Petrography and whole-rock geochemical characteristics of Vastervik granitoids to syenitoids, southeast Sweden: constraints on petrogenesis and tectonic setting at the southern margin of the Svecofennian domain

摘要

Granitoid to syenitoid petrogenesis along the Laurentia–Baltica continental margin occurred in two cycles. Within the Fennoscandian Shield, the first cycle is correlated with the 1.95–1.8 Ga Svecofennian domain and the second is correlated with the 1.85–1.65 Ga Transscandinavian Igneous Belt (TIB). Affiliation of Palaeoproterozoic granitoids to syenitoids (～1.8 Ga) in a transitional zone between the Svecofennian domain to the north and the TIB to the south in the Vastervik area in southeast Sweden, is highly debated. In this study, the Vastervik granitoids to syenitoids have been studied for major and trace element, whole-rock Sm–Nd and U–Pb single zircon isotopic compositions. Field observations, petrography and geochemical composition allow discrimination into five groups ranging from tonalitic to syenogranitic compositions. Geochemical signatures indicate magnesian, metaluminous cordilleran character for three of them, whereas the two most differentiated groups show a transition into ferroan, peraluminous granites with postorogenic or A-type characters. The former three groups may be correlated with the TIB-1 suite at 1.81–1.77 Ga. The older of the A-type groups may be correlated with the TIB-0 suite that intruded at 1.87–1.84 Ga whereas the younger group possibly is related to the socalled granite–pegmatite suite. An earlier tectonic model implies a migration of the subduction zonesystem to the present southwest. The defined granitoid to syenitoid groups of the Vastervik region fit nicely into the model and support the idea of a new tectonic cycle to the south of the Bergslagen region.