Geology of the west dome sector of the Dun mountain ophiolite belt.
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The West Dome sector of the Dun Mountain ophiolite belt has been mapped on a scale of 4 inches to 1 mile (approximately 1:16000). The area covers approximately 200 sq.km. (80 sq. ml.) and has been subdivided into northwest - southeast trending structural units that are correlated with the terranes of Coombs et al.(1976a). Minor but significant variations from this trend occur. Major faults are present at the contacts of these units.
The following stratigraphy (from northeast to southwest) is recognised at West Dome:
A. CAPLES - PELORUS TERRANE: This terrane contains sedimentary and igneous rocks which are in fault contact. Local informal nomenclature is proposed for these strata at West Dome.
Acton Downs melange: This unit comprises tectonic inclusions of meta-igneous rocks and minor sedimentary ones that have undergone at least one period of cataclasis, set in an obscure matrix.
Gyzeh volcanics: They contain mainly non-pillowed metabasalts and together with the Acton Downs melange, probably represent part of a much dismembered ophiolite sequence.
Diston stream sedimentary unit: It can be subdivided into a lower, mainly coarse grained sandstone assemblage, and an upper, mainly fine grained assemblage of thinly interbedded fine sandstone and mudstone.
B. HUMBOLDT GROUP: This group occurs as a dismembered ophiolite sequence at West Dome, the ultramafic portion giving rise to well developed knocker topography.
Red Mountain Ultramafite: This subgroup is almost completely serpentinised and is present as a melange. The tectonic inclusions are dominated by metagabbros and metadolerites, but minor metabasalts rare sedimentary rocks and albite amphibolites occur in a serpentinite matrix. The meta-igneous rocks are derived from the overlying Livingstone Subgroup but the sedimentary inclusions, some containing
Atomodesma, are of unknown origin.
Livingstone Subgroup: This subgroup has been divided into four informal units of which units (ii) - (iv) are considered to be stratigraphically continuous and directly comparable with the upper part of an ophiolite sequence.
unit (i) comprises non-pillowed metabasalts locally intruded by metagabbros.
unit (ii) is dominated by metadolerites with minor amounts of metagabbros and metabasalts. Where contacts have been observed finer grained rocks intrude coarsed grianed ones.
unit (iii) contains metadolerites and meta-pillow-basalt, the latter increasing in abundance stratigraphically upwards, and passing into unit (iv) in which meta-pillow-basalt and meta-pillowbreccia are present in subequal proportions.
C. BRYNEIRA GROUP: This group has been redefined to include the following formations:
Upukerora Formation: comprising basic volcanogenic conglomerates, sandstones and mudstones.
Howden Formation: comprising limestones and basic volcanogenic sandstones and mudstones.
Annear Formation: comprising significantly more quartz-rich sandstones and siltstones
Tapara Formation: comprising basic volcanogenic sandstones, siltstones and claystones. This formation has been divided into eight informal units based on distinctive lithology.
Winton Formation: comprising very thin interbedaed volcanogenic sandstones ru~d mudstones.
D. STEPHENS GROUP: This is a new group uniting the Countess and Snowdon Formations. These formations have similar lithologies, mainly massive sandstones, with fewer thin mudstones and thick conglomerates, and indicate a notable change in depositional character from that of the underlying Bryneira Group.
A Mid- to Late-Permian age for the sedimentary strata is suggested by the occurrence of Atomodesma trabeculum Waterhouse in the Diston stream sedimentary unit, by A. trechmanni Marwick in the Annear Formation, and a rich late Permian fauna at the top of the Countess Formation. This fauna includes fragments and moulds of brachiopods, corals, bivalves, gastropods, bryozoans, echinoderms and a trilobite.
Calcite prisms, regarded as comminuted Atomodesma shells, are present in the Upukerora, Howden, Annear and Snowdon Formations, but more significantly, also in unit (iv) of the Livingstone Subgroup. Their presence strongly suggests a Permian age for the Humboldt Group.
The major lithfacies recognised are conglomerate, sandstone, interbedded sandstone and mudstone, with minor amounts of limestone, pebbly sandstone and intraformational conglomerate. The lithofacies are considered to have been deposited in deep water by mass flow and turbidity currents. Modal analyses indicate the sandstones to be mainly lithic volcarenites derived from a basic to intermediate source area. The Annear Formation is exceptional because the sandstones are very significantly enriched in quartz. Bryneira and Stephens Group strata are considered to have been deposited in the axial trough of an elongate basin.
