Several tectonothermal events in the pre-Carboniferous basement of Svalbard during Caledonian and Proterozoic times have been dated recently by radiometric age determinations. Three or four stages have been recognized in the Caledonian period; a post-orogenic graben formation during the Devonian, a late Caledonian event in the Middle Silurian, an early Caledonian event in the Middle Ordovician and possibly an earliest event in the Middle to Late Cambrian. A latest Proterozoic group of ages, 600- 660 Ma, from igneous rocks are not clearly correlatable to a recognized tectonic event, though it is coeval with the Baikalian event at the southeastern coast of the Barents Sea. The Grenvillian event, 950-1270 Ma, has been well established by both radiometric ages and unconformities in Nordaustlandet and southwestern Spitsbergen. Sveco-Karelian ages, 1670-1750 Ma, also have been obtained from Ny Friesland, northeastern Spitsbergen and there is geological support for this event, although it has not been studied in detail yet. Two even older ages (zircon U-Pb upper intercept ages), 2.1 and 3.2 Ga, may suggest the presence of still older crust in Svalbard and adjacent areas.
Y. Ohta, Norsk Polarinstitutt, P.O. Box 158, 1330 Oslo Lufthavn, Norway.