Park Volcanics, Murihiku Terrane, New Zealand: Petrology, petrochemistry, and tectonic significance
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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 fO 2during 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 SiO2behaviour, 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 litanian 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:
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A study of amphiboles and associated minerals in samples of Soufrière Hills andesite erupted from 1995 to 2002 shows significant compositional variations within hornblende phenocrysts, a separate set of small pargasitic crystals in the groundmass, and two types of reaction rims on the phenocrysts. The composition of the amphiboles and coexisting phases defines the thermal history of the erupting magma. As many as seven zones (<200 µm wide) in the hornblende phenocrysts begin with a sharp increase in Mg and Si, and then change gradually to a more Fe- and Al-rich hornblende, a transition that is consistent with a temperature rise. Analyses of the hornblende phenocrysts and associated Fe–Ti oxides verify previous conclusions that the pre-eruption magma was at 130 MPa and 830 ± 10°C, but was variably heated before eruption. The heating occurred within ∼30 days of eruption for all magmas erupted, based on the width of Ti-rich rims on titanomagnetite phenocrysts. Experimental phase equilibria for the andesite confirm that the natural hornblende phenocrysts would be stable between 825 and 855°C at a PH2O of 130 MPa, and would be even more Al rich if crystallized at higher pressure. Pargasite is not stable in the andesite, and its presence, along with high-An plagioclase microphenocrysts, requires mafic magma mingling and hybridization with pre-existing andesite. Experimental melts of the andesite at 130 MPa and 830 and 860°C compare well with melt inclusions in quartz and plagioclase, respectively. Reaction rims on a few hornblende crystals in each andesite sample are rich in high-Ca pyroxene and are produced experimentally by heating the andesite above the stability limit for hornblende. Decompression-induced breakdown rims occur in some samples, and the rate of this reaction has been experimentally calibrated for isothermal andesite magma ascent at 830–860°C. The average ascent rate of magma during much of the 1995–2002 eruption has been >0·02 m/s, the rate that allows hornblende to erupt free of decompression-induced reaction rims.
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Abstract Andesite volcanoes of the North Island of New Zealand are divided on the basis of their potash contents into two groups; a low potash group which includes the Northland, Coromandel, and Taupo Volcanic Zone andesites, and a high potash group which contains the volcanoes of western Taranaki. There are no significant differences among the chemistries of the low potash andesites from the different regions, leading to the proposition that they have a similar origin. It is suggested that an andesite "source zone" developed in the upper mantle beneath the old arcuate structure during Miocene time and has propagated eastwards across the North Island. The high potash andesites appear to be associated with events beneath the old arcuate structure rather than with the active margin of the east coast of the North Island. The chemistries of the New Zealand andesites are compared with analyses from other active regions and the invariance of the soda content is contrasted with the large potash variation.
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Petrographical and geochemical characteristics of calc-alkaline andesites on Shodo-Shima Island, SW Japan, having bulk compositions largely identical to the continental crust, are presented. The following petrographic observations suggest a role for magma mixing in producing such andesite magmas: (1) two types of olivine phenocrysts and spinel inclusions, one with compositions identical to those in high-Mg andesites and the other identical to those in basalts, are recognized in terms of Ni–Mg and Cr–Al–Fe3+ relations, respectively; (2) the presence of orthopyroxene phenocrysts with mg-number >90 suggests the contribution of an orthopyroxene-bearing high-Mg andesite magma to production of calc-alkaline andesites; (3) reversely zoned pyroxene phenocrysts may not be in equilibrium with Mg-rich olivine, suggesting the involvement of a differentiated andesite magma as an endmember component; (4) the presence of very Fe-rich orthopyroxene phenocrysts indicates the association of an orthopyroxene-bearing rhyolitic magma. Contributions from the above at least five endmember magmas to the calc-alkaline andesite genesis can also provide a reasonable explanation of the Pb–Sr–Nd isotope compositions of such andesites.
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Abstract Titiraupenga is an early Pleistocene volcano sited approximately 20 km northwest of Lake Taupo, North Island, New Zealand, at the junction of the Taupo Volcanic Zone and a line of andesite volcanoes extending from Ohakune in the south to Waiheke Island in the north. Exposures on the volcano are restricted to a central plug (type 1 andesite) and rare exposures of flows on the lower slopes (type .2 andesite). Both type 1 and type 2 andesites are porphyritic with phenocrysts of orthopyroxene, augite, plagioclase, and scarce olivine in a groundmass of plagioclase, pyroxene, and iron OXIde. Pigeonite occurs only in the groundmass and as reaction rims to orthopyroxene phenocrysts in type 1 andesites. Chemically, most samples are low-Si andesites, and variation is probably due to varying degrees of pyroxene accumulation within the magma reservoir. Type 2 andesites are inhomogeneous and probably result from mixing of magmas from different parts of the reservoir. Aphyric lenses (type 3 andesite) within type 2 andesites are thought to represent a wholly liquid portion of the reservoir which was incorporated in the type 2 magma during eruption. Slight differences in isotopic composition of type 3 magma may indicate some crustal contamination.
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Abstract Thirty-three new partial or complete chemical analyses are presented for pyroxene, amphibole, and biotite phenocrysts separated from three andesitic, three dacitic and thirteen rhyolitic volcanic rocks. The orthopyroxenes are mostly of hypersthene composition, with the rhyolitic hypersthenes generally being slightly more Fe-rich compared to the andesitic types. No detailed correlation, however, is evident between the Fe/Mg ratio of the rhyolitic orthopyroxenes and their parent rocks or co-existing groundmasses. A correlation between Ca content and the type of co-existing ferromagnesian assemblage was found. The clinopyroxenes are augites, and occur most commonly in the andesites and dacites. Two amphiboles are found, a calcic amphibole and cummingtonite. The former range from a tschermakitic hornblende (andesitic) to magnesio-hornblende in the rhyolites. One dacitic ferro-hornblende was found. The data illustrate the increasing substitution of Al and increasing Na in the andesitic hornblende. A close correlation of the Niggli mg ratio, and also MnO, exists between the analysed co-existing orthopyroxenes and hornblendes. Again, however, no detailed correlation between Fe/Mg ratio of hornblende and parent rock (or co-existing groundmass) was found to exist.
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