IS THERE A FRAGMENT OF THE LAURENTIAN MIDCONTINENT RIFT IN EAST ANTARCTICA?

The geophysical signature of the Laurentian mid-continent rift and related Keweenawan large igneous province can readily be traced southwestward from Lake Superior to Kansas. A continuation to Trans-Pecos Texas has been inferred on the basis of both geophysical and geologic evidence. For example the ca. 1120 Ma Red Bluff granite suite in the Franklin Mountains immediately north of El Paso has a Pb isotope signature indistinguishable from that of the Keweenawan lavas. Uranium-lead dating of zircons from flat-lying granophyre and rhyolite in Coats Land, East Antarctica, at the head of the Weddell Sea (~77°49’S, 035°07’W) yielded an age of 1112±4Ma (Gose et al., Journal of Geophysical Research, 1997), equivalent to the earliest stages of Keweenawan magmatism. This is also the age of the Umkondo large igneous province in the Kalahari craton of Africa, which lay close to Coats Land in the reconstructed Gondwana supercontinent. However, the Pb isotope ratios of the Umkondo igneous rocks are markedly more radiogenic than those of Coats Land. The latter are indistinguishable from the ratios of the Keweenawan and Red Bluff suites. Moreover, a high quality paleomagnetic pole obtained from a Coats Land lava which yielded a titanite age of 1106±3 Ma is permissive of the small but geophysically distinct Coats Land crustal block of East Antarctica having been located adjacent to the present southern margin of the Laurentian craton at that time. Hence, as pointed out by Loewy et al. (Geology, 2011), the igneous rocks of Coats Land may be a continuation of those associated with the Laurentian mid-continent rift. This has implications for the reconstruction of the Rodinian supercontinent. The Coats Land block is separated from the Grunehogna block of Dronning Maud Land (~72°45’S, 003°30’W), an unquestioned fragment of the Kalahari craton, by the Maud orogen of Grenvillian age. Hence there appears to have been a Kalahari-Laurentia continent-continent collision at ca. 1.0 Ga. Final separation of Laurentia and the Gondwana supercontinent does not appear to have taken place until the early Cambrian (Dalziel, Geology, 2014). It was that event that separated the possible Coats Land continuation of the mid-continent rift from its parent craton.