Geology and metamorphism near the head of Lake Wakatipu.
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Abstract:
The geology and metamorphism of an area between Hollyford Valley and the western flank of the Richardson Mountains at the head of lake Wakatipu are described. The area covered in this study includes Rockburn, Routeburn, Caples River, a part of Greenstone River, most of the Ailsa Mountains, souther half of the Humboldt Mountains, Mt. Earnslaw massif, Mt. Alfred and Mt. Alaska. It will be loosely referred to as upper Wakatipu although it extends beyond the edges of the Upper Wakatipu Survey District.
The area is divided into three sub-areas, western, middle, and eastern, by two major, north-striking faults.
The western side of western sub-area is bounded by the Livingstone fault with a serpentinite of the Red Mountain Ultramafites lying immediately beyond. The western sub-area comprises four formations forming the eastern limb of a partly overturned anticline. The western limb of this anticline has been removed by the Livingstone fault.
The Harris Saddle Formation, which is the lowest in the western sub-area sequence, consists mainly of mafic schists with minor pelitic, psammitic siliceous and ferruginous schists. The mafic schists are derived from mafic tuff, subordinate lava flows or sills, and tuff breccia. Distinct schistosity and lineation are developed. Locally, superposed folding is recognized. General attitude is N-S strike with an eastward dip. The thickness is over 3000m., the base being unknown.
The Momus Formation, conformably overlying the Harris Saddle Formation, comprises a rather regular intercalation of psammitic and pelitic rocks, psammites predominating. Cleavage is distinct to poor and lineation is poorly developed or absent. Sedimentary structures such as load casts, slumping, and grading are common. The strike is generally northwards with eastward dips in the west, and westward dips in the east. Facing directions are invariably to the east. The thickness is about 3000m.
Kay's Creek Formation, conformably overlying the Momus Formation, consists of mafic to intermediate volcanic sandstone and tuffite. Schistosity and lineation are almost absent. Sedimentary structures such as slumping and convolute lamination are abundant. Bedding generally strikes N-S with vertical westward dips. The thickness is about 1200m. at Caples River but appears to become thinner towards the north.
Upper Peak Formation conformably overlies Kay's Creek Formation, and consists mainly of regularly intercalated psammites and pelites. Graded bedding is common. Almost isoclinal folding is conspicuous especially toward the east where the formation is in the fault contact with the Routeburn-Caples Ultramafite Belt. Axial plane cleavage and lineation are poorly developed. The strike is generally northwards, and the dip moderate to the west. The thickness is about 2000m. or less.
Except for the Harris Saddle Formation, the same formations are found in the same order of superposition to the north of a fault running parallel to the Rockburn.
Routeburn-Caples Ultramafite Belt, which separates the middle sub-area from the western sub-area is a structural belt running N-S, and noth hitherto recognized as a continuous unit. It is composed of very divergent rocks including serpentinitem rodingite, metagabbro, amphibolite, mafic lava, mafic, pelitic, and psammitic schists, and metasomatized mafic, pelitic, and psammitic schists. Hardening of pelitic rocks in contact with serpentinite is not uncommon. Serpentinite invariably occurs throughout the belt. Other rock types in the above list are interpreted mostly as tectonic inclusions in serpentinite though the amount of serpentinite rarely exceeds a half of the total exposed rocks. No dunite or peridotite is recognized. Many faults are observed bounding both eastern and western sides of the belt as well as within the belt. The width of the belt varies from less than a hundred meters to over 1000m.
The middle sub-area is bounded on the west by the Routeburn-Caples Ultramafite Belt, and on the east by an inferred fault running N-S along the eastern flank of the Humboldt Mountains. This sub-area is occupied by the Bold Peak Formation which consists mainly of psammitic rocks with minor pelites and rare mafic rocks. The structure appears to by synclinal. Cleavage is weakly developed and lineation is nearly absent. Thickness is about 2000m.
The eastern sub-area is occupied by rocks of the Haast Schist Group. They consist mainly of pelitic and lesser amounts of psammitic schists with minor mafic and siliceous schists. The Haast Schist Group of this sub-area forms a syncline gently plunging to the south. Schistosity and lineation are well developed. Quartzo-feldspathic laminae are well developed in the lower part of the sequence. Thickness in this area is about 2500m. The base is not known.
