Some aspects of the geology of the George Sound Track area, North-Central Fiordland
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This report describes the geology of approximately 1000km2 of basement rocks exposed in the George Sound Track area, north-central Fiordland.
Two regions are recognised which are broadly comparable to the Eastern and Central regions of Oliver and Coggon (1979).
The Eastern Units described in Part A consist of weakly metamorphosed acid to basic intrusive rocks, together with a series of previously unknown volcanic rocks, termed the Loch Burn Formation.
Within the intrusive rocks, five units are recognised: Halfway Peak Hornblende Gabbro, Lake Roxburgh Tonalite, Howitt Peaks Gabbro, Lugar Burn Quartz Monzonite and North Fiord Granite.
The Loch Burn Formation consists of a fault-bound sliver of lavas, volcaniclastic sediments and dyke rocks. They have been metamorphosed to conditions of the lower-pressure part of the greenschist facies. The biotite isograd has been attained at relatively low pressures for a limited range of bulk compositions.
Field relations among the Eastern Units indicate that the Howitt Peaks Gabbro is probably the oldest unit, as it is intruded by the Lake Roxburgh Tonalite. In turn, clasts of Lake Roxburgh Tonalite have been tentatively identified within the Loch Burn Formation. Zircon U-Pb data for the Loch Burn Formation indicate an igneous age around 210 Ma. The Loch Burn Formation, although largely faulted against the surrounding intrusives, appears to be intruded by the Halfway Peak Hornblende Gabbro in North Fiord. Granitoids of the Lugar Burn Quartz Monzonite and the North Fiord Granite intrude the Halfway Peak Hornblende Gabbro, an intrusive age of around 110 Ma is envisaged for the North Fiord Granite, based on discordant U-Pb age data. It is concluded that repeated extrusive and intrusive activity in southern New Zealand occurred throughout almost the entire Mesozoic.
The Central Units are separated from the Eastern Units by a major fault: the Howitt Peaks Fault. They consist of four units; Lake Hankinson Complex, Lake Wade Diorite, Lake Wapiti Paragneiss and Mount Kurrell Amphibolite. The Lake Hankinson Complex consists largely of tonalitic orthogneiss, with subordinate amphibolite. It is thought to intrude both the Mount Murrell Amphibolite and the Lake Wapiti Paragneiss, whereas the Lake Wade Diorite appears to intrude the Lake Hankinson Complex. Contacts between the Lake Hankinson Complex and the Western Fiordland Orthogneiss are largely faulted in the George Sound Track area.
The Lake Wade Diorite is a largely homogenous body of amphibolitised diorite and is correlated with the Mount George Gabbro of Codling (1977) and Gibson (1979).
The Lake Wapiti Paragneiss is a complex unit and consists of calcareous gneisses (including marbles), calc-silicate gneisses, biotite hornblende gneisses, amphibolite and minor K-poor aluminous gneiss.
Textural evidence for kyanite pseudomorphous after sillimanite is present in the K-poor aluminous gneiss.
This predominantly metasedimentary sequence has been intruded by leucocratic granitoid gneisses. which contain xenoliths of basic and ultrabasic material.
The Mount Hurrell Amphibolite represents fragments of a metamorphosed layered basic to ultrabasic igneous complex and occurs as several small xenolithic bodies less than 2km across, enclosed in Lake Hankinson Complex. The Mount Murrell Amphibolite is correlated with the Black Giants Complex of Gibson (1979).
Relicts of the magmatic stage of the Mount Murrell Amphibolite include symplectite corona structures which have developed at olivine-plagioclase grain boundaries.
Aluminous bulk compositions have resulted in metamorphic assemblages which, together with mineralogical data, are used to estimate P-T conditions of deformation and recrystallisation. Textural and other data are presented which suggest that an early, relatively anhydrous equilibration, at pressures up to 9-10 kB, generated assemblages involving some or all of the following: pargasitic amphibole, calcic plagioclase, kyanite, corundum, staurolite, margarite and clinozoisite. These rocks have been overprinted to various degrees by relatively hydrous assemblages involving sodic to intermediate plagioclase, clinozoisite, margarite and quartz. P-T conditions for this later equilibration are estimated to be 560°C, 7kB based on experimentally and theoretically derived equilibria in the system CaO-Al2O3-SiO2-H2O.
