Age and emplacement of late Sveconorwegian monzogabbroic dykes, SW Sweden

Abstract

The monzogabbroic (jotunitic) Göteborg dykes in SW Sweden display igneous mineralogy and texture, with chilled margins that crosscut all ductile structures in the wall rock. Morphological features suggest emplacement into brittle crust along a set of pre-existing WNW- to W-trending fractures. U–Pb dating of baddeleyites from the largest of these dykes, the Tuve dyke, yields a late Sveconorwegian (Grenvillian) age of 935±3 Ma, which is considered as the crystallisation age of the dyke swarm. The Göteborg dykes and the Hakefjorden Norite–Anorthosite Complex on the Swedish west coast show many similarities with the jotunitic rocks in the Rogaland Anorthosite Complex of south-western Norway, in terms of age, major element composition and Fe–Ti oxide concentrations. The Blekinge–Dalarna dyke swarm east of and along the Protogine Zone in south-eastern Sweden, and the Hunnedalen dykes in south-western Norway, also record the characteristic high Ti–P–K, evolved “jotunitic” character. A distinct feature of all these late Sveconorwegian mafic rocks is the low content of normative diopside (low Ca), but in other aspects they resemble modern continental flood basalts. Ages around 930 Ma for many late Sveconorwegian intrusions correspond in time with the latest phase of extensional movements along crustal-scale shear zones, suggesting that this magmatism is related to extensional collapse and exhumation of the Sveconorwegian orogen.