The Auckland basalts are close to OIB average, but are very magnesian (10-13%), have a small compositional spread and show some ankaramite trends.
Once again we see the ability of alkaline basalts to be intruded into or at the edge of continental masses without modification.
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Prehistoric (10-15,000yr) centres in the Auckland Volcanic Field. |
| Browns Id. This consists of an older sea-eroded tuff cone plus a younger centre. Notice the wild Th point. These wild values are something of a puzzle! Or is it a misprint?? |
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Ngatutura Basalts. These occur south of Auckland but are similar,
though rather less magnesian.A plio-Pleistocene field south of Auckland, NZ. |
Several old Pliocene alkali basalts domes are found on the east coast of the South Island of New Zealand and more on subantarctic islands. All are deeply eroded and partly submerged due to post-glacial sea-level rise, often forming magnificent harbours.
Banks Peninsula
This promontory on the east coast of the South Island, NZ, is formed of two overlapping Miocene-Pliocene volcanoes, Lyttleton and Akaroa . Both have been deeply eroded and then flooded by post glacial sea rise, the radiating valleys forming numerous harbours.
As seen in the satellite view, the major bay to the north forms the harbour of Lyttleton and once drained the Lyttleton caldera. The Gebbies pass ryholite referred to below occurs at the southern limit of the Lyttleton Harour and valley. The harbour of Akaroa to the south is of similar origin. At the extreme northern end of Akaroa is a small peninsula within which is found the Onawe Gabbro and Syenite.
The flows of both centres are mainly of hawaiite, with basalt, mugearite, benmoriite and trachyte occurrring mainly as dykes. The Lyttleton dome or shield was formed between 12 and 9.7 myr ago ( Price, R.C. & Taylor,S.R., 1980, (C.Min.Pet 72, 1-18). The larger Akaroa dome was formed between 9 and 7.5 myr.
Price and Taylor believed that Akaroa is a more alkaline centre, but the data is minimal and while considerable scattering of alkali data is seen it is not consistent with the lack of variation in the REE.
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Variation diagram for both volcanoes, data from Price and Taylor. |
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The REE normalized to EMORB. The sample the marked negative Eu anomaly is the odd Gebbies Pass rhyolite, very depleted and possibly a crustal melt. The samples of lowest and highest Lu are the Onawe Gabbro and Syenite |
Lyttleton-Akaroa Volcanoes
These lie immediately east of the city of Christchurch, the two centres overlap with a basal diameter of about 30 miles. No trace element data is yet available. There is a wide spread of composition, with end products ranging from trachyte to rhyolite. No basanites or phonolites are found. The "rhyolites" are more likely to be quartz-bearing commendites.
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Lyttleton and Akaroa Volcanoes. Very old data, no trace elements. |
On an Alks/SiO2 diagram there appear to be three series, some quartz-bearing "rhyolites", an alkali basalt - mugearite - benmoreiite - trachyte series, and a basanite trachybasalt - trachy phonolite series. However these do not show distinctly on the variation diagram nor are they confined to one centre. Even the Gebbies Pass Rhyolite while of lower alumina fits in with the general trends. More data is needed!
Dunedin Volcano
(or the East Otago Volcanic Province).
This major Pliocene province consists of a large deeply eroded central volcano centred near the city itself with many peripheral vents (some estimates go as high as 150) with lopoliths, domes, massive flows and residual plugs of alkali basalt, basanite, theralite, trachyte and numerous phonolites of extreme composition, (See "Rhyolites, Trachytes, Phonolites").
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Dunedin Volcanics, (or as Benson termed it, The East Otago Province.)
Small alkaline to tholeiitic flows and breccias dot the eastern foothills of the Alps for 130 miles down to Dunedin, a major centre now about 6 miles across and of middle-upper Eocene to lower Pliocene age. More alkaline than Banks peninsula, the Dunedin Volcanics are mainly basanite to phonolite with some trachyte. Note the high alumina extending to more than 20% and the high soda (10% in the phonolites.)
Rb is high in the phonolites (~300 ppm) as is Th (~67 ppm) with a Th/U of 7.1. |
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Zr/Nb diagram for Dunedin Volcanics. Zr/Nb averages 5 with extremes of about 4-7, higher than seen in nephelinites. Nb in some of the phonolites especially Mt Mopanui, is very high (~300 ppm) as is Rb (~300 ppm) though Ba seldom exceeds 1000 ppm and falls to almost zero in the phonolites. |
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Zr/AE diagram for Dunedin Volcanics. |
Historical Note
The lavas of the Dunedin or East Otago Volcanic Province surround the university city of Dunedin, and have been intensively studied over the years beginning with the pioneering work of the well known historical figure of Professor Benson in the First World War era. Benson was only one of a group of famous petrologists trained by Sir Edgeworth David of Sydney. David and his student Mawson were of course with Shackleton in the Antarctic in 1907 when "Shackles" reached a point only 90 miles from the South Pole, while David and Mawson walked to the South Magnetic Pole! "Benny" did the petrography of all the rocks collected including erratic samples of Ferrar Dolerite. In 5 years of work I seldom found anything mineralogical that "Benny" had missed, and he was using an ancient brass microscope with plates of calcite as nicol-prisms!
Auckland Is
These consist of the eroded remnants of a chain of alkali basalt centres forming islands 300 miles south of New Zealand. The most obvious centre is that at Carnely Harbour, where the data of Gamble shows numerous ankaramites to be present. No REE available.
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Carnely Harbour, Auckland Islands
This volcanic island group is only 24 miles long, built of shallow dipping flows of about Pliocene age, deeply indented by Pleistocene glacial fiords on the north-east, pounded by the westerlies into steep cliffs on the south-west and west sides.
Carnely Volcano is an eroded centre on the southern end the island, the one-time crater forming a large harbour, as in Lyttleton-Akaroa.
Data is rather old and variation in K suggests alteration. Only mildly alkaline as shown by the low if variable soda. No TE or REE. |
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Cross-section of the Toomba flow at Lowes Basin. The 130,000 year old Toomba flow is resting on fossil-rich Devonian limestone.
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