Dunedin volcanic group

The time allocated for running scripts has expired.The time allocated for running scripts has expired. The Dunedin volcanic group is a volcanic group that covers over The time allocated for running scripts has expired. of Otago in the South Island of New Zealand.^[2] It is a recent^{[note 1]} reclassification of the group previously known as the Waiareka-Deborah volcanic field due to common magma melt ancestries of the Dunedin Volcano^[7] with the overlapping alkali basaltic monogenetic volcanic field.^[1] Excluded from the group are a group of volcanics of different composition (sub-alkaline basalt to basaltic andesite) and older age (36.4 to 27.6 million years ago) near Oamaru, which have been given the name previously used for the Dunedin group.^[8] The older Waiareka-Deborah volcanic field overlaps the new Dunedin volcanic group geographically; though Dunedin Volcano has been well studied from the 1880s since New Zealand's first school of geology was established at the University of Otago, detailed studies of north-central volcanoes such as the Crater near Middlemarch were done much later,^[6]^[9] and high-quality composition studies still need to be done to properly classify many volcanics near Oamaru.

Geography

The volcanic features of the Dunedin group define the city and harbour of Dunedin. Multiple monogenetic basalt volcanoes alter the mainly sedimentary Otago landscape. An eruptive centre provides access to the fossil site of Foulden Maar. The northernmost eruptive centre is at Arnmore, near Ngapara, and the group extends south to near Kaitangata The time allocated for running scripts has expired. away. While the easternmost on-land occurrence is at Lookout Bluff in north Otago, seismic data provides evidence of offshore underwater centres. Haughton Hill in the Maniototo is the most northwestern eruptive centre.^[1]

Geology

The time allocated for running scripts has expired. Over 150 flows, or eruptive centres, are known, and these are mainly small volume

alkaline basanites.^[1] The Dunedin volcanic group shows evidence of a magma melt hot spot centred near Portobello on the Otago Peninsula that has presumably been accumulating in the last 10 million years since active volcanism ceased,^[10] as backed up by surface helium measurements.^[11]^[1] In this region of Otago, there is a metamorphosed belt known as the Otago Schist that formed as part of the Zealandia continent, dated from the Jurassic through the Cretaceous period.^[2] In this belt, Oligocene to Miocene aged volcanic group basement rocks including pumpellyite and actinolite greyschist have erupted through upper and lower greenshist belts, which are present from Wanaka down to Dunedin.^[2] The Otago Schist is overlain by a sedimentary sequence, deposited from Late Cretaceous to Miocene time.^[1] The peak of marine transgression is marked by an Oligocene carbonate platform that represents near-total continent submergence, followed by the late Oligocene faulting that lifted Otago above sea level.^[1]

Studies of the group were previously constrained by gaps in age mapping;[6]^[9] current age mappings range from 25 million to approximately 9 million years ago.^[1] The oldest rocks in the group range towards the west. The Crater near Middlemarch, now the high-level vent diatreme facies of an approximately The time allocated for running scripts has expired. wide maar is the oldest dated component at 24.8 ± 0.6 Ma.^[9]^[1] The oldest eruptions of the Dunedin Volcano are likely to have been near Allans Beach on the Otago Peninsula at 16.0 ± 0.4 Ma.^[1] The age of Mount Cargill is measured at 11 Ma. About two-thirds of the eruptions in the entire group took place in the period between 16 and 11 Ma, which contains all of the Dunedin Volcano eruptions as well.^[1] The youngest rocks, at Lookout Bluff, may be as recent as 8 Ma, though measurements in this area are considered provisional.^[5]^[1]

Basaltic monogenetic volcanoes were previously arbitrarily separated from the Dunedin Volcano, as determined by subsequent rock analysis.

plutonic dykes; however, there are no other plutonic rocks in the group area.^[1]

Second Beach and on the slopes of Mount Cargill

.

Scoria, tuff and basanite overlie Eocene-Oligocene marine sediments in the northwest Yellow Hill-Summer Hills area. Ram Rock, an eroded basanitic pipe to the northeast of this area, is associated with peridotite and pyrometamorphosed Otago Schist xenoliths.^[2]

The Swinburn area to the south side of State Highway 85 has coarse doleritic textured basaltic volcanics overlying scoria-fall deposits, which in turn overlie Eocene-Oligocene marine sediments. The basalt contains rare xenoliths of peridotite, schist and porcellanitised Cenozoic sediment. North of State Highway 85, thin aphanitic basanitic lavas overlie Miocene sediments. These particular lavas have all been tilted and now mostly dip to the north and northeast.^[1]

In the

Pigroot area at Trig L, lavas overlie marine sediments and include exotic mantle peridotite and crustal gabbro xenolith-bearing phonolite as the cap.^[1]

The main eruptive centre in the northwestern area of the Kakanui Range in north Otago is Siberia Hill, with a basanite cap on top of trachybasalt covering a The time allocated for running scripts has expired. layer of Eocene to Oligocene sediments above the Otago Schist. Kattothyrst, about The time allocated for running scripts has expired. to its east, is a columnar-jointed basanitic plug. Mount Dasher, about The time allocated for running scripts has expired. to the northwest of Kattothyrst, has basanite lavas with an intervening phonotephrite flow on the northeast side. The Obelisk has a south end of trachybasalt and basalt at the northern end.^[1]

Just southwest of Dunedin, the

clinopyroxenite and peridotite have also been found on this western shore.^[1]

Relationship to other volcanoes

New Zealand's South Island has many extinct volcanic centres without agreed upon

Canterbury, and on the Chatham Islands. The largest single eruptive centre in the South Island is the Banks Peninsula Volcano, followed by the Dunedin Volcano. Despite being smaller, the Dunedin volcanic group contains a much larger monogenetic volcanic field of related eruptives than the Banks Peninsula Volcano.^[1]

These volcanic centres can be dormant for tens of millions of years between eruptions. This implies that the mechanism of formation may be connected to the

decompression melting. This theoretically could cause volcanic activity that is locked to the moving lithosphere over many millions of years, as observed in the Dunedin group.^[14]

Quarantine Island

(centre of the image).

Notes

^ The term Dunedin Volcanic Group was used in independent academic literature since at least the mid-90s,^[3] being first defined as Miocene Otago volcanics near Dunedin excluding the Dunedin Volcano in 1986.^[4] however, such use was outside the then-current volcanic classification methodologies, which have changed in the last 20 years. Its current meaning builds on works such as that of Coombs.^[5] and Németh.^[6] The formal recommendation to adopt the term came in 2020 as a result of a peer-reviewed reanalysis exercise of over a century of Otago geological specimens.^[1]

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References

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