12. Kirwan Volcanic Group

Fig. 14.2 Queen (or Dronning) Maud Land contains groups of nunataks composed of the Jurassic Kirwan Volcanics. The groups of nunataks are identified by number 1, Vestfjella 2, Fossilryggen 3, Mannefallknausane 4, Heimefrontfjella 5, Bjornnutane 6, Sembberget and 7, Kirwan Escarpment. These groups of nunataks rise above the surface of the East Antarctic ice sheet which covers the entire area on this map (Adapted from Fig. 1 of 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 numerous basalt dikes which intruded the flows of southern Vestfjella are unaltered in contrast to the flows (Fumes and Mitchell 1978). Nevertheless, Fumes et al. (1982) reported that the initial Sr/ Sr ratios of ten dikes range from 0.70347 to 0.70687 and average 0.70460 0.00061 (2a). This result reinforces the evidence that the initial Sr/ Sr ratios of basalt flows and dikes of the Kirwan Volcanics in Queen Maud Land are not constant and that average values of this ratio are lower than the initial Sr/ Sr ratios of the basalt flows and dolerite sills of the Ferrar Group in the Transantarctic Mountains. 

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))...

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