Chemical analyses of red and non-red strata indicate that the red colouration varies with Fe2O3 FeO ratio. Red and non-red beds probably reflect variations in the palaeoclimate directly or indirectly attributable to Gondwanan Permian ice-age.
Relict igneous minerals and ghost textures in the Red Mountain Ultramafite indicate a mainly harzburgite parent. Microprobe analyses of olivine, orthopyroxene and chromian clinopyroxene give compositions of Fo90 ; En90-92 ; Wo49 En48 Fs3 respectively and also indicate that the original opaque phase was chromian spinel. These values are directly comparable to mineral compositions from ophiolite sequences and Alpine - type ultrabasic complexes.
Kizardite, chrysotile and magnetite are the major metamorphic minerals in the Red Mountain Ultramafite. Serpentinisation was probably initiated during an ocean-floor metamorphic episode, further alteration taking place during and possibly after the Rangitata Orogeny. The present mineral assemblage indicates temperatures of formation of less than 300°C and a few kilobars pressure, conditions that are comparable to those producing prehnite - pumpellyite facies assemblages in adjacent strata.
Igneous textures are usually well preserved in the Livingstone Subgroup but metamorphic reconstitution is such that only three relict igneous phases now occur. Only Ca-rich pyroxene has been observed and it shows increasing alteration stratigraphically downwards, being most altered in the tectonic inclusions. Microprobe analyses of these clinopyroxenes indicate tholeiitic affinities for these rocks and using the criteria of Nisbet and Pearce (1977) the analyses suggest that the rocks were formed in an ocean-floor environment. Brown hornblende is restricted to the tectonic inclusions and is considered to have crystallised from an evolving magma. Opaque minerals invariably show alteration to sphene and this, together with microprobe data, indicates an original iron - titania oxide phase.
Zeolite facies metamorphic assemblages in the southwest of the area pass northeastwards through a narrow zone of lawsonite - albite - quartz facies into an extensive area of prehnite - pumpellyite facies mineral assemblages. This low-grade regional metamorphism is associated with the Mesozoic Rangitata Orogeny. A temperature gradient of about 23C/km was operative during this event. Maximum pressures of approximately 5 kb and temperatures of approximately 380°C were attained.
Prehnite - pumpellyite facies mineral assemblages overprint greenschist facies mineral assemblages in the metadolerites, metagabbros and albite amphibolites. Microprobe analyses of amphiboles from these rocks show a compositional gap within and between grains, but a miscibility gap is not developed. Amphibole compositions are governed initially by precursor mineral composition, later being modified by host rock composition. These results suggest that conditions of metamorphic equilibrium have not been attained.
The meta-igneous rocks were metamorphosed under ocean-floor conditions of low pressure and of temperatures up to at least 400C, possibly reaching 600C locally. This metamorphic event was of an early Permian age, and was later overprinted during the Rangitata Orogeny.
Co-existing phases in the Livingstone Subgroup include pumpellyite(Fe), albite, iron-rich epidote, chlorite, celadonite, prehnite, sphene, quartz and calcite. (Microprobe analyses are given for all but quartz and calcite). Andradite amd intermediate members of the andradite - grossularite series occur with prehnite, epidote, chlorite, quartz, calcite and an opaque phase in vesicle infillings of a non-pillowed metabasalt. Conditions of formation of this mineral assemblage involved temperatures of less than 300°C, possibly even less than 100C and pressue pressures of a few kilobars.
Several models are discussed regarding the environment in which the strata at West Dome originated, and the ensuing tectonic conditions that brought about the juxtaposition of these rocks. The simplest model is that of a single Permian - Cretaceous volcanic arc - arc-trench gap - trench in which Humboldt Group (already metamorphosed under ocean-floor conditions) formed the upper plate of the subduction zone and on which Bryneira and Stephens Group strata were deposited in the arc-trench gap; Caples ·· Pelorus terrane rocks probably formed in part of the trench. All of these rocks were folded and regionally metamorphosed during the Mesozoic Rangitata Orogeny.
The following stratigraphy (from northeast to southwest) is recognised at West Dome:
A. CAPLES - PELORUS TERRANE: This terrane contains sedimentary and igneous rocks which are in fault contact. Local informal nomenclature is proposed for these strata at West Dome.
Acton Downs melange: This unit comprises tectonic inclusions of meta-igneous rocks and minor sedimentary ones that have undergone at least one period of cataclasis, set in an obscure matrix.
