On Some Age Relations in the Pre-Cambrian of Southwestern Sweden

Abstract

In view of forthcoming radiological data on the ages of rocks from S.W. Sweden an attempt is made to reconstruct the geologic history of this region on the basis of purely geological and petrological evidence.

It is shown that there exist two paragenetically different groups of granitic pegmatites. One of these is, in its distribution, clearly related to the Bohus granite. The Högsbo pegmatite deposit near Gothenburg paragenetically belongs to this particular group.

The other group of pegmatites is related to the Askim granite on the basis of field evidence in the Gothenburg area.

The Askim granite is shown to be older than the Bohus granite, since it is cut by swarms of meta-dolerites (the Koster dikes), which in turn are older than the Bohus granite.

On the other hand the Askim granite is shown to be younger than quartzdiorite dikes which intrude the Frölunda granite-gneiss. Likewise it is younger than bodies of metahyperite which are most probably intrusive into the Lundby granite-gneiss. Hence the Askim granite is younger than the Åmål—Kroppefjäll group of granites with which these two granite-gneisses can be correlated.

It is also suggested that the region went through a stage of intense metamorphism prior to the emplacement of the Askim granite. The charnockites of Varberg indicate that it attained the grade of granulite facies in some areas. At the time of the Askim granite the metamorphic grade was again reduced corresponding to epidote-amphibolite facies.

Finally the recognition of orogenic cycles in the older bedrock of the region is discussed. It is concluded that at present there exists no safe basis for outlining the lower limit of the Gothian cycle.