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massifs UB

Ultrabasic massifs of New Caledonia

The very large occurrence of ultrabasic rocks is the original feature of the folded arc of Grande Terre. These rocks make large and high massifs in the South and along the West Coast.

They cover an area of 5500 km² (1/3 of Grande Terre). For a long time the large ultrabasic massif of the South was the largest ultrabasic body known on Earth. It has been supplanted in the seventies by an ultrabasic body in Oman.

Ultrabasic rocks originate from the mantle (deep layer under the crust and above the earth core). When plates collided generating the last New Caledonian folding, a large sheet of mantle was upraised and overthrust above other terrains. Erosion and tectonics then divided it in several massifs.

Ultrabasic rocks are iron and magnesium rich and also contain nickel, cobalt and chromium. They are the source of the nickel, cobalt and chromite deposits in New Caledonia, hence their name of "terrains miniers" (mining grounds). This name was extended to the ultrabasic massifs then named "massifs miniers".

Peridotites and serpentinites

Among ultrabasic rocks there are peridotites and serpentinites.

Peridotites are coarse grained rocks which make most of the New Caledonian ultrabasic massifs.

Serpentinites, are rocks with a similar chemical composition but with different and hydrated minerals which result from an alteration of the Peridotites minerals.

Rocks and minerals

The most widespread rock is the harzburgite, a rock consisting in olivine and pyroxen (bronzite) the later often in large cristals. Harzburgite is the major rock forming the ultrabasic massifs of New Caledonia. Dunite is another important rock consisting almost entirely in olivine.

Serpentines, or more correctly serpentinites, result from an alteration of peridotites. Water and peculiar physical conditions have created new minerals. These ones are mostly antigorite, vermiculite, talc, actinolite. New Caledonia peridotites are generally partially serpentinised (antigorite replacing olivine). A thick bed of serpentinite occurs at the massifs bottom. It results from the metamorphism of peridotites when the ultrabasic sheet was thrust over the other terrains which are mostly basalts from an oceanic floor. Serpentinites also occur as long dykes in metamorphic rocks of the North. The "serpentine" of the New Caledonian miners ("petits mineurs") is actually a weathered dunite, often with a very rich nickel content. Vocabulary becoming more technical, the miners name is progressively abandoned, the official geological terms becoming now prevalent.

All these minerals are iron and magnesium silicates with, for some, associated calcium (amphiboles).

A special mention for the tremolite - actinote family and for chrysotile. These minerals are generally in fiber shapes and are then called asbestos. Tremolite - actinote are nowadays infamous for the pathology resulting from usages which lead to inhalate their tiny fibers.

Visitors interested in geology can access to a geological and structural map of New Caledonia with complementary information on the main events which led to the actual archipelago.

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