Geochronology of porphyries and related rocks in northern and western Dalarna, south-central Sweden

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.