Gyzeh volcanics: They contain mainly non-pillowed metabasalts and together with the Acton Downs melange, probably represent part of a much dismembered ophiolite sequence.
Diston stream sedimentary unit: It can be subdivided into a lower, mainly coarse grained sandstone assemblage, and an upper, mainly fine grained assemblage of thinly interbedded fine sandstone and mudstone.
B. HUMBOLDT GROUP: This group occurs as a dismembered ophiolite sequence at West Dome, the ultramafic portion giving rise to well developed knocker topography.
Red Mountain Ultramafite: This subgroup is almost completely serpentinised and is present as a melange. The tectonic inclusions are dominated by metagabbros and metadolerites, but minor metabasalts rare sedimentary rocks and albite amphibolites occur in a serpentinite matrix. The meta-igneous rocks are derived from the overlying Livingstone Subgroup but the sedimentary inclusions, some containing
Atomodesma, are of unknown origin.
Livingstone Subgroup: This subgroup has been divided into four informal units of which units (ii) - (iv) are considered to be stratigraphically continuous and directly comparable with the upper part of an ophiolite sequence.
unit (i) comprises non-pillowed metabasalts locally intruded by metagabbros.
unit (ii) is dominated by metadolerites with minor amounts of metagabbros and metabasalts. Where contacts have been observed finer grained rocks intrude coarsed grianed ones.
unit (iii) contains metadolerites and meta-pillow-basalt, the latter increasing in abundance stratigraphically upwards, and passing into unit (iv) in which meta-pillow-basalt and meta-pillowbreccia are present in subequal proportions.
C. BRYNEIRA GROUP: This group has been redefined to include the following formations:
Upukerora Formation: comprising basic volcanogenic conglomerates, sandstones and mudstones.
Howden Formation: comprising limestones and basic volcanogenic sandstones and mudstones.
Annear Formation: comprising significantly more quartz-rich sandstones and siltstones
Tapara Formation: comprising basic volcanogenic sandstones, siltstones and claystones. This formation has been divided into eight informal units based on distinctive lithology.
Winton Formation: comprising very thin interbedaed volcanogenic sandstones ru~d mudstones.
D. STEPHENS GROUP: This is a new group uniting the Countess and Snowdon Formations. These formations have similar lithologies, mainly massive sandstones, with fewer thin mudstones and thick conglomerates, and indicate a notable change in depositional character from that of the underlying Bryneira Group.
A Mid- to Late-Permian age for the sedimentary strata is suggested by the occurrence of Atomodesma trabeculum Waterhouse in the Diston stream sedimentary unit, by A. trechmanni Marwick in the Annear Formation, and a rich late Permian fauna at the top of the Countess Formation. This fauna includes fragments and moulds of brachiopods, corals, bivalves, gastropods, bryozoans, echinoderms and a trilobite.
Calcite prisms, regarded as comminuted Atomodesma shells, are present in the Upukerora, Howden, Annear and Snowdon Formations, but more significantly, also in unit (iv) of the Livingstone Subgroup. Their presence strongly suggests a Permian age for the Humboldt Group.
The major lithfacies recognised are conglomerate, sandstone, interbedded sandstone and mudstone, with minor amounts of limestone, pebbly sandstone and intraformational conglomerate. The lithofacies are considered to have been deposited in deep water by mass flow and turbidity currents. Modal analyses indicate the sandstones to be mainly lithic volcarenites derived from a basic to intermediate source area. The Annear Formation is exceptional because the sandstones are very significantly enriched in quartz. Bryneira and Stephens Group strata are considered to have been deposited in the axial trough of an elongate basin.
Chemical analyses of red and non-red strata indicate that the red colouration varies with Fe2O3 FeO ratio. Red and non-red beds probably reflect variations in the palaeoclimate directly or indirectly attributable to Gondwanan Permian ice-age.
Relict igneous minerals and ghost textures in the Red Mountain Ultramafite indicate a mainly harzburgite parent. Microprobe analyses of olivine, orthopyroxene and chromian clinopyroxene give compositions of Fo90 ; En90-92 ; Wo49 En48 Fs3 respectively and also indicate that the original opaque phase was chromian spinel. These values are directly comparable to mineral compositions from ophiolite sequences and Alpine - type ultrabasic complexes.
Kizardite, chrysotile and magnetite are the major metamorphic minerals in the Red Mountain Ultramafite. Serpentinisation was probably initiated during an ocean-floor metamorphic episode, further alteration taking place during and possibly after the Rangitata Orogeny. The present mineral assemblage indicates temperatures of formation of less than 300°C and a few kilobars pressure, conditions that are comparable to those producing prehnite - pumpellyite facies assemblages in adjacent strata.