The rocks of the western sub-area may possibly be lithostratigraphically correlated with the Upper Permian rocks of the Livingstone fault. If this correlation is correct, it is notable that the thickness in this area is much greater than on the western side of the Livingstone fault.
Twenty-nine representative rocks of mafic, pelitic, psammitic, ferruginous, and siliceous compositions from the area have been chemically analysed. The psammites are characterized by rather low SiO2 and K2O, high TiO2, Al2O3, total iron, and Na2O compared with averages in the literature. The pelitic rocks are characterized by high Na2O and low K2/Na2O ratio. High Al2O3 in psammitic rocks may be explained by high percentage of micaceous minerals in the matrix. SiO2 and Al2O3 are inversely related for more mature rocks of pelitic and psammitic composition in the literature, a relation not well shown by some of the rocks from the present study, suggesting low maturity.
All types of analysed rocks of the present study have conspicuously higher Fe2O3/FeO ratios than analyses of higher grade rocks (i.e. above biotite grade) of the Haast Schist Group. Total iron content of pelitic and psammitic rocks in this study is higher than most of the analysed rocks from the New Zealand geosyncline.
Analysed mafic rocks of the upper Wakatipu area plot in all three fields of alkali-olivine basalt, hight alumina basalt, and tholeiite in the (Na2O+K2O)-SiO2 diagram of Kuno (1959, 1968). The K2O/Na2O ratio of mafic rocks is fairly low. Plots on the AFM diagrams show similarities to the differentiation trend of of tholeiites and mildly alkaline rock series. Nearly two-thirds of all analysed mafic rocks have normative quartz.
Relict clinopyroxenes in mafic and psammitic rocks have been analysed by electron microprobe analyser (EPMA). They are rich in SiO2 and poor in Al2O3 and plot within non-alkaline and normal-alkaline host rock fields of LeBas (1962).
Five metamorphic zones in increasing metamorphic grade are discriminated on the basis of mineralogical changes.
Zone I is characterized by the occurrence of prehenite and is found within a limited area around Mt. Bolpland-Bold Peak in the middle sub-area.
Zone II is defined by the occurrence of lawsonite with no actinolite or prehnite. The Upper Peak, Kay's Creek and upper part of the Momus Formation belong to this zone.
Zone IIIa is characterized by actinolite and predominantly blue-green pumpellyite. Lower parts of the Bold Peak and Momus Formations and the Harris Saddle Formation belong to this zone. Alkali-amphiboles mantled by actinolite occur in this zone.
Zone IIIb is characterized by actinolite and predominantly colorless pumpellyite. The higher members occupying a synclinal trough in the local development of the Haast Schist Group belongs to this zone.
Zone IV is characterized by the absence of pumpellyite, epidote being abundant. The lower part of the local development of the Haast Schist Group belongs to this zone.
Zone I corresponds to Ch.1 or transitional between Ch.1 and 2 in textural zonation, Zone II partly to Ch.1 and partly to Ch.2, Zone IIIa partly to Ch.2 and partly Ch1.3, Zone IIIb to Ch1.3 and Zone IV to Ch1.4.
Quartz is nearly ubiquitous and is absent only in such rocks as metasomatized rocks adjacent to serpentinite.
All examined plagioclase feldspars are found to be albite of low temperature structural state.
Muscovite is ubiquitous for all Zones except in few mafic rocks. Two muscovites are analysed by electron probe microanalyser (EPMA). The celadonite content is found to be 23.4-29.0%. Refractive indices of muscovites from all five zones are similar and show rather low values compared with those of eastern Otago and the Sambagawa zone indicating lower celadonite content than in those terranes. Basal spacing of muscovites do not show much difference irrespective of the differences of host rock or metamorphic grade. The only polytype detected is 2M, No 1M or 1Md micas have been found.
Epidote is common throughout the area, especially in mafic rocks. Epidotes from nine rocks have been partially analysed for Al2O3, FeO3 and CaO by EPMA, and a number of epidotes have been examined for Fe+3 content by Myer's (1965, 1966) X-ray diffraction method. Chemical and X-ray analyses seem to match very well. The frequency maximum in epidote composition lies at about Fe+3/(Fe+3 + Al) = .29, in contrast with Brown's (1967) data for Eastern Otago, which show a maximum at 0.19 in rocks of slightly higher grade than those of the upper Wakatipu area. An abrupt change in the epidote composition may thus occur in rocks of grade intermediate between the two areas. This would correspond roughly to the Ch1.3-4 boundary and the disappearance of pumpellyite. Refractive indices of epidote may be slightly higher than those of hydrothermal, metasomatic and other environments.
