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Kaluwe carbonatite, Zambia: an alternative to natrocarbonatite

Department of Geoscience, Reading Univeristy, PO Box 227, Reading RG6 2AB, UK

Fine grained carbonatite in the Kaluwe complex has been inferred, on textural grounds, to be calcified natrocarbonatite. Electron microprobe analysis, however, shows that the brown laths thought to be pseudo-nyerereite are dolomitic, with Sr values much higher than normal in low-temperature carbonates. This chemistry effectively rules out near-surface replacement of an original natrocarbonatite, and the compositions must closely reflect that of the magma from which they crystallized. The observed fabrics can be interpreted in terms of the system CaO–MgO–FeO–CO₂, where CaO:MgO and cooling history have largely determined the crystallization paths.

Carbonatite pedogenesis has long been a controversial topic and there is still debate about how far observed carbonatite compositions represent the magma from which they crystallized. Lava eruptions from Oldoinyo Lengai (Dawson 1962) removed any doubt that carbonate melts could exist at the Earth's surface. This lava, composed of the Na-K-Ca minerals nyerereite [(Na_0.82K_0.18)₂Ca(CO₃)₂] and gregoryite [(Na_0.78K_0.05)₂(Ca_0.17CO₃)] with whole rock Na₂O up to 30%, is referred to as natrocarbonatite whereas other carbonatites are low in alkalis.

This discovery led to the idea that many older extrusive carbonatites were erupted as natrocarbonatite, from which the alkalis were leached by meteoric water, and the nyerereite and gregoryite pseudomorphed by secondary calcite (Dawson et al. 1987 and references therein). Some workers, however, have questioned whether alkali carbonate volcanics could have retained their original structure during total replacement by ground-water calcite (Keller 1981; Bailey in Turner 1988).

One of the most recent candidates for replacement of original natrocarbonatite is the extensive

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				D. C. TURNER and D. C. REX Volcaniclastic carbonatite at Kaluwe, Zambia: age and relations to sedimentary rocks in the Zambezi rift valley Journal of the Geological Society, February 1, 1991; 148(1): 13 - 15. [Abstract] [PDF]