The Jurassic mafic and ultramafic dikes of Antarctica: implications for Gondwana break-up
processes and mantle sources of Karoo continental flood basalts
Jussi S. Heinonen1, Arto V. Luttinen1, Teal R. Riley2, Richard W. Carlson3, Matti Kurhila4, and
Mary F. Horan3
1. Finnish Museum of Natural History, University of Helsinki, Finland
2. British Antarctic Survey, Cambridge, UK
3. Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington DC, US
4. Department of Geosciences and Geography, University of Helsinki, Finland
The Karoo continental flood basalt province was emplaced across the land masses of southern
Africa and western Dronning Maud Land, Antarctica, when they still were part of the Gondwana
supercontinent (Fig. 1). Voluminous (~2 000 000 km3) magmatic activity at ~180 Ma ago was
followed by the opening of the Indian Ocean at ~170–160 Ma ago. The widespread flood basalts of
the African Karoo have been extensively studied (e.g., Erlank, 1984), but their origin has remained
a matter of debate. This stems from the fact that even the most primitive magma types from African
Karoo show geochemical evidence of a significant lithospheric component.
Our recent studies in the Antarctic Karoo (Fig. 1) have revealed three magma types that
represent uncontaminated highly magnesian melts formed deep in the mantle (Riley et al., 2005;
Heinonen et al., 2010; Heinonen, 2011). These rocks provide key information on the Karoo flood
basalt magmatism: 1) The depleted ferropicrites of Vestfjella are coeval with the Karoo main
pulse (~180 Ma) and have Sr, Nd, Pb, and Os isotopic signatures that are compatible with a
depleted upper mantle source. Based on geochemical modeling, this source may also have produced
the parental magmas for most of the contaminated flood basalts of Vestfjella. 2) The depleted
picrites of Ahlmannryggen show similar Nd and Os isotope compositions with the Vestfjella
depleted ferropicrites, but exhibit more enriched Sr and Pb isotopic compositions and relative
enrichment of high-field strength elements (e.g., TiO2 > 3 wt. %) suggestive of a distinctive
depleted mantle source. 3) The enriched ferropicrites of Vestfjella that show OIB-like trace
element and Sr, Nd, Pb, and Os isotopic characteristics likely sampled subordinate mantle
heterogeneities, such as recycled oceanic crust or melt-metasomatized portions of the
sublithospheric or lithospheric mantle.
The highly magnesian depleted ferropicrites of Vestfjella represent the hottest and most
primitive magma type described from the Karoo province. Their geochemical compositions,
however, are entirely compatible with a predominant depleted upper mantle source and do not
require deep mantle plume components. Overall, our data for uncontaminated magma types and our
geochemical modeling of voluminous flood basalts lend support to generation of Karoo magmas
mainly as a consequence of heating of the upper mantle beneath the Gondwana supercontinent. The
role of enriched (plume/recycled?) sources remains to be constrained.
Figure 1. Distribution of Mesozoic CFBs in reconstructed Gondwana supercontinent. In the case of the Karoo province,
the known extent of intrusive equivalents (found outside CFBs) is also shown. See Heinonen (2011) for references.
References
Erlank, A.J. (Ed.), 1984. Petrogenesis of the Volcanic Rocks of the Karoo Province.
Geological Society of South Africa, Special Publication, v. 13.
Heinonen, J.S., 2011. Geochemistry and petrology of the ferropicrite dikes and associated
rocks of Vestfjella, western Dronning Maud Land, Antarctica. Academic Dissertation, Department
of Geosciences and Geography A9, Unigrafia, Helsinki, 50 p.
https://helda.helsinki.fi/handle/10138/26267
Heinonen, J.S., Carlson, R.W. and Luttinen, A.V., 2010. Isotopic (Sr, Nd, Pb, and Os)
composition of highly magnesian dikes of Vestfjella, western Dronning Maud Land, Antarctica: A
key to the origins of the Jurassic Karoo large igneous province? Chemical Geology, v. 277, 227–
244.
Riley, T.R., Leat, P.T., Curtis, M.L., Millar, I.L., Duncan, R.A. and Fazel, A., 2005. EarlyMiddle Jurassic dolerite dykes from Western Dronning Maud Land (Antarctica): Identifying mantle
sources in the Karoo Large Igneous Province. Journal of Petrology, v. 46, 1489–1524.
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