Igneous textures are usually well preserved in the Livingstone Subgroup but metamorphic reconstitution is such that only three relict igneous phases now occur. Only Ca-rich pyroxene has been observed and it shows increasing alteration stratigraphically downwards, being most altered in the tectonic inclusions. Microprobe analyses of these clinopyroxenes indicate tholeiitic affinities for these rocks and using the criteria of Nisbet and Pearce (1977) the analyses suggest that the rocks were formed in an ocean-floor environment. Brown hornblende is restricted to the tectonic inclusions and is considered to have crystallised from an evolving magma. Opaque minerals invariably show alteration to sphene and this, together with microprobe data, indicates an original iron - titania oxide phase.
Zeolite facies metamorphic assemblages in the southwest of the area pass northeastwards through a narrow zone of lawsonite - albite - quartz facies into an extensive area of prehnite - pumpellyite facies mineral assemblages. This low-grade regional metamorphism is associated with the Mesozoic Rangitata Orogeny. A temperature gradient of about 23C/km was operative during this event. Maximum pressures of approximately 5 kb and temperatures of approximately 380°C were attained.
Prehnite - pumpellyite facies mineral assemblages overprint greenschist facies mineral assemblages in the metadolerites, metagabbros and albite amphibolites. Microprobe analyses of amphiboles from these rocks show a compositional gap within and between grains, but a miscibility gap is not developed. Amphibole compositions are governed initially by precursor mineral composition, later being modified by host rock composition. These results suggest that conditions of metamorphic equilibrium have not been attained.
The meta-igneous rocks were metamorphosed under ocean-floor conditions of low pressure and of temperatures up to at least 400C, possibly reaching 600C locally. This metamorphic event was of an early Permian age, and was later overprinted during the Rangitata Orogeny.
Co-existing phases in the Livingstone Subgroup include pumpellyite(Fe), albite, iron-rich epidote, chlorite, celadonite, prehnite, sphene, quartz and calcite. (Microprobe analyses are given for all but quartz and calcite). Andradite amd intermediate members of the andradite - grossularite series occur with prehnite, epidote, chlorite, quartz, calcite and an opaque phase in vesicle infillings of a non-pillowed metabasalt. Conditions of formation of this mineral assemblage involved temperatures of less than 300°C, possibly even less than 100C and pressue pressures of a few kilobars.
Several models are discussed regarding the environment in which the strata at West Dome originated, and the ensuing tectonic conditions that brought about the juxtaposition of these rocks. The simplest model is that of a single Permian - Cretaceous volcanic arc - arc-trench gap - trench in which Humboldt Group (already metamorphosed under ocean-floor conditions) formed the upper plate of the subduction zone and on which Bryneira and Stephens Group strata were deposited in the arc-trench gap; Caples ·· Pelorus terrane rocks probably formed in part of the trench. All of these rocks were folded and regionally metamorphosed during the Mesozoic Rangitata Orogeny.
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178 leaves : ill., 3 maps (fold) ; 30 cm.
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1979Hyden_G
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Location (WKT, WGS84):
POLYGON ((168.209693133000087 -45.665156176999972,168.188347208000096 -45.664532452999936,168.186946168000077 -45.6613425099999,168.180039790000023 -45.645621837999954,168.163645185000064 -45.608280301999969,168.163984342000049 -45.608294340999976,168.126187456000025 -45.504027253999936,168.129684672000053 -45.5041328099999,168.194297864000077 -45.506042373999946,168.198291142000016 -45.506157237999957,168.198741855000094 -45.506171281999968,168.316475437000122 -45.544962533999978,168.343043887000022 -45.553694694999933,168.338410751000083 -45.615379730999962,168.337693856000101 -45.615351159999932,168.337080913000023 -45.624312624999959,168.336980158000074 -45.624367193999944,168.321991751000041 -45.632687198999974,168.276571844000046 -45.657879342999934,168.260794339000086 -45.666619754999942,168.22099839100008 -45.665478830999973,168.210260803000097 -45.665170406999948,168.209693133000087 -45.665156176999972))
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Citation
Hyden, Graham., “Geology of the west dome sector of the Dun mountain ophiolite belt.,” Otago Geology Theses, accessed February 7, 2025, https://theses.otagogeology.org.nz/items/show/121.