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Abstract
Volcanic rocks and granites belonging to the Transscandinavian Igneous Belt (TIB) form important constituents of the bedrock in the northern and western parts of the Province of Dalarna, south-central Sweden. The volcanic rocks comprise rhyolitic to quartz-trachytic porphyries as well as mainly intermediate to mafic porphyrites, and contain intercalations of sedimentary rocks. U-Pb zircon datings of 11 porphyries, 2 porphyrites, and 3 granites suggest 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 be called Older and Younger TIB porphyries, respectively. Some of the porphyries and granites representing the younger event have a distinct rapakivi affinity, as defined by both chemical and textural criteria, whereas in others such features are lacking or only weakly developed. The rocks formed during the older event lack rapakivi characteristics. Our new chronological sub-division 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 suggested lithostratigraphy which included Gothian porphyries, a Lower Dala group, and an Upper Dala group. Nd isotopic signatures (∊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 in-corporate significant amounts of Archaean crustal material.
