Rb-Sr isotope systematics in the magmatic rocks of the Oslo rift.

Abstract

Fifty-one different units of Oslo Rift magmatic rocks have been investigated by the Rb-Sr isotope method. Isochron ages obtained for all but three of the units vary between 295 Ma and 240 Ma. The ages from the southern (Vestfold) segment of the rift are generally older (294\u00B16 Ma to 266\u00B15 Ma) than those from the northern (Akershus) segment (291\u00B18 Ma to 241\u00B13 Ma), suggesting a northward migration of the magmatic activity with time. A similar migration within each of these two segments of the rift is also suggested by the data. Thomb-porohyry eruptions seem to have started almost simultaneously in the southern and central parts of the rift at around 295-290 Ma. Lava eruption ended earlier (280 Ma) in the southern than in the central area (276 Ma). Caldera formation represents a distinct magmatic stage in the Vestfold segment (269-266 Ma). In the Akershus segment caldera formation seems to have taken place episodically over a longer period of time. Magmatic stages that are distinct in both segments, like the emplacement of multiple larvikite plutons, occurred at a later time in the Akershus (273-266 Ma), than in the Vestfold segment (281-276 Ma). 87Sr\/86 Sr initial ratios obtained for rhomb-porphyry lavas, larvikites and related rocks.