Lawsonite has anonymously low refractive indices. This may be interpreted by low Fe content of the analysed material which is otherwise quite normal in composition compared with other lawsonites in the literature.
Pumpellyite is predominantly blue-green in Zones I, II, and IIIa while it is predominantly colorless in Zone IIIb. It does not occur in Zone IV. The change of color suggests less ferriferous (more aluminous and/or more magnesian) compositions with increasing metamorphic grade. This trend is supported by a few measurements of optical properties.
Prehnite is identified in Zone I, but is not studied further.
All the carbonates examined in this study are calcite. No aragonite, dolomite, etc. are found.
Actinolite occurs in Zones IIIa, IIIb, and IV. Six actinolites are analysed by EPMA. There is no obvious correlation between the composition of actinolites and metamorphic grade. The composition of actinolite seems to be largely controlled by the host rock composition. On the 2V-Y diagram trend of actinolite may be said to be barroisitic (subcalcic), which is one of the characteristic metamorphic minerals in the glaucophanitic metamorphic terranes (Miyashiro, 1968).
The alkali-amphiboles from Zone IIIa are analysed by EPMA. They are riebeckite and Mg-riebeckite in composition. Glaucophane and crossite are identified optically, but they invariably occur mantled by actinolite or other alkali-amphibole species. Thus, it is concluded that glaucophane and crossite are unstable while riebeckite and Mg-riebeckite are stable in Zone IIIa. Crossite occurs in amphibolite in the Routeburn-Caples Ultramafite Belt.
Ferristilpnomelane is one of the commoner minerals throughout Zone I to IV. Ferristilpnomelane occur only rarely.
Chlorite is one of the commonest and most abundant minerals irrespective of host rocks or metamorphic grades. Si-Al substitution is very limited while Fe-Mg substitution is fairly large. The composition of chlorite is largely related to that of host rock, thought it may be also related to metamorphic grade. Thus chlorite in psammitic and pelitic rocks have higher Fe content than in mafic rocks. The basal spacing of chlorite shows a good inverse correlation with Al content. The basal spacings in psammitic and pelitic rocks are smaller than in mafic rocks corresponding to higher Al content in the former. Variation in chlorite composition with metamorphic grade is not clear, although comparison with the chlorite from eastern Otago (Brown, 1967), where metamorphic grade is slightly higher than this area, suggests that chlorite may become less magnesian, more ferroan, and more aluminous with increasing grade of metamorphism.
Insufficiant data are obtained to discuss partition of elements between coexisting mineral pairs.
Tourmaline is commonly found in pelitic and psammitic rocks though it is minor in amount.
Axinite occurs as veinlets in rocks of Zones I, II, and IIIa. It was not found in the Haast Schist Group (Zones IIIb and IV).
One analysed garnet in Zone IIIa is anomalously rich in andradite molecule. Spessartine (Zone IV), and grossularite and hydro-garnet (both in Routeburn-Caples Ultramafite Belt) are also recorded.
Optical properties of relict hornblendes and clinopyroxenes are determined. Judging from Mg/Fe ratio estimate by optical properties the source rocks of relict hornblendes may be intermediate rocks.
Biotite, zircon, allanite, epidote, muscovite, garnet, apatite, tourmaline, and orthoclase are among the relict minerals recorded.
Serpentine minerals in the Routeburn-Caples Ultramafite Belt are identified as orth- and clino-crysotile. No antigorite nor lizardite has been found. Talc, picotite, magnetite, and asbestiform tremolite are not uncommon.
The metamorphic mineral assemblages in each Zone are given.
Possible chemical reactions defining isogrades are discussed.
The isograd between Zones II and IIIa are discussed in some detail using uH2O-uCO2 diagram for the system Al2O3-(Mg, Fe)O-CaO-H2O-CO2.
P-T conditions of this area are discussed referring to the latest experimental data by Liou (in press a, b).
The present area is considered to be the first recorded example of the facies series prehenite-actinolite, and greenschist facies.
