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 Cia crustal-scale Hassela Shear Zone (HSZ). The LjB has emplacement ages of 1.86-1.84 Cia, is mainly alkali-calcic, metaluminous, has epsilon(Nd) 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 epsilon(Nd) 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 F-2 folds, and the fabric is truncated by 1.80 Cia pegmatites.