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Emplacement temperatures of pyroclastic and volcaniclastic deposits in kimberlite pipes in southern Africa

Fontana, G; Mac Nio Caill, C; Brown, RJ; Sparks, RSJ; Field, M

Emplacement temperatures of pyroclastic and volcaniclastic deposits in kimberlite pipes in southern Africa Thumbnail


Authors

G Fontana

C Mac Nio Caill

RSJ Sparks

M Field



Abstract

Palaeomagnetic techniques for estimating the emplacement temperatures of volcanic deposits have been applied to pyroclastic and volcaniclastic deposits in kimberlite pipes in southern Africa. Lithic clasts were sampled from a variety of lithofacies from three pipes for which the internal geology is well constrained (the Cretaceous A/K1 pipe, Orapa Mine, Botswana, and the Cambrian K1 and K2 pipes, Venetia Mine, South Africa). The sampled deposits included massive and layered vent-filling breccias with varying abundances of lithic inclusions, layered crater-filling pyroclastic deposits, talus breccias and volcaniclastic breccias. Basalt lithic clasts in the layered and massive vent-filling pyroclastic deposits in the A/K1 pipe at Orapa were emplaced at >570°C, in the pyroclastic crater-filling deposits at 200–440°C and in crater-filling talus breccias and volcaniclastic breccias at <180°C. The results from the K1 and K2 pipes at Venetia suggest emplacement temperatures for the vent-filling breccias of 260°C to >560°C, although the interpretation of these results is hampered by the presence of Mesozoic magnetic overprints. These temperatures are comparable to the estimated emplacement temperatures of other kimberlite deposits and fall within the proposed stability field for common interstitial matrix mineral assemblages within vent-filling volcaniclastic kimberlites. The temperatures are also comparable to those obtained for pyroclastic deposits in other, silicic, volcanic systems. Because the lithic content of the studied deposits is 10–30%, the initial bulk temperature of the pyroclastic mixture of cold lithic clasts and juvenile kimberlite magma could have been 300–400°C hotter than the palaeomagnetic estimates. Together with the discovery of welded and agglutinated juvenile pyroclasts in some pyroclastic kimberlites, the palaeomagnetic results indicate that there are examples of kimberlites where phreatomagmatism did not play a major role in the generation of the pyroclastic deposits. This study indicates that palaeomagnetic methods can successfully distinguish differences in the emplacement temperatures of different kimberlite facies.

Citation

Fontana, G., Mac Nio Caill, C., Brown, R., Sparks, R., & Field, M. (2011). Emplacement temperatures of pyroclastic and volcaniclastic deposits in kimberlite pipes in southern Africa. Bulletin of Volcanology, 73(8), 1063-1083. https://doi.org/10.1007/s00445-011-0493-9

Journal Article Type Article
Publication Date Jun 1, 2011
Deposit Date Jan 16, 2012
Publicly Available Date Mar 29, 2024
Journal Bulletin of Volcanology
Print ISSN 0258-8900
Electronic ISSN 1432-0819
Publisher Springer
Peer Reviewed Peer Reviewed
Volume 73
Issue 8
Pages 1063-1083
DOI https://doi.org/10.1007/s00445-011-0493-9
Keywords Kimberlite, Emplacement temperature, Palaeomagnetism, Pyroclastic deposits, Thermoremanent magnetization, Explosive eruption

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