The area is divided into three sub-areas, western, middle, and eastern, by two major, north-striking faults.
The western side of western sub-area is bounded by the Livingstone fault with a serpentinite of the Red Mountain Ultramafites lying immediately beyond. The western sub-area comprises four formations forming the eastern limb of a partly overturned anticline. The western limb of this anticline has been removed by the Livingstone fault.
The Harris Saddle Formation, which is the lowest in the western sub-area sequence, consists mainly of mafic schists with minor pelitic, psammitic siliceous and ferruginous schists. The mafic schists are derived from mafic tuff, subordinate lava flows or sills, and tuff breccia. Distinct schistosity and lineation are developed. Locally, superposed folding is recognized. General attitude is N-S strike with an eastward dip. The thickness is over 3000m., the base being unknown.
The Momus Formation, conformably overlying the Harris Saddle Formation, comprises a rather regular intercalation of psammitic and pelitic rocks, psammites predominating. Cleavage is distinct to poor and lineation is poorly developed or absent. Sedimentary structures such as load casts, slumping, and grading are common. The strike is generally northwards with eastward dips in the west, and westward dips in the east. Facing directions are invariably to the east. The thickness is about 3000m.
Kay's Creek Formation, conformably overlying the Momus Formation, consists of mafic to intermediate volcanic sandstone and tuffite. Schistosity and lineation are almost absent. Sedimentary structures such as slumping and convolute lamination are abundant. Bedding generally strikes N-S with vertical westward dips. The thickness is about 1200m. at Caples River but appears to become thinner towards the north.
Upper Peak Formation conformably overlies Kay's Creek Formation, and consists mainly of regularly intercalated psammites and pelites. Graded bedding is common. Almost isoclinal folding is conspicuous especially toward the east where the formation is in the fault contact with the Routeburn-Caples Ultramafite Belt. Axial plane cleavage and lineation are poorly developed. The strike is generally northwards, and the dip moderate to the west. The thickness is about 2000m. or less.
Except for the Harris Saddle Formation, the same formations are found in the same order of superposition to the north of a fault running parallel to the Rockburn.
Routeburn-Caples Ultramafite Belt, which separates the middle sub-area from the western sub-area is a structural belt running N-S, and noth hitherto recognized as a continuous unit. It is composed of very divergent rocks including serpentinitem rodingite, metagabbro, amphibolite, mafic lava, mafic, pelitic, and psammitic schists, and metasomatized mafic, pelitic, and psammitic schists. Hardening of pelitic rocks in contact with serpentinite is not uncommon. Serpentinite invariably occurs throughout the belt. Other rock types in the above list are interpreted mostly as tectonic inclusions in serpentinite though the amount of serpentinite rarely exceeds a half of the total exposed rocks. No dunite or peridotite is recognized. Many faults are observed bounding both eastern and western sides of the belt as well as within the belt. The width of the belt varies from less than a hundred meters to over 1000m.
The middle sub-area is bounded on the west by the Routeburn-Caples Ultramafite Belt, and on the east by an inferred fault running N-S along the eastern flank of the Humboldt Mountains. This sub-area is occupied by the Bold Peak Formation which consists mainly of psammitic rocks with minor pelites and rare mafic rocks. The structure appears to by synclinal. Cleavage is weakly developed and lineation is nearly absent. Thickness is about 2000m.
The eastern sub-area is occupied by rocks of the Haast Schist Group. They consist mainly of pelitic and lesser amounts of psammitic schists with minor mafic and siliceous schists. The Haast Schist Group of this sub-area forms a syncline gently plunging to the south. Schistosity and lineation are well developed. Quartzo-feldspathic laminae are well developed in the lower part of the sequence. Thickness in this area is about 2500m. The base is not known.
The rocks of the western sub-area may possibly be lithostratigraphically correlated with the Upper Permian rocks of the Livingstone fault. If this correlation is correct, it is notable that the thickness in this area is much greater than on the western side of the Livingstone fault.
Twenty-nine representative rocks of mafic, pelitic, psammitic, ferruginous, and siliceous compositions from the area have been chemically analysed. The psammites are characterized by rather low SiO2 and K2O, high TiO2, Al2O3, total iron, and Na2O compared with averages in the literature. The pelitic rocks are characterized by high Na2O and low K2/Na2O ratio. High Al2O3 in psammitic rocks may be explained by high percentage of micaceous minerals in the matrix. SiO2 and Al2O3 are inversely related for more mature rocks of pelitic and psammitic composition in the literature, a relation not well shown by some of the rocks from the present study, suggesting low maturity.