Structural elements of the Central Units indicate that axial planar structures, which developed during a first phase of deformation, have been folded about a second generation fold axis. As this fold axis is coincident with linear structures in the Lake Hankinson Complex, Mount Kurrell Amphibolite, Lake Wapiti Paragneiss and Lake Wade Diorite, these four units are thought to have undergone this pervasive and coherent second phase of deformation. The second phase of deformation has strongly overprinted all earlier structures and largely obliterated them.
The Lake Wapiti Paragneiss is thought to constitute the country-rock of a large part of the mapping area. Contact relations indicate that both the Lake Wapiti Paragneiss and the Mount Murrell Amphibolite are older than the Lake Hankinson Complex. Partially melted pelitic, semipelitic and calc-silicate material within the Mount Hurrell Amphibolite is thought to represent fragments of the country-rock into which the Mount Murrell Amphibolite was intruded: this country rock is not, however, considered to be equivalent to the Lake Wapiti Paragneiss.
Zircon U-Pb data for the Lake Hankinson Complex do not unequivocally define an igneous age for the intrusive part of the body. However, age data for the Mount George Gabbro, here considered to be a correlative of the Lake Wade Diorite, may indicate a minimum age of around 140 Ma for the Lake Hankinson Co1nplex, which is intruded by the Lake Wade Diorite.
In a synthesis of the structural and metamorphic history of the Central Units, the Mount Kurrell Amphibolite is considered to have been intruded at pressures around 9kB. The Lake Hankinson Complex was intruded near the tonalite solidus (excess water); P-T conditions for this are envisaged as being between 8-9 kB at 6500 to 725° C. Deformation of all units had effectively ceased below P-T conditions of 5600, 7kB.
It is concluded that widespread plutonism and penecontemporaneous metamorphism at about 140 Ka appears to have been the most important event in the evolution of the Central Units.
Two regions are recognised which are broadly comparable to the Eastern and Central regions of Oliver and Coggon (1979).
The Eastern Units described in Part A consist of weakly metamorphosed acid to basic intrusive rocks, together with a series of previously unknown volcanic rocks, termed the Loch Burn Formation.
Within the intrusive rocks, five units are recognised: Halfway Peak Hornblende Gabbro, Lake Roxburgh Tonalite, Howitt Peaks Gabbro, Lugar Burn Quartz Monzonite and North Fiord Granite.
The Loch Burn Formation consists of a fault-bound sliver of lavas, volcaniclastic sediments and dyke rocks. They have been metamorphosed to conditions of the lower-pressure part of the greenschist facies. The biotite isograd has been attained at relatively low pressures for a limited range of bulk compositions.
Field relations among the Eastern Units indicate that the Howitt Peaks Gabbro is probably the oldest unit, as it is intruded by the Lake Roxburgh Tonalite. In turn, clasts of Lake Roxburgh Tonalite have been tentatively identified within the Loch Burn Formation. Zircon U-Pb data for the Loch Burn Formation indicate an igneous age around 210 Ma. The Loch Burn Formation, although largely faulted against the surrounding intrusives, appears to be intruded by the Halfway Peak Hornblende Gabbro in North Fiord. Granitoids of the Lugar Burn Quartz Monzonite and the North Fiord Granite intrude the Halfway Peak Hornblende Gabbro, an intrusive age of around 110 Ma is envisaged for the North Fiord Granite, based on discordant U-Pb age data. It is concluded that repeated extrusive and intrusive activity in southern New Zealand occurred throughout almost the entire Mesozoic.
The Central Units are separated from the Eastern Units by a major fault: the Howitt Peaks Fault. They consist of four units; Lake Hankinson Complex, Lake Wade Diorite, Lake Wapiti Paragneiss and Mount Kurrell Amphibolite. The Lake Hankinson Complex consists largely of tonalitic orthogneiss, with subordinate amphibolite. It is thought to intrude both the Mount Murrell Amphibolite and the Lake Wapiti Paragneiss, whereas the Lake Wade Diorite appears to intrude the Lake Hankinson Complex. Contacts between the Lake Hankinson Complex and the Western Fiordland Orthogneiss are largely faulted in the George Sound Track area.
