Mesoproterozoic bimodal magmatism along the Protogine Zone, S Sweden: three magmatic pulses at 1.56, 1.22 and 1.205 Ga, and regional implications

摘要

The Protogine Zone is a major tectonic zone situated at the eastern boundary of the Sveconorwegian orogenic belt in southern Sweden. This structure hosts Mesