Metamorphism, structure and post-metamorphic intrusives of the Haast river area, South Westland, New Zealand

Author:

Cooper, Alan (Alan Frederick)

Year:

Project type:

Abstract:

This thesis reports the study of a metamorphic terrane at Haast River, South Westland, the type section of the Haast Schist Group.
A sequence of eugeosynclinal sediments, metamorphosed and intensely deformed during the Rangitata orogeny, have been subdivided into four metamorphic zones on the basis of the mineralogical adjustment of the dominant lithology, metagreywacke, in response to progressive metamorphism. Metagreywacke is interlaminated with metabasic {greenschist), metachert, marble and ultramafic serpentinite horizons.
In the Burke River section, separated from the Haast River zonal sequence by the major, east-west trending Burke Fault, the chlorite zone has been further subdivided on a textural basis. Both sections, north and south of the Burke Fault show an east to west rise in metamorphic grade, although in the Haast River area zonal distribution has been complicated by the repetition of oligoclase, garnet, and possibly biotite zones through postmetamorphic folding.
Quartzofeldspathic schist from the Chl.II subzone occasionally shows relict clastic textures, although by Chl.IV subzone the rocks are more fully reconstituted, with only rare relict minerals allanite and hornblende, mantled by epidote and actinolite respectively persisting to slightly higher grades. Upgrade of the Chl.III-IV subzone boundary quartzofeldapathic schists characteristically exhibit a coarse segregation banding, with interlaminated quartz-plagioclase-calcite and muscovite-chlorite-epidote-sphene-apatite-opaque assemblages. Actinolite, and more rarely stilpnomelane are restricted to the lowest grade chlorite textural subzones.
At the biotite isograd muscovite and chlorite interact to produce coarse deep brown flakes of biotite, which occurs in the great majority of biotite zone quartzofeldspathic schists. In contrast, the transition between biotite and an upgrade garnet zone is only poorly defined in the Haast River area since garnet appears only rarely in quartzofeldspathic schist lithologies, due to unfavourable bulk rook composition. The mechanism of formation of garnet is not evident from textural petrographic considerations, although the reduction or even absence of chlorite in garnet zone schists speaks for its involvement in garnet producing reactions. The garnet developed at the garnet isograd is typically zoned, with spessartine and grossularite components decreasing in amounts towards the rim of the grain. Overall compositions are almandine rich, a typical rim composition showing Alm68 •7And1•3Groas10 •6Pyr4•7spese14 •7, in marked contrast to the spessartine-rich garnets of metachert, which occur at much lower metamorphic grades.
Isogradic with the appearance of almandine-rich garnet in quartzofeldapathic schist, albite, typically An0-1 at lower metamorphic grades is rimmed by thin zones of a second plagioclase, oligoclase, of composition An24_25. Throughout the garnet zone modal albite steadily decreases, becoming more calcic in composition, up to An2-3, while oligoclase increases, becoming more sodic in composition, down to An21. Co-existing plagioclase compositions outline the form of the peristerite solvus. The albite-oligoclase conversion releases excess silica, which forms lobate quartz inclusions in oligoclase, defining a distinctive myrmekitic texture. Albite completely disappears at the second oligoclase isograd defining a transition to an oligoclase zone of amphibole facies status.
In the lower grade part of the oligoclase zone a second form of myrmekitic texture indicates the incoming of microcline, commonly forming intergranularly, and in coarser patches frequently exhibiting a perthitic texture. Microcline is generally derived during progressive metamorphism by the complete breakdown of muscovite with production of a more aluminous phase, normally sillimanite. This mechanism is however inapplicable in the Haast River area, where assemblages lack an Al2Sto5 polymorph. Muscovite, however, is frequently tattered and corroded above the microcline isograd, and a reaction of the form muscovite + 2 epidote + 2 quartz = K feldspar + 4 anorthite + 4H20 is thought more likely.
