Geology of New Caledonia

The geology of New Caledonia includes all major rock types (igneous, sedimentary, and metamorphic), which here range in age from ~290 million years old (Ma) to recent.

[2] The mantle-derived Eocene Peridotite Nappe is the most significant and widespread unit (labelled as "Ophiolites" and coloured in bright green in Fig.

Thrusting, exhumation and folding of some of the units have contributed to the rugged topography of the elongated main island, with Mont Panié and Mount Humboldt being the highest points (over 1,600 m).

[6] The geology of the Collectivity (autonomous region) is divided into two relatively separated parts: the larger Grande Terre in the West and the Loyalty Islands in the East.

[2] Rocks in Koh-Central, Teremba and Boghen Terranes on the island suggest the units were emplaced and compressed between a volcanic arc and the associated subduction zone (known as a forearc basin along a collisional plate boundary).

[2] Northern Zealandia separated into the Lord Howe Rise in the west and the narrow strip of Norfolk Ridge in the east, of which New Caledonia is the northernmost tip.

[19] In the fore-arc basin tectonic setting, the suite of Koh Ophiolite is known to be the earliest terrane to be formed on the island in the early Permian.

[2] The clastic rocks are mainly greywacke, which could share the same source as those in the Koh-Central Terrane;[23] the composition of the volcanics varies from rhyolitic to andesitic.

[2] Likely due to a westward plate subduction, the complex further metamorphosed under high pressure but low temperature in the Late Jurassic to form blueschist.

[7] However, a shift of fossil records to marine bivalves and gastropods indicate the subsidence of the landmass turned the sedimentary environment into an estuarine one.

[7] Formation à charbon is overlaid by the Paleocene Koumac Terrane, which primarily consists of sedimentary black chert and also some argillite.

[2] In Eocene, on one hand there is an abrupt change of white micrite into pink marl, followed by the deposition of limestone breccia and conglomerates in the North of the Island.

[10] The underlying Poya Terrane on the western coast consists of pillow and massive undepleted mid-oceanic ridge basalts (MORB) with uplifts of m- to km- scale of thickness.

[2][38] Models suggest the complex have isostatically rebounded to form the Mount Panie Antiform during an extensional event with Diahot in the west, and Pouebo in the east, after obduction later on.

[2][40] Similar conglomerates and breccias are also found in the Gwa (or Goa) N'doro Formation near Houaïlou, and represents the remnants of a river network developed since Miocene.

[2] Combined analysis of the three terranes suggests the Grande Terre formed at the eastern edge of a volcanic forearc related to subduction.

[27] Also, as the deep-sea sedimentary rocks show, the region was located at the southeast Gondwana marginal basin system that opened between the Permian and the Cretaceous, with Australia being the source of the sediments.

[2][7] On a macro-scale, as evidenced by the presence of endemic fossil species, Zealandia drifted away from the rest of Gondwana relatively to the northeast, particularly away from the Australian and the Antarctic land masses.

[2] The moisture released from metamorphism caused partial melting, initiating a series of extrusive volcanism to form the arc of Loyalty Ridge/Islands.

[2][46] Instead, the oceanic lithosphere obducted above the basement of the Norfolk Ridge, thus bringing units like the Poya Terrane and the Peridotite Nappe onto Grande Terre.

[2][36] After obduction, the uplift and exhumation of the metamorphic core complex between Oligocene and Miocene are either simultaneous, or resultant of an extensional event i.e. unloading due to slab detachment.

[11] Intrusion of granitoids into the Peridotite Nappe also happened in the Oligocene, possibly due to a short-term reactivated subduction from the New Caledonia Basin.

[39] Nepoui Series witnessed the uplift of Grande Terre in the Miocene, possibly by the forearc bulge in response to the subduction nearby to Vanuatu.

[47] Due to the obduction of the Peridotite Nappe, origininating in the metal-enriched mantle, metal ores are concentrated as the main earth resources in the region.

[2] Nickel (Ni) and cobalt (Co) ore minerals are enriched in laterite, a material forming at the lower part of the weathering profile of the Peridotite Nappe.

[8] Being the most important earth resource in the territory, the nickel mining industry has contributed to a high local GDP per capita in past decades.

[53] Particularly, the deposits in Poum Massif are believed to be crystallized in a supra-subduction setting from boninitic to tholeiitic magmas, then transported by an asthenospheric (lherzolite) intrusion along a transform fault (hence, the Eocene oblique subduction in Fig.

[2][53] Note that the podiform chromitites are also enriched in platinum group elements like osmium, iridium and ruthenium relatively, but the concentrations are still not high enough for extraction.

[55] Coal in the Formation à charbon was mined near Moindou on the west coast for a short period between 1924 and 1930 and was consumed in the first Ni smelters in Nouméa.

[55] Furthermore, there are potentials for petroleum and gas extraction off the west coast of the Grande Terre, which belongs to the Tasman Frontier area of northern Zealandia.

Fig.1 A simplified geological map of New Caledonia with a cross-section, modified by Xu and Liu (2019) [ 1 ]
Fig. 2 Topographic map of New Caledonia and the neighboring Vanuatu .
Fig. 3 The position of New Caledonia (top) in Zealandia
Fig. 4 Laterite in the Goro mine .
Fig. 5 Simplified cross-sectional evolution of the Eocene and granitoid units with respect to New Caledonia tectonics at that period, modified from Cluzel et al. (2005 and 2012), Patriat et al. (2018). [ 14 ] [ 2 ] [ 11 ] Things are not drawn to scale.
Fig. 6 Outcrop of carbonate rocks on Lifou Island, Loyalty.
Fig. 7 Simplified tectonic evolution of New Caledonia; not drawn to scale, and only related continental blocks are colored. Modified from Cluzel et al. (2012). [ 2 ]
Fig. 8 Simplified map of New Caledonia tectonics in Eocene, modified from Cluzel et al. (2019) and Patriat et al. (2018). [ 2 ] [ 14 ] For the cross-sectional evolution with line marked in blue from west (W) to east (E), please refer to Fig. 5.
Fig. 9 Népouite , a clay silicate lateritic nickel ore deposit named after the local Népoui Mine, is found on the Peridotite Nappe. [ 48 ] The nickel element in 2+ oxidation state gives the green colour.
Fig. 10 Chromite distribution in New Caledonia, by USGS (2012). [ 53 ]