7.5.2019

Science Blog: Ancient Supercontinents in East Antarctica – Africa Border Zone Revealed by Xenoliths from Vestfjella

Teksti:
Ilona Romu, Geologist

Exploration and research in the remote and long inaccessible Arctic and Antarctic regions became possible at the late era of sailing vessels. In the western Dronning Maud Land of East Antarctica, Vestfjella, known also as the Kraul Mountains, was first listed on the map in the 1930s and some of the first printed geological notes were published in 1959. The 120 km long range of Vestfjella Mountains is limited between the shelf ice of the Weddell Sea and the continental ice sheet of East Antarctica. However, this is only the latest geographical position for Vestfjella. The bedrock geology of Vestfjella, which has been studied by Finnish geologists and geophysicists since 1989 under the strict Antarctic Treaty regulations, is dominated by continuous flood basalt lava fields aged 180 Ma.

Beneath these formations, which cover over 250 000 km2 on the present continents of Africa, Antarctica, South America and the Indian subcontinent, the Vestfjella bedrock is concealed. In southern Vestfjella, rare ultrapotassic intrusions transported rock fragments, xenoliths, from inaccessible crustal depths of 10-52 km ca. 160 Ma ago. The story recorded by these Vestfjella xenoliths progresses from mountain building in the Mesoproterozoic era (1350 Ma) to the late stage of Gondwana continent break-up and related crustal melting in the Jurassic period (164 Ma). During those one billion years, organic life of Earth transformed from multicellular life forms to early mammals and cold-blooded animals, while the concealed bedrock of Vestfjella was deformed, repeatedly heated and tectonised. As organic life evolved, also the lithosphere evolved.

To model the geochemical evolution of the lithosphere, i.e. the rocks and minerals it comprises, the geochemistry, mineralogy and specific isotopic features need to be determined and interpreted. My PhD thesis involved the initial ground-breaking steps of geochemical modelling of meta-igneous and meta-sedimentary Vestfjella xenoliths and zircon U-Pb dating. Petrologists and geochemists studying igneous geochemistry are very familiar with genetic classifications based on incompatible fluid-immobile elements. The protoliths of the xenoliths studied were predominantly aged 1000-1300 Ma. These rock types originated during the Grenville orogeny, which welded the present Africa, Antarctica, Australia, North and South America and parts of Eurasia to form the supercontinent Rodinia. Over 4000 km of mountain ranges rose to heights as great perhaps as the Andes today. Our project group also observed the youngest zircon age of Vestfjella, 164 Ma, possibly indicating the end of Jurassic crustal heating by the Bouvet mantle plume. This project was funded by the Academy of Finland and the Adult Education Fund of Finland. The Finnish Antarctic Research Programme (FINNARP) and FINNARP 2007 Crew provided vital logistics and field assistance. Invaluable laboratory support was provided by the University of Helsinki (Department of Geology), Museum of Natural History, Stockholm (NORDSIMS) and the Geological Survey of Finland and their personnel.

References:

Romu, I. 2019. Origin of the concealed continental crust of Vestfjella, western Dronning Maud Land, Antarctica – Evidence from xenoliths hosted by Jurassic lamproites. Geological Survey of Finland, Bulletin 409, 100 pages, 25 figures, 10 tables and 2 appendices (submitted).

Romu, I., Kurhila, M. & Luttinen, A. 2016. Formation of continental crust at rifted Gondwana margin of Western Dronning Maud Land, Antarctica: Petrological, geochemical and geochronological evidence. NordVulk summer school in Tectono-magmatic evolution of the NE Atlantic – from continental break-up to passive margins, June 12-20, 2016, Keilir Reykjanesbær, Iceland. (Poster presentation).

Romu, I. & Luttinen, A., 2007. Antarctica: A keystone in a changing world. In: Online Proceedings for the 10th International Symposium on Antarctic Earth Sciences. Cooper, A. & Raymond, C. (eds).  Extended Abstract 080 5 p. (USGS Open-File Report 2007/1047).

Fig. 1. Silicic crustal xenoliths hosted by an ultrapotassic mica-rich boulder. Photographed on-site on the saddle of Kjakebeinet nunatak, southern Vestfjella. Photo: Ilona Romu (KRRO), University of Helsinki.

 

Fig. 2. At the end of a busy field day, one of 22 days spent on southern Vestfjella and Heimefrontfjella. Kjakebeinet nunatak can be seen in the background. Photo: Mika Kalakoski, FINNARP.

 

Fig. 3. The FINNARP 2007 expedition crew preparing for their trip to the coast of the Weddell Sea. On the ground at the Finnish-Swedish Nordenskiöld Base Camp, Basen nunatak. Photo: Ilona Romu, University of Helsinki.
Ilona Romu

Teksti: Ilona Romu

Ilona Romu (Ms.) is a geologist working for GTK’s Unit of Regional Geodata and Interpretation in Kuopio. She has been working on tasks related to bedrock and resources since 2011.