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

Content Provider	Springer Nature Link
Author	Fontana, Giovanni Mac Niocaill, Conall Brown, Richard J. Sparks, R. Stephen J. Field, Matthew
Copyright Year	2011
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.
Starting Page	1063
Ending Page	1083
Page Count	21
File Format	PDF
ISSN	02588900
Journal	Bulletin of Volcanology
Volume Number	73
Issue Number	8
e-ISSN	14320819
Language	English
Publisher	Springer-Verlag
Publisher Date	2011-06-15
Publisher Place	Berlin/Heidelberg
Access Restriction	One Nation One Subscription (ONOS)
Subject Keyword	Kimberlite Emplacement temperature Palaeomagnetism Pyroclastic deposits Thermoremanent magnetization Explosive eruption Geophysics/Geodesy Geology Sedimentology Mineralogy
Content Type	Text
Resource Type	Article
Subject	Geochemistry and Petrology

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