All types of analysed rocks of the present study have conspicuously higher Fe2O3/FeO ratios than analyses of higher grade rocks (i.e. above biotite grade) of the Haast Schist Group. Total iron content of pelitic and psammitic rocks in this study is higher than most of the analysed rocks from the New Zealand geosyncline.
Analysed mafic rocks of the upper Wakatipu area plot in all three fields of alkali-olivine basalt, hight alumina basalt, and tholeiite in the (Na2O+K2O)-SiO2 diagram of Kuno (1959, 1968). The K2O/Na2O ratio of mafic rocks is fairly low. Plots on the AFM diagrams show similarities to the differentiation trend of of tholeiites and mildly alkaline rock series. Nearly two-thirds of all analysed mafic rocks have normative quartz.
Relict clinopyroxenes in mafic and psammitic rocks have been analysed by electron microprobe analyser (EPMA). They are rich in SiO2 and poor in Al2O3 and plot within non-alkaline and normal-alkaline host rock fields of LeBas (1962).
Five metamorphic zones in increasing metamorphic grade are discriminated on the basis of mineralogical changes.
Zone I is characterized by the occurrence of prehenite and is found within a limited area around Mt. Bolpland-Bold Peak in the middle sub-area.
Zone II is defined by the occurrence of lawsonite with no actinolite or prehnite. The Upper Peak, Kay's Creek and upper part of the Momus Formation belong to this zone.
Zone IIIa is characterized by actinolite and predominantly blue-green pumpellyite. Lower parts of the Bold Peak and Momus Formations and the Harris Saddle Formation belong to this zone. Alkali-amphiboles mantled by actinolite occur in this zone.
Zone IIIb is characterized by actinolite and predominantly colorless pumpellyite. The higher members occupying a synclinal trough in the local development of the Haast Schist Group belongs to this zone.
Zone IV is characterized by the absence of pumpellyite, epidote being abundant. The lower part of the local development of the Haast Schist Group belongs to this zone.
Zone I corresponds to Ch.1 or transitional between Ch.1 and 2 in textural zonation, Zone II partly to Ch.1 and partly to Ch.2, Zone IIIa partly to Ch.2 and partly Ch1.3, Zone IIIb to Ch1.3 and Zone IV to Ch1.4.
Quartz is nearly ubiquitous and is absent only in such rocks as metasomatized rocks adjacent to serpentinite.
All examined plagioclase feldspars are found to be albite of low temperature structural state.
Muscovite is ubiquitous for all Zones except in few mafic rocks. Two muscovites are analysed by electron probe microanalyser (EPMA). The celadonite content is found to be 23.4-29.0%. Refractive indices of muscovites from all five zones are similar and show rather low values compared with those of eastern Otago and the Sambagawa zone indicating lower celadonite content than in those terranes. Basal spacing of muscovites do not show much difference irrespective of the differences of host rock or metamorphic grade. The only polytype detected is 2M, No 1M or 1Md micas have been found.
Epidote is common throughout the area, especially in mafic rocks. Epidotes from nine rocks have been partially analysed for Al2O3, FeO3 and CaO by EPMA, and a number of epidotes have been examined for Fe+3 content by Myer's (1965, 1966) X-ray diffraction method. Chemical and X-ray analyses seem to match very well. The frequency maximum in epidote composition lies at about Fe+3/(Fe+3 + Al) = .29, in contrast with Brown's (1967) data for Eastern Otago, which show a maximum at 0.19 in rocks of slightly higher grade than those of the upper Wakatipu area. An abrupt change in the epidote composition may thus occur in rocks of grade intermediate between the two areas. This would correspond roughly to the Ch1.3-4 boundary and the disappearance of pumpellyite. Refractive indices of epidote may be slightly higher than those of hydrothermal, metasomatic and other environments.
Lawsonite has anonymously low refractive indices. This may be interpreted by low Fe content of the analysed material which is otherwise quite normal in composition compared with other lawsonites in the literature.
