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Dolomitic volcanism in Zambia: Cr and K signatures and comparisons with other dolomitic melts from the mantle

By
D.K. Bailey
D.K. Bailey
Department of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK
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S. Kearns
S. Kearns
Department of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK
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Published:
June 01, 2011

Volcaniclastic carbonatites in southeast Zambia contain dolomitic melt lapilli, in which high-Cr chromite phenocrysts indicate direct eruption from the mantle. There are no lava flows. Vent tuffisite, with dolomite lapilli in a matrix of dolomite + iron oxides, provides the least contaminated samples of primary erupted material. This tuffisite has high trace-element levels typical of carbonatite, but its high Cr and Ni make it exceptional. Fragmental phlogopite, sanidine, and orthoclase are indicators of potassium (K) mineralization in deeper parts of the conduit. This was the only known case of dolomite eruption directly from source until 2005, when dolomite volcanism in Spain and France provided the first opportunity for comparing these characteristics. Eruptions are fragmental: typically with rounded lapilli of low-Fe, high-Mn dolomite. Outstanding common features are euhedral chrome spinels, and a high-temperature, platy habit of the dolomite crystals. In all cases, the sparse interstitial residuum between the dolomite plates is potassic, and in Spain and France, the Cr spinels are zoned to high-Ti rims, analogous to those in high-temperature kimberlites. Experiments have long predicted that dolomite should be the initial melt from carbonated mantle below 70 km: In Spain and France, the eruptions carry mantle xenoliths, and in all three provinces, the dolomite volcanic rocks have Mg# >0.65, indicative of primary magmas. Thus, for the first time, fresh constraints are emerging from a multicomponent natural system. Bearing in mind the differences between the European (Cenozoic) and Zambian (Cretaceous) provinces (including lithosphere structures, history, and tectonics), their common features indicate that dolomite melts in the mantle are in equilibrium with chromite, and contain K-Al-Si melts (without Na). The Zambian vents are part of the large, classic Chilwa carbonatite province, which was first described in Malawi and Mozambique, where, around the intrusions, K-metasomites typically exceed carbonatite in amount. When this K is included in the eruptive budget, the total introduced material invites comparison with high-K carbonate melts formed at high pressure, e.g., carbonatitic fluid inclusions in diamonds that were trapped in the diamond stability field are dolomitic and characterized by high-K contents. Intrusive and volcanic carbonatites show a bimodal distribution in Sr-Nd isotopes, similar to group 1 and 2 kimberlites, respectively. Diamond inclusions span the same range. Together with the indications of a deep mantle source for the volcanic facies at Rufunsa, this raises the possibility that the Chilwa complexes may be highlighting a key aspect of carbonatite activity in which potassium has a special role.

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GSA Special Papers

Volcanism and Evolution of the African Lithosphere

Luigi Beccaluva
Luigi Beccaluva
Dipartimento di Scienze della Terra, Università di Ferrara, Italy
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Gianluca Bianchini
Gianluca Bianchini
Dipartimento di Scienze della Terra, Università di Ferrara, Italy
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Marjorie Wilson
Marjorie Wilson
School of Earth & Environment, The University of Leeds, Leeds, UK
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Geological Society of America
Volume
478
ISBN print:
9780813724782
Publication date:
June 01, 2011

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