High pressure petrology and evolution of the Haast schist, New Zealand


Green, David Colin, 1960-


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Petrogenesis of the system Na20-CaO-MgO-Al203-Si02-H20 (NCMASH) is examined at 18 to 30kbar, 700°C for the purpose of modelling quartz-bearing eclogitic gneiss. Phases synthesised are quartz, kyanite, paragonite, omphacite, Na-amphibole and talc. Trioctahedral Na-mica is not synthesised in NCMASH and Na-amphibole is stable in water-saturated conditions. Omphacite progressively changes composition from jadeite49diopsides1 at 18kbar to jadeite7odiopside3o at 30kbar and Na-amphibole becomes enriched in the glaucophane end member, although the range is compressed such that all compositions are more than 95% glaucophane, 5% tremolite.
At 18kbar the stable assemblage is omphacite+paragonite+Na-amphibole+quartz+H20 and probably garnet. Kyanite appears in a quartz+H20-saturated system between 18 and 22kbar via the reaction paragonite+garnet+Na-amphibole=kyanite. Above 24kbar, paragonite is lost via the reaction paragonite=2omphacite+jadeite exchange (NaA1Ca_tMg_t)+kyanite+H20 and NaAlCa-tMg_l becomes unbuffered. Na-amphibole is lost between 27 and 30kbar by the reaction Na-amphibole (near glaucophane composition)=2omphacite+0.64jadeite exchange+ talc. The paragonite and Na-amphibole breakdown reactions in the NCMASH experimental system are compatible with observations of correlative natural assemblages. A petrogenetic grid derived from the NCMASH experimental results is applic:able to quartzofeldspathic and quartzbearing basic compositions in the eclogite facies. The petrogenetic grid is integrated with others interpreted from natural paragenetic relationships showing the relationships between different quartz-bearing eclogite facies terrains in terms of chiefly continuous reactions.
Haast Schist at Mariners Peak in south Westland was metamorphosed during the Mesozoic Rangitata Orogeny producing an amphibolite facies assemblage in quartzofeldspathic schist. Compositional variation with increasing metamorphic grade across a structural section is interpreted in terms of 3 continuous equilibria involving the consumption of epidote, quartz, NaK_t and AlAl(Fe,Mg)_tSLt and the production ofplagioclase, garnet, CaAlNa_tSLt. TiSLt. MnFe_l and CaFe_l· The major compositional zoning of garnet is CaFe_l enrichment toward the rim and is compatible with pro grade growth in the matrix assemblage.
During the Rangitata Orogeny, the tectonic burial of schist now exposed within 5km of the Alpine Fault was rapid and isothermal, at least for the final lOkm of burial to about 20-30km. Rangitatan deformation was a continuum of synmetamorphic, nappe-forming, ductile Matukituki folding and later, largely post-metamorphic, ductile Manorburn folding.
After Rangitatan tectonism ceased, the geotherm relaxed and the crust was thinned. The high grade schist did not re-equilibrate during uplift, but pegmatite with a cooling age of 60-65Ma indicates localised partial melting at a deeper level.
Mid to late Cenozoic Kaikouran wrenching and uplift associated with the Pacific-Australian plate boundary folded and sheared the schist, but reequilibration was limited to incipient biotite growth in the cores of some folds. A zone of mylonitisation adjacent to the Alpine Fault is bounded by a strain rate contour and the brittle-ductile transition, both of which evolved as rapid uplift advected heat. The total Kaikouran exhumation at the Alpine Fault is 10-17km and declines rapidly in a lOkm wide zone adjacent to the fault. Relative uplift was accommodated by spaced slip parallel to schistosity without retrogressive reaction. At present, much of the tectonic shortening may be accommodated by thickening of the upper crust.
At Mariners Peak the Rangitata Orogen involved rapid vertical tectonics, but did not generate a thermal regime compatible with eclogite facies metamorphism. In contrast, it is likely that eclogite facies conditions currently exists under the Southern Alps. The thermal regimes of both orogens must be accounted for if a single model encompasses all continental collisions.

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xi, 277 p. : ill., maps ; 30 cm.


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Green, David Colin, 1960-, “High pressure petrology and evolution of the Haast schist, New Zealand ,” Otago Geology Theses, accessed October 25, 2020, http://theses.otagogeology.org.nz/items/show/260.

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