Pumpellyite is predominantly blue-green in Zones I, II, and IIIa while it is predominantly colorless in Zone IIIb. It does not occur in Zone IV. The change of color suggests less ferriferous (more aluminous and/or more magnesian) compositions with increasing metamorphic grade. This trend is supported by a few measurements of optical properties.
Prehnite is identified in Zone I, but is not studied further.
All the carbonates examined in this study are calcite. No aragonite, dolomite, etc. are found.
Actinolite occurs in Zones IIIa, IIIb, and IV. Six actinolites are analysed by EPMA. There is no obvious correlation between the composition of actinolites and metamorphic grade. The composition of actinolite seems to be largely controlled by the host rock composition. On the 2V-Y diagram trend of actinolite may be said to be barroisitic (subcalcic), which is one of the characteristic metamorphic minerals in the glaucophanitic metamorphic terranes (Miyashiro, 1968).
The alkali-amphiboles from Zone IIIa are analysed by EPMA. They are riebeckite and Mg-riebeckite in composition. Glaucophane and crossite are identified optically, but they invariably occur mantled by actinolite or other alkali-amphibole species. Thus, it is concluded that glaucophane and crossite are unstable while riebeckite and Mg-riebeckite are stable in Zone IIIa. Crossite occurs in amphibolite in the Routeburn-Caples Ultramafite Belt.
Ferristilpnomelane is one of the commoner minerals throughout Zone I to IV. Ferristilpnomelane occur only rarely.
Chlorite is one of the commonest and most abundant minerals irrespective of host rocks or metamorphic grades. Si-Al substitution is very limited while Fe-Mg substitution is fairly large. The composition of chlorite is largely related to that of host rock, thought it may be also related to metamorphic grade. Thus chlorite in psammitic and pelitic rocks have higher Fe content than in mafic rocks. The basal spacing of chlorite shows a good inverse correlation with Al content. The basal spacings in psammitic and pelitic rocks are smaller than in mafic rocks corresponding to higher Al content in the former. Variation in chlorite composition with metamorphic grade is not clear, although comparison with the chlorite from eastern Otago (Brown, 1967), where metamorphic grade is slightly higher than this area, suggests that chlorite may become less magnesian, more ferroan, and more aluminous with increasing grade of metamorphism.
Insufficiant data are obtained to discuss partition of elements between coexisting mineral pairs.
Tourmaline is commonly found in pelitic and psammitic rocks though it is minor in amount.
Axinite occurs as veinlets in rocks of Zones I, II, and IIIa. It was not found in the Haast Schist Group (Zones IIIb and IV).
One analysed garnet in Zone IIIa is anomalously rich in andradite molecule. Spessartine (Zone IV), and grossularite and hydro-garnet (both in Routeburn-Caples Ultramafite Belt) are also recorded.
Optical properties of relict hornblendes and clinopyroxenes are determined. Judging from Mg/Fe ratio estimate by optical properties the source rocks of relict hornblendes may be intermediate rocks.
Biotite, zircon, allanite, epidote, muscovite, garnet, apatite, tourmaline, and orthoclase are among the relict minerals recorded.
Serpentine minerals in the Routeburn-Caples Ultramafite Belt are identified as orth- and clino-crysotile. No antigorite nor lizardite has been found. Talc, picotite, magnetite, and asbestiform tremolite are not uncommon.
The metamorphic mineral assemblages in each Zone are given.
Possible chemical reactions defining isogrades are discussed.
The isograd between Zones II and IIIa are discussed in some detail using uH2O-uCO2 diagram for the system Al2O3-(Mg, Fe)O-CaO-H2O-CO2.
P-T conditions of this area are discussed referring to the latest experimental data by Liou (in press a, b).
The present area is considered to be the first recorded example of the facies series prehenite-actinolite, and greenschist facies.
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xxii,219,Illustrations, Tables, Maps (folded in pocket) ; 27cm
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POLYGON ((168.223649439701205 -44.683315621267596,168.340815005458268 -44.688356823877271,168.377561325552165 -44.77089256800965,168.330461431679936 -44.849875226854031,168.220507202841247 -44.835610988500392,168.223649439701205 -44.683315621267596))
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Kawachi, Y., “Geology and metamorphism near the head of Lake Wakatipu.,” Otago Geology Theses, accessed April 23, 2025, https://theses.otagogeology.org.nz/items/show/47.