LATE PROTEROZOIC AND CARBONIFEROUS ULTRAMAFIC MAGMATISM OF CARBONATITIC AFFINITY IN SOUTHERN NORWAY

Ultramafic dikes with carbonatitic affinities (''damtjernites'') in so uthern Norway were generated during two magmatic events separated by a bout 275 Ma. The older event is late Proterozoic and the younger is mi d Carboniferous. More than 50 satellitic damtjernite intrusions occur within a 1500 km(2) large region surrounding the Fen Central Complex, Phlogopite macrocrysts from 10 of these satellites yield a Rb-Sr isoch ron age of 578+/-24 Ma (2 sigma expanded errors). This demonstrates th at the late Proterozoic carbonatitic magmatism centered at the Fen Cen tral Complex occurred on a regional scale. This region is termed ''the Fen Province''. The emplacement of the magmas in the Fen Province mos t likely occurred in connection with minor extensional tectonic activi ty on the Baltic platform during the drift-phase after the Proto-Atlan tic opening. Sr-isotopic data also show that a dike mineralogically an d chemically similar to the Fen damtjernites was emplaced at 324+/-4 M a (mid Carboniferous). This dike very likely dates the initiation of m agmatism in the Oslo rift. Consequently very similar carbonate-bearing ultramafic magmas were generated within the south Norwegian mantle du ring the relatively minor Fen event and in the initial extensional per iod when the magma production in the Oslo rift was still low.