T. Lundqvist et Po. Persson, Geochronology of porphyries and related rocks in northern and western Dalarna, south-central Sweden, GFF, 121, 1999, pp. 307-322
Volcanic rocks and granites belonging to the Transscandinavian Igneous Belt
(TIB) form important constituents of the bedrock in the northern and weste
rn parts of the Province of Dalarna, south-central Sweden. The volcanic roc
ks comprise rhyolitic to quartz-trachytic porphyries as well as mainly inte
rmediate to mafic porphyrites, and contain intercalations of sedimentary ro
cks. U-Pb zircon datings of 11 porphyries, 2 porphyrites, and 3 granites su
ggest a chronostratigraphic subdivision into two major magmatic events, at
1.8 and 1.7 Ga. We propose that the porphyries of these two events should b
e called Older and Younger TIB porphyries, respectively. Some of the porphy
ries and granites representing the younger event have a distinct rapakivi a
ffinity, as defined by both chemical and textural criteria, whereas in othe
rs such features are lacking or only weakly developed. The rocks formed dur
ing the older event lack rapakivi characteristics. Our new chronological su
bdivision conforms with the age pattern in other parts of the TIB, although
the youngest phase of the belt (TIB 3 at 1.65 Ga) is poorly represented in
the Province of Dalarna. This subdivision should replace the previously su
ggested lithostratigraphy which included Gothian porphyries, a Lower Dala g
roup, and an Upper Dala group. Nd isotopic signatures (epsilon(Nd) between
+0.2 and +1.7) indicate that the 1.8 and 1.7 Ga igneous rocks belonging to
the TIB in Dalarna and adjoining areas do not incorporate significant amoun
ts of Archaean crustal material.