Karoo basalts

Fig. 14.1 Queen (or Dronning) Maud Land is located in East Antarctica along the east coast of the Weddell Sea. It is not considered to be an extension of the Transantarctic Mountains, although it does include flat-lying sandstones of the Permian Amelang Formation which are overlain by continental flood basalts of the Jurassic Kirwan Volcanics. The chemical and isotopic compositions of the basalt flows and sills resemble the Karoo Basalt of southern Africa which was adjacent to Queen Maud Land before the break-up of Gondwana (Adapted from Fumes et al. (1987))...

The Karoo Basalt, like the Kirwan Volcanics, formed from mantle-derived magma that differentiated by fractional crystallization and concurrent contamination in an open system in contact with granitic gneisses of Precambrian age. The proximity of Queen Maud Land to southern Africa in pre-Jurassic time supports the hypothesis that the Kirwan Volcanics are more closely related to the Karoo Basalt than to the basalt and dolerite of the Ferrar Group (Neethling 1972b Harris et al. 1991 Faure 2001, Fig. 5.67). 

The basalt plateans that form by the extrusion of lava through deep fractures in the crust may be tom apart when the rifts widen into oceans (e.g., the Kirwan Volcanics of Queen Mand Land and the Karoo Basalt of Southern Africa). Other examples of paired basalt plateaus that are now separated by an ocean are the Edendeka Basalt of Namibia and the Parana Basalt of Brazil as well as the diabase dikes of Liberia in West Africa and the diabase and basalt along the east coast of North America. Additional, examples were described by Faure (2001). 

The break-up of Gondwana was initiated by the arrival of a mantle plume in the area between Queen Maud Land of East Antarctica and southern Africa. The stresses imposed on the lithosphere by this plume resulted in the development of a triple junction consisting of crustal rifts. The decompression melting of the rocks in the plume head and in the adjacent lithospheric mantle formed basalt magma which was erupted at the surface to form the Kirwan Volcanics of Queen Maud Land and parts of the Karoo Basalt of southern Africa. The rift between Africa and East Antarctica subsequently widened by seafloor spreading into the Weddell Sea and the Indian Ocean. 

Karoo basalt, Lesotho JP 58, South Africa (Erlank 1984). 

Ellam R. M. and Cox K. G. (1991) An interpretation of Karoo picrite basalts in terms of interaction between asthenospheric magmas and the mantle lithosphere. Earth Planet. Sci. Lett. 105, 330-342. 

Ellam R. M., Carlson R. W., and Shirey S. B. (1992) Evidence from Re-Os isotopes for plume-lithosphere mixing in Karoo flood basalt genesis. Nature 359, 718-721. 

They include continental flood basalts of the type found in the Deccan plateau in India or the Karoo province of southern Africa, and, where the volcanism is within oceanic crust, oceanic plateaus. Most Mesozoic and Cenozoic LIPs had an original areal extent of more than a million km2 and represent magma volumes of up to 4 million km3. The Ontong Java oceanic plateau has a lava volume of 6 million km3 and when extrusive and intrusive components are aggregated a total volume of 44.4 million km3 (Ernst et al., 2005). 

Duncan A.R., 1987, The Karoo igneous province — a problem area for inferring tectonic setting from basalt geochemistry. J. Vole. Geotherm. Res., 32, 13-34. 

The initial Sr/ Sr ratios reported by Harris et al. (1990) have a somewhat higher average than the basalt on nunatak B reported by Faure et al. (1979). Nevertheless, the initial Sr/ Sr and 1/Sr ratios of the eight freshest samples selected by Harris et al. (1990) out of a total of 26 also form a mixing line with a negative slope and a correlation coefficient of 0.79. These authors noted that a similar phenomenon occurs in the basalts of the Sabie River Formation (Karoo Group) in southern Lebombo (Cox and Bristow 1984). 

Fig. 14.16 (a) The Archean gneisses of the Grunehogna craton in Queen Maud Land of East Antarctica are an extension of the Kaapvaal craton of southern Africa when both are restored to the positions they occupied when the Kirwan Basalt and the Karoo... 

Fig. 14.19 The chemical types (CT) of flows and dikes on Plogen and Basen are defined in terms of concentration ratios of Ti/P vs. Ti/Zr (a) and Nb/Ta vs. Ti/Zr (b). The chemical compositions of CT-1 and CT-2 resemble certain basalt flows of the Karoo Group in southern Africa, whereas the CT-3 samples resemble the Kirwan Volcanics on the nunataks of southern Vestfjella. A fourth chemical type (CT-4) was later added by Luttinen et al. (1998) (Data and interpretation from Luttinen and Siivola (1997))...

Fig. 25. Variations in lanthanide patterns between the southern (Lesotho) and northern (Nuanetsi) sectors ol the Karoo plateau basalt province, probably reflecting major changes in mantle chemistry. There is no change in crustal thickness or composition across this boundary. [Data are from Erlank (1984) and from table 16.]...

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