Oligoclase-zone quartzofeldspathic schists, now more gneissic than schistose in texture retain the assemblage quartz-oligoclase +- microcline-biotite-muscovite +- garnet to the highest metamorphic grades observed.
The mineralogical changes of interbedded greenschist horizons in response to progressive metamorphism have been intensively studied, and can be used successfully to supplement the zonation defined for dominant quartzofeldspathic schist. In-the chlorite zone, green-schists are characteristically foliated, yet lack the segregation banding typical of metagreywacke. In contrast to other areas of the Haast Schist Group, stilpnomelane is a rare mineral at Haast River, and greenschist assemblages are dominated by albite, chlorite, epidote, muscovite, calcite, and rarely actinolite.
Chlorite at low metamorphic grades is consistently an aluminous ripidolite, having Mg/Fe <1, and a formula approximating (Mg5Fe5Al2) Si5.3Al2.7O20(OH)16. As the metamorphic grade increases, chlorite is progressively depleted, being absent from some garnet-zone greenschist assemblages, and of only sporadic occurrence in oligoclase zone amphibolites. The gradual elimination of chlorite is probably largely due to its involvement in reactions producing biotite and hornblende. Chlorite becomes progressively more magnesian towards higher metamorphic grades, and in the upper grade portion of the biotite zone changes from an optically -ve, Fe-Mg chlorite to an optically +ve Mg-Fe variety. Oligoclase zone chlorites, although still ripidolites, have an Mg/Fe ratio> 2 e.g. (Mg6•8re3•1Al2 •2 )si2 •2Al2 •8o20<oH) 16.
Epidote, an essential greenschist mineral in chlorite, biotite and garnet zones becomes sporadic towards highest metamorphic grades, probably reflecting progressive dissociation with enrichment of plagioclase in the anorthite component. Epidote compositions range in Fe+3 content from Pa13•6 to Pa30, spanning and plotting within the miscibility gap between clinozoisite and epidote, centred at Pa22_ 23 previously suggested by Strens, 1963, 1965. The composition appears to be insensitive to metamorphic grade and is dependent more upon the oxidation states prevailing during metamorphism. Pistacite-rich epidotes, often accompanied by magnetite and a deep coloured re+3 rich hornblende are typically developed in greenschists which have recrystallised during metamorphism under conditions of high P02. Epidotes from the 2 Haast River area are poor in FeO (0.46-0.57%) relative to 'high index' epidotes from the Maitai-Bryneira and Caples formations of North-west Otago, where high P/T metamorphism is evidenced by lawsonite-bearing assemblages.
Biotite enters the greenschist assemblage in advance of the quartzofeldspathic schist isograd, and persists throughout the facies series, showing little systematic variation in composition with metamorphic grade. The composition, averaging K2 (Ms2.a9re2 •52Al, Ti0 •59 Hst5•53Al2 •47 >o20 COB)4 is dependent more on bulk rock composition, and greenschist biotites are consistently less aluminous than those from quartzofeldapathic schist.
A pale green or pale bluish green actinolite is found sporadically in low grade chlorite and biotite zone greenschists, and in the latter commonly exhibits exsolution textures with thin (<3u) lamellae of a green hornblendic amphibole along the (101) plane of the host. At the garnet isograd actinolite is superseded in most greenschists by a dark coloured, often deep bluish green hornblende which occasionally forms an optically distinct outer zone to pale actinolitic cores. Hornblendes also frequently contain exsolution lamellae along (101), although generally not in samples which co-exist with actinolite, and it is possible that the lamellae here are of cummingtonite composition, since analyses commonly contain cations in excess of the 5.00 atoms per formula unit in the Y site. The sudden break in composition of the amphibole at the garnet isograd is believed to be controlled largely by an underlying miscibility gap in the calciferous amphibole solid solution series. Towards higher metamorphic grades the hornblende progressively changes colour from deep bluish green ( Z-absorption tint) to dark green, and compositionally shows a marked increase in the Ti substitution and increase in the content of the A site.
