Pirajno reported in 2015 that intracontinental alkaline complexes and carbonatites commonly occur in rift-related settings, best exemplified by the Cenozoic–present day East African Rift System. Examples in the ancient geological record are more difficult to unravel due to subsequent overprinting during tectono-thermal events unrelated to the original rifting. In the present paper, with the exception of the Bayan Obo carbonatite, the author has used examples of anorogenic alkaline complexes and carbonatites that are effectively pristine and unaffected or little influenced by later geological events. In this context, them author reports on two alkaline complexes and associated mineralization in Namibia (Brandberg and Erongo) related to the opening of the South Atlantic and the Paraná–Etendeka large igneous province.
With regard to carbonatites and associated mineralisation, the author discusses the exceptionally well-preserved Kruidfontein and Goudini in South Africa, Mount Weld and Gifford Creek ferrocarbonatite complex in Western Australia, the controversial Bayan Obo in northern China and the carbonatites of the Mianning–Dechang belt in SW China.
The author proposed that all these igneous systems are formed by processes of partial melting in metasomatized lithospheric mantle, induced by upwelling mantle plumes. Furthermore, it is contended that these alkaline and carbonatite igneous systems are generally formed in the distal sectors of a mantle plume due to lateral transfer of volatiles, resulting in low degrees of partial melting in the metasomatised sub-continental lithospheric mantle.
The author concludes that anorogenic and intraplate alkaline magmatism, which includes silica oversaturated and silica-undersaturated complexes and carbonatites is the spotlight of increasing attention due to its potential to host economically viable critical and rare metals. Mineralization in alkaline and carbonatite rocks is formed through at least three principal ore-forming processes, namely:
1) primary magmatic, in which the ore minerals are precipitated from residual fluids in the subsolidus range of temperature and pressure;
2) hydrothermal, from late-stage fluids evolved from 1) and their mixing with heated meteoric groundwater; hydrothermal mineralization commonly overprints
the primary magmatic; and
3) supergene, usually developed in the weathering profiles of carbonatite in tropical climates and under conditions of high Eh and low pH.
The products of intraplate and rift-related magmatism are volumetrically small when compared to the large granitic batholiths formed at subduction fronts. Nevertheless, in spite of this, rift-related alkaline magmas and carbonatites provide excellent and most useful insights into mantle and sub-continental lithospheric mantle dynamics. With this in mind, the author has chosen to showcase some of the relevant aspects of intraplate alkaline magmatism, associated carbonatites and related mineral systems. In so doing the author has reviewed a selection of anorogenic alkaline plutonic and volcano-plutonic complexes:
- Brandberg and Erongo in Namibia,
- Carbonatites (Kruidfontei and Goudini in South Africa,
- Mount Weld and Gifford Creek in Western Australia,
- Bayan Obo and Maoniuping in China).
The Brandberg and Erongo anorogenic complexes are part of the ~138–130 Ma Paraná–Etendeka LIP, outcropping in present-day South America and southwest Africa and linked to the Tristan da Cunha hotspot.Theseanorogenic complexes were emplaced along major transform and later strike-slip faults, resulting from the drifting movement of the South American and African plates.
The author concluded that alkaline with syenitic–nephelinitic melts and associated carbonatites are dominantly emplaced in rift structures formed by the impingement of mantle plumes at the base of the sub-continental mantle lithosphere. This is abundantly shown in the geological record as well as present-day
intracontinental rifts (e.g. East African Rift System). The model put forward by Woolley (1987) and re-enforced by Martin (2006) shown in Fig. 31 is considered quite pertinent for the topic treated in this contribution.
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