The Lake Wade Diorite is a largely homogenous body of amphibolitised diorite and is correlated with the Mount George Gabbro of Codling (1977) and Gibson (1979).
The Lake Wapiti Paragneiss is a complex unit and consists of calcareous gneisses (including marbles), calc-silicate gneisses, biotite hornblende gneisses, amphibolite and minor K-poor aluminous gneiss.
Textural evidence for kyanite pseudomorphous after sillimanite is present in the K-poor aluminous gneiss.
This predominantly metasedimentary sequence has been intruded by leucocratic granitoid gneisses. which contain xenoliths of basic and ultrabasic material.
The Mount Hurrell Amphibolite represents fragments of a metamorphosed layered basic to ultrabasic igneous complex and occurs as several small xenolithic bodies less than 2km across, enclosed in Lake Hankinson Complex. The Mount Murrell Amphibolite is correlated with the Black Giants Complex of Gibson (1979).
Relicts of the magmatic stage of the Mount Murrell Amphibolite include symplectite corona structures which have developed at olivine-plagioclase grain boundaries.
Aluminous bulk compositions have resulted in metamorphic assemblages which, together with mineralogical data, are used to estimate P-T conditions of deformation and recrystallisation. Textural and other data are presented which suggest that an early, relatively anhydrous equilibration, at pressures up to 9-10 kB, generated assemblages involving some or all of the following: pargasitic amphibole, calcic plagioclase, kyanite, corundum, staurolite, margarite and clinozoisite. These rocks have been overprinted to various degrees by relatively hydrous assemblages involving sodic to intermediate plagioclase, clinozoisite, margarite and quartz. P-T conditions for this later equilibration are estimated to be 560°C, 7kB based on experimentally and theoretically derived equilibria in the system CaO-Al2O3-SiO2-H2O.
Structural elements of the Central Units indicate that axial planar structures, which developed during a first phase of deformation, have been folded about a second generation fold axis. As this fold axis is coincident with linear structures in the Lake Hankinson Complex, Mount Kurrell Amphibolite, Lake Wapiti Paragneiss and Lake Wade Diorite, these four units are thought to have undergone this pervasive and coherent second phase of deformation. The second phase of deformation has strongly overprinted all earlier structures and largely obliterated them.
The Lake Wapiti Paragneiss is thought to constitute the country-rock of a large part of the mapping area. Contact relations indicate that both the Lake Wapiti Paragneiss and the Mount Murrell Amphibolite are older than the Lake Hankinson Complex. Partially melted pelitic, semipelitic and calc-silicate material within the Mount Hurrell Amphibolite is thought to represent fragments of the country-rock into which the Mount Murrell Amphibolite was intruded: this country rock is not, however, considered to be equivalent to the Lake Wapiti Paragneiss.
Zircon U-Pb data for the Lake Hankinson Complex do not unequivocally define an igneous age for the intrusive part of the body. However, age data for the Mount George Gabbro, here considered to be a correlative of the Lake Wade Diorite, may indicate a minimum age of around 140 Ma for the Lake Hankinson Co1nplex, which is intruded by the Lake Wade Diorite.
In a synthesis of the structural and metamorphic history of the Central Units, the Mount Kurrell Amphibolite is considered to have been intruded at pressures around 9kB. The Lake Hankinson Complex was intruded near the tonalite solidus (excess water); P-T conditions for this are envisaged as being between 8-9 kB at 6500 to 725° C. Deformation of all units had effectively ceased below P-T conditions of 5600, 7kB.
It is concluded that widespread plutonism and penecontemporaneous metamorphism at about 140 Ka appears to have been the most important event in the evolution of the Central Units.
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xvii. 227 p. ill. Maps. 30cm.
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1984King
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King, RP, “Some aspects of the geology of the George Sound Track area, North-Central Fiordland,” Otago Geology Theses, accessed August 8, 2026, https://theses.otagogeology.org.nz/items/show/178.