In most greenschists, reactions involving the production of hornblende at the garnet isograd are linked with the appearance of oligoclase, which co-exists with albite throughout the garnet zone. Towards the second oligoclase isograd, as with quartzofeldspathic schist plagioclase, compositions outline the form of part of the peristerite solvus, although the final disappearance of albite is occasionally at a slightly lower grade than in interbedded quartzofeldspathic schist. Oligoclase-forming reactions are suppressed in some greenschists characterised by high Pco2, and garnet-zone hornblende-bearing greenschists sometimes contain only albite plus calcite. Zoning is a common feature in oligoclase-zone plagioclases and is generally sharp, with a break between an oligoclase core An25-29 and andesine rim An37-43, possibly reflecting a discontinuity in the plagioclase solid solution series at An3, suggested by Doman et. al., 1965.
Garnet is sporadic in lower grade garnet-zone greenschists, becoming more common towards the upper grade portions and attaining the role of an essential mineral only in oligoclase-zone amphibolites, where it is commonly porphyroblastic. Electron microprobe data show garnet to be zoned, although the zoning is probably lees pronounced than in garnets from the quartzofeldspathic schist. However, grains are again rich in spessartine and grossularite components in the core (e.g. Alm54.5speaa15 •1Groae26., And2 •2Pyr1•9 ), becoming progressively richer in almandine and pyrope towards the rim (e.g. Alm74.2And2.9Gross12.4Pyr7.9spess2.7).
Sphene, apatite and ilmenite are constant accessories through the facies series, and these are commonly joined at higher metamorphic grades by rutile. Magnetite, pyrite and pyrrhotite are sometimes found. Tourmaline, generally a schorl variety, is also widespread, commonly in the form of zoned prisms, with blue Fe+2 rich cores and brown-green Fe+3 rich rims.
Metachert horizons are subordinate to greenschist and quartzofeldspathic schist, yet show a wide distribution. Mineralogies are dominated by quartz, with interlaminated muscovite-biotite-chlorite-spessartine rich layers. Biotite is developed approximately isogradically with biotite in quartzofeldspathic schist, while a spessartine-rich garnet is present in chlorite textural subzone metacherts, being reported elsewhere in the Haast Schist Group from Chl.2-Chl.3 subzone rocks. A garnet analysis from a garnet-zone ferruginous metachert showing a composition Alm30.8And10.4Gross4.9Pyr3.4Spess50.5.
In metacherts which have recrystallised during metamorphism under high PO2 conditions, evidenced by the presence of hematite, the epidote of spessartine-rich metacherts is replaced by piemontite. Biotite in these rocks, again probably due to high P02 conditions is a pale-coloured phlogopitic variety. Piemontite has a composition (Ca1.75Mno.25)(Al2.23Fe+30.53Mn0.24)Si3O12OH, similar to other hydrothermal and low grade metamorphic piemontite, grouped about a composition Ca2(Fe,Mn)Al2Si3O12OH.
Cummingtonite is occasionally present in garnet and oligoclase-zone metacherts. Optical properties indicate a total range composition from grunerite (Mg2.1Fe4.9Si8O22(OH)2) to cummingtonite (Mg3.9Fe3.1Si8O22(OH)2). A member of the cummingtonite-grunerite series occasionally co-exists with hornblende or actinolite, the two amphibole species forming optically distinct zones or irregular patches in composite grains. Grunerite and actinolite host amphiboles are also traversed by (101) actinolite and grunerite exsolution lamellae respectively. Analyses presented of one co-existing pair span the miscibility gap between calciferous and non-calciferous clino-ampbiboles.
Marble horizons are poorly represented in the Haast River area, generally occurring in association with greenschist and metachert; garnet-zone occurrences are characterised by an assemblage calcite-quartz-albite-tremolite-muscovite.
Ultramafic schists occurring in some greenschists are derived by the metamorphism of strongly serpentinized peridotite pods. The largest pod, in Douglas Creek, retains a serpentinite core which is enclosed within a series of thin, concentric, mono and bi-mineralic shells, characterised by the dominance of talc and magnesite, talc, tremolite, chlorite, and biotite, passing outwards into greenschist.
These zones reflect elemental migrations of Si, Ca, and Co2 from greenschist to peridotite and Mg from peridotite to greenschist during metamorphism. Cr appears to be immobile during metamorphism and a sharp break in the bulk rock Cr-content at the inner margin of the monomineralic chlorite zone is taken to delineate the original mafic-ultramafic contact.
Structurally the schists of the Haast River area have suffered multiphase deformation during the Rangitata orogeny and four folding phases have been recognised. The earliest folding phase, designated F1 , produced isoclinal folds, which on a mesoscopic scale gives rise to an axial plane foliation. S1, parallel, or subparallel to the original bedding lamination S0. The simple outcrop pattern of greenschist horizons and the consistent westerly vergence of mesoscopic folds suggests that the Haast River area lies on one limb of a recumbent fold of considerable amplitude.
In the east of the area, early isoclinal F1 folds have been coaxially refolded about a later F2 fold axis which gave rise to more open style south-westerly plunging mesoscopic structures. Quartz-albite veining frequently defines a south easterly dipping axial plane foliation. Mesoscopically F2 folds define a +ve vergence boundary marking the axis of a macroscopic, recumbent, antiformal fold, with probably a paired synformal structure further to the east.
To the west, F1 structures have been overprinted by a third folding phase F3 , resulting, from east to west, in open sub-horizontal, south-westerly trending, post metamorphic folds, the Mount Victor Monocline, Haast Antiform and Thomas Synform. A second post-metamorphic folding phase, F4 results in angular chevron or kink style folds which are of localised occurrence and are genetically related to faulting.
Two generations of faulting have been recognised, the earlier set orientated approximately east-west, typified by the Burke Fault of reverse dip-slip character, and a later north-east trending set, cross cutting and displacing the earlier structures.
Deformational phase F1 and F2 are synmetamorphic predating the attainment of the metamorphic maximum, while F3 folds the mineralogical isograds, and causes repetition of the metamorphic zones.
Shortly after the F3 and F4 deformational phases the area was intensively intruded by an alkalic-lamprophyric dyke-swarm of carbonatitic affinities. The Haast River area forms only part of a regional dyke swarm, extending 105km from Paringa River in the north, where the swarm is bounded by the Alpine Fault, to the Matukituki River in the south. Similar minor intrusives are found to the north of the Alpine Fault, extending 170-200km from Mt. Bonar in the south to the Buller River in the north.
In the map area east-west trending dykes and north-east trending sills constitute a consanguineous series, ranging in composition from ultramafic hornblende-mica peridotite, through dominant camptonitic lamprophyre to feldspar porphyry, sodalite tinguaite, potassium trachyte and carbonatite. Many alkalic intrusives are rich in Nb and Zr, while carbonatites are characterised by a Ba, Sr, rare earth association, giving rise in one dyke to essential Ba-Mg carbonate norsethite and accessory monazite.
Adjacent to all intrusions the country-rock schist has suffered metasomatic alteration, the affected zone reaching a maximum width for alkalic and carbonatitic varieties. In extreme cases quartzofeldspathic schist is totally mineralogically reconstituted to albite-aegirine-blue amphibole schist by a process likened to fenitization. Metasomatic fenitization mechanisms involve the introduction Na2O, Co2, S, Nb, and Th to the country rock schist, which itself is possibly depleted in K2O (and in SiO2 , if all the quartz is replaced by K-feldspar).

Thesis description:

xvii, 331 leaves : illus., maps (5 fold. in pocket) ; 30 cm.

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OU geology Identifier:

1970Cooper

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Cooper, Alan (Alan Frederick), “Metamorphism, structure and post-metamorphic intrusives of the Haast river area, South Westland, New Zealand ,” Otago Geology Theses, accessed February 8, 2025, https://theses.otagogeology.org.nz/items/show/43.

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