Continental margin magmatism and migmatisation in the west-central Fennoscandian Shield

The Ljusdal Batholith (LjB) is a major component of the central Svecofennian Domain in Sweden. It is separated from the Bothnian Basin to the north by the 1.82–1.80 Ga crustal-scale Hassela Shear Zone (HSZ). The LjB has emplacement ages of 1.86–1.84 Ga, is mainly alkali-calcic, metaluminous, has ɛNd values between − 0.3 and + 1.2 and was formed in a magmatic arc setting. During the Svecokarelian orogeny the LjB was affected by at least three fold episodes. Large-scale folded screens of migmatised metasedimentary rocks occur in the eastern part of the batholith, and to the north of the HSZ, there is a 50 km wide diatexite belt. The Transition Belt (TrB), consisting of 1.88–1.85 Ga granitoids, is located at the northwestern extension of this belt. A calc-alkaline and peraluminous composition combined with negative ɛNd values (− 1.7 to − 0.8) indicates a large proportion of metasediments in the source for these granitoids. U–Pb SIMS data on zircon rims from migmatites and leucogranites to the north and east of LjB yield ages of 1.87–1.86 Ga, i.e. coeval with the granitoids of the LjB and the TrB. There is thus a close relationship between the LjB, the TrB and the migmatites in both space and time. Syn-migmatitic shearing along the HSZ indicates that a proto-HSZ was initiated already at c. 1.86 Ga, and the location of the proto-HSZ is inferred to be controlled by two older nuclei present in the lower parts of the crust. As crustal-scale shear zone systems are known to act as ascent pathways for sheet-like flow in active orogenies the TrB may represents accumulations of melts that were attracted and extracted by the proto-HSZ and intruded in a block that was not pervasively affected by subsequent shear along the HSZ. An active continental margin setting for the LjB implies subduction at c. 1.86 Ga, and provides a heat source for both the migmatites and the TrB. A later migmatisation at 1.82 Ga has been recorded to the south of the HSZ. Within the LjB the 1.82 Ga stromatic migmatites are folded by F2 folds, and the fabric is truncated by 1.80 Ga pegmatites.