Chemical features and petrography of early granitoids in the Göteborg-Borås area, south-western Sweden

Abstract

In south-western Sweden, two areas separated by a major tectonic zone were selected for study of the earliest known granitoid groups. This was done in a recently mapped region. In both reference areas, the early granitoids, now dominantly gneissic, comprise the majority of rocks by volume. The western granitoid group is cale-alkaline, with granodiorites being predominant. The eastern granitoid group is distinetly more acid and potassium-rich. The chemical trend is cale-alkaline with alkali-calcie inclination. The niggli ratio of qz vs. si and also the Sr/CaO ratio differ between the two rock groups, the latter being highest in the eastern group. The granitoids have intruded supracrustals of intermediary composition in the western region. In the eastern region, the side-rocks are scattered amphibolites and a few xenoliths of supracrustal gneisses. The western granitoids thus intruded into a sial crust, while the eastern granitoids apparently display a more paradoxical situation: intrusion into a dominantly basic crust by granitoids which are rather evolved. The latter aspect seems to indicate either that the sial crust was assimilated, which is not supported by field evidence, or that the differentiation process was prolonged. The latter interpretation seems to be supported by the higher Sr/CaO ratio for the eastern granitoid group, which indicates an intermediary or greater (>40 km) depth of magma formation.

Key Words:

• Granitoid formation
• granitoid chemistry
• calc-alkaline
• alkali-calcic
• alkaline
• crustal growth
• Sr
• niggli qz
• niggli si
• intrusion pattern
• Proterozoic
• south-western Sweden
• N5725 N5808 E1317 E1159