Dun Mountain-Maitai Terrane
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The Dun Mountain-Maitai Terrane comprises the Dun Mountain Ophiolite Belt (also called the Mineral Belt), Maitai Group and Patuki Mélange. The Dun Mountain Ophiolite is an
ophiolite An ophiolite is a section of Earth's oceanic crust and the underlying upper mantle that has been uplifted and exposed above sea level and often emplaced onto continental crustal rocks. The Greek word ὄφις, ''ophis'' (''snake'') is found ...
of
Permian The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Paleo ...
age located in New Zealand's
South Island The South Island, also officially named , is the larger of the two major islands of New Zealand in surface area, the other being the smaller but more populous North Island. It is bordered to the north by Cook Strait, to the west by the Tasman ...
. Prehistorically this ophiolite was quarried by Māori for both metasomatized
argillite :''"Argillite" may also refer to Argillite, Kentucky.'' Argillite () is a fine-grained sedimentary rock composed predominantly of indurated clay particles. Argillaceous rocks are basically lithified muds and oozes. They contain variable amounts ...
and
pounamu Pounamu is a term for several types of hard and durable stone found in southern New Zealand. They are highly valued in New Zealand, and carvings made from pounamu play an important role in Māori culture. Name The Māori word , also used ...
( jade) which was used in the production of tools and jewellery. In the late 1800s, the Dun Mountain Ophiolite Belt was surveyed for its economic potential. During this time the rock types
dunite Dunite (), also known as olivinite (not to be confused with the mineral olivenite), is an intrusive igneous rock of ultramafic composition and with phaneritic (coarse-grained) texture. The mineral assemblage is greater than 90% olivine, with ...
and rodingite (after Dun Mountain and the Roding River) were first named. Discovery of economic deposits of chromite near
Nelson Nelson may refer to: Arts and entertainment * ''Nelson'' (1918 film), a historical film directed by Maurice Elvey * ''Nelson'' (1926 film), a historical film directed by Walter Summers * ''Nelson'' (opera), an opera by Lennox Berkeley to a lib ...
lead to the building of New Zealand’s first railway, however, extraction only occurred between 1862 and 1866. In the 20th century,
serpentinite Serpentinite is a rock composed predominantly of one or more serpentine group minerals, the name originating from the similarity of the texture of the rock to that of the skin of a snake. Serpentinite has been called ''serpentine'' or ''se ...
was mined for fertiliser and the ophiolite remains one of New Zealand's main sources of
pounamu Pounamu is a term for several types of hard and durable stone found in southern New Zealand. They are highly valued in New Zealand, and carvings made from pounamu play an important role in Māori culture. Name The Māori word , also used ...
( jade), but all other mineral exploration has failed to find economic deposits.


Description

The Dun Mountain Ophiolite Belt is composed of a typical ophiolite sequence of
ultramafic Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are composed ...
rocks overlain by a
plutonic Intrusive rock is formed when magma penetrates existing rock, crystallizes, and solidifies underground to form '' intrusions'', such as batholiths, dikes, sills, laccoliths, and volcanic necks.Intrusive RocksIntrusive rocks accessdate: March ...
then volcanic sequence, and finally by conglomerates and other sedimentary rocks of the Maitai Group. The unaltered ultramafic rocks are restricted to three massifs, Dun Mountain, the Red Hills and Red Mountain, elsewhere they are highly serpentinized. This ophiolite sequence is structurally underlain by the ophiolitic Patuki
Mélange In geology, a mélange is a large-scale breccia, a mappable body of rock characterized by a lack of continuous bedding and the inclusion of fragments of rock of all sizes, contained in a fine-grained deformed matrix. The mélange typically cons ...
. The Dun Mountain Ophiolite Belt likely formed in a
forearc Forearc is a plate tectonic term referring to a region between an oceanic trench, also known as a subduction zone, and the associated volcanic arc. Forearc regions are present along a convergent margins and eponymously form 'in front of' the vo ...
environment. *Mantle lithologies **Dun Mountain Ultramafics Group **Wairere Serpentinite **Upukerora Mélange *Crustal igneous rocks **Otanomomo Complex **Livingstone Volcanics Group *Crustal sedimentary rock **Maitai Group ***Upukerora Breccia ***Wooded Peak Limestone *** Tramway Sandstone *** Greville Formation *** Little Ben Sandstone *** Stephens Subgroup *Basal mélange **Windon Mélange **Paruki Mélange


Distribution

The Dun Mountain Ophiolite Belt is a locally intact approximately section through
oceanic crust Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafic ...
. It is exposed between D'Urville Island in Marlborough District and St Arnaud in
Tasman District Tasman District () is a local government district in the northwest of the South Island of New Zealand. It borders the Canterbury Region, West Coast Region, Marlborough Region and Nelson City. It is administered by the Tasman District Council, ...
, and Jackson Bay in the West Coast Region and Balclutha in Otago. The Dun Mountain Ophiolite Belt is exposed in the South Island and is inferred to exist at depth under the North Island. It is in two sections, as it is offset by the
Alpine Fault The Alpine Fault is a geological fault that runs almost the entire length of New Zealand's South Island (c. 480 km) and forms the boundary between the Pacific Plate and the Indo-Australian Plate. The Southern Alps have been uplifted on the f ...
, with sections to the west of the fault having been displaced northwards. The Dun Mountain-Maitai Terrane also extends at depth into the North Island as far as Northland.


See also

* Geology of the Tasman District * Stratigraphy of New Zealand **
Torlesse Composite Terrane The Torlesse Composite Terrane is a plate tectonic terrane forming part of the South Island of New Zealand. It contains the Rakaia, Aspiring and Pahau Terranes and the Esk Head Belt. Greywacke (or Torlesse Greywacke) is the dominant rock type of ...
**
Takaka Terrane The Takaka Terrane is a Paleozoic terrane that outcrops in the South Island of New Zealand. It is most extensively exposed within the Kahurangi National Park in the Tasman District. The terrane is mostly made up of marble and volcanic rocks but ...


References


External links


New Zealand Stratigraphic Lexicon: Dun Mountain Ophiolite Belt

Nelson Council: Protecting Dun Mountain and the Mineral Belt
{{Tasman District Terranes Geology of New Zealand Ophiolites Paleozoic Oceania Geography of Nelson, New Zealand Geography of the Marlborough Region Geography of Otago Geography of Southland, New Zealand Geography of the Tasman District