terrane fragments in the Median Tectonic Zone of southern New Zealand, may be arc-front remnants of the same arc system.

  

Keywords Park Volcanics Group; Gowan Andesite; Glenham Porphyry; Pinney Dacite; Barnicoat Andesite; Murihiku Terrane; Triassic; Jurassic; high-K andesites; trachydacites; Median Tectonic Zone; mineral analyses

New Zealand Journal of Geology and Geophysics, 1996, Vol. 39: 469-492

0028-8306/96/3904-0469 $2.50/0 (c) The Royal Society of New Zealand 1996

PDF file of entire paper: medium quality (2168K); (scanned from paper original: notes about this process)

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