| Dunedin volcanic group | |
|---|---|
| Stratigraphic range:Oligocene-Miocene (25–8 Ma)[1] | |
| Type | Igneous |
| Overlies | Rakaia Terrane |
| Area | 7,800 square kilometres (3,000 sq mi)[1][2] |
| Lithology | |
| Primary | Basalt,trachyte,phonolite andbreccia |
| Location | |
| Coordinates | 45°39′S170°18′E / 45.65°S 170.3°E /-45.65; 170.3 |
| Region | Otago |
| Country | New Zealand |
| Type section | |
| Named for | Dunedin |
 Map of selected surface volcanic features of theSouth Island centred onOtago. Legend
| |
TheDunedin volcanic group is a volcanic group that covers over 7,800 km2 (3,000 sq mi) ofOtago in theSouth Island ofNew Zealand.[2] It is a recent[note 1] reclassification of the group previously known as theWaiareka-Deborah volcanic field due to common magma melt ancestries of theDunedin Volcano[7] with the overlapping alkali basalticmonogenetic 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) nearOamaru, 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 theUniversity of Otago, detailed studies of north-central volcanoes such as the Crater nearMiddlemarch were done much later,[6][9] and high-quality composition studies still need to be done to properly classify many volcanics near Oamaru.
The volcanic features of the Dunedin group define the city and harbour ofDunedin. Multiple monogenetic basalt volcanoes alter the mainly sedimentaryOtago landscape. An eruptive centre provides access to the fossil site ofFoulden Maar. The northernmost eruptive centre is at Arnmore, nearNgapara, and the group extends south to nearKaitangata 160 km (99 mi) 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]
Over 150 flows, or eruptive centres, are known, and these are mainly of small volumealkalinebasanite eruptives.[1] The Dunedin volcanic group shows continued evidence of amagma melt hot spot centred nearPortobello on theOtago Peninsula that has presumably been accumulating in the last 10 million years since active volcanism ceased.[10] This is because of high surface heliumisotope measurements, consistent with a magma pocket.[11][1]
In this region of Otago, there is a metamorphosed belt known as theOtago Schist that formed as part of theZealandia continent, dated from theJurassic through theCretaceous period.[2] In this belt,Oligocene toMiocene aged volcanic groupbasement rocks includingpumpellyite andactinolite greyschist have erupted through the upper and lower greenschist belts, which are present fromWānaka down to Dunedin.[2] The Otago Schist is overlain by a sedimentary sequence, deposited from Late Cretaceous to Miocene time.[1] The peak ofmarine transgression is marked by an Oligocene carbonate platform that means there was at this time near-total submergence under water. This was later followed by late Oligocene faulting that lifted the area of Otago above sea level.[1]
Studies of the volcanic group were previously limited as the ages of the volcanoes were not known well;[6][9] now the distribution of ages from 25 million to approximately 9 million years ago is better understood by location.[1] The oldest rocks in the group trend towards the west. The Crater, a volcano near Middlemarch, is the high-level ventdiatremefacies of an approximately 1.5 km (0.93 mi) widemaar and has an age of 24.8 ± 0.6Ma for its oldest component.[9][1] The oldest eruptions of the Dunedin Volcano are likely to have been near Allans Beach on theOtago Peninsula at 16.0 ± 0.4 Ma.[1] The age ofMount 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 includes 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]
The local basaltic monogenetic volcanoes were historically arbitrarily separated from the Dunedin Volcano, as has been determined by subsequent rock analysis.[12] The associated Dunedin volcanic group basalts are likely derived from several subtly different but related isotopic reservoirs.[2] Other long-standing knowledge, such as the occurrence ofrhönite,[13] a very rare mineral, with the mineralsphonolite andtrachyte in the Dunedin Volcano, confused matters until international collaboration with detailed analysis of samples from the whole group area was possible.[1] The central vent at Port Chalmers, which is likely a largediatreme, is filled with breccia in an oval about 1.3 by 2.5 km. The vent contains volcanic clasts of mainlyphonolite, with minor proportions ofbasanite, basaltic-trachyandesite,trachyandesite,syenite,gabbro,pyroxenite, andhornblendite, as well as fragments ofOtago Schist and Cenozoic sedimentary rocks.[1] Within the Dunedin Volcano area atHoopers Inlet arenephelinesyeniteplutonicdykes; however, there are no other plutonic rocks in the group area.[1]
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 withperidotite and pyrometamorphosed Otago Schistxenoliths.[2]
TheSwinburn area to the south side ofState 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, thinaphanitic basanitic lavas overlie Miocene sediments. These particular lavas have all been tilted and now mostly dip to the north and north-east.[1]
In thePigroot area at the volcano called Trig L, lavas overlie marine sediments and includeexotic mantle peridotite and crustal gabbro xenolith-bearingphonolite as the cap.[1]
The main eruptive centre in the northwestern area of theKakanui Range in north Otago is Siberia Hill, with abasanite cap on top oftrachybasalt covering a 40 m (130 ft) layer ofEocene to Oligocene sediments above the Otago Schist. Kattothyrst, about 1 km (0.62 mi) to its east, is a columnar-jointed basanitic plug. Mount Dasher, about 1.5 km (0.93 mi) to the northwest of Kattothyrst, has basanite lavas with an interveningphonotephrite flow on the northeast side. The Obelisk has a south end oftrachybasalt and basalt at the northern end.[1]
Just southwest of Dunedin, theSaddle Hill, Jaffray Hill, and Scroggs Hill volcanoes are composed of tuff, basaltic trachyandesite, phonotephrite and basanite lava, emplaced through coal-bearing sediments andCenozoicmudstone. Basanite is dominant on Saddle Hill, and phonolitic pyroclastic rock underlies the lava at Scroggs Hill. A historic underground coal mine is reported to have encountered the feeder pipe at Jaffray Hill.[1] Further to the southwest are volcanoes on the western side ofLake Waihola, with at least three vents present.Olivinetheralite (nepheline gabbro),clinopyroxenite andperidotite have also been found on this western shore.[1]
New Zealand's South Island has many extinct volcanic centres without agreed upontectonic mechanisms of formation. They extend in age from theCretaceous to thePliocene and outcrop throughoutOtago,Canterbury, and on theChatham Islands. The largest single eruptive centre in the South Island is theBanks Peninsula Volcano, followed by the Dunedin Volcano. Despite being smaller, the Dunedin volcanic group contains a much largermonogenetic 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 thelithosphere, unlike some other intraplate volcanoes such as theHawaii island chain, which are rooted in theasthenosphere. One possible mechanism for the creation of these volcanoes is the flaking off of the base ofZealandia's lower lithosphere into the asthenosphere. Zealandia has a thin lithosphere, as it has been extended whilerafting away from Australia. If large sections of this already thin lithosphere sank into the asthenosphere, it would be replaced with hotter rock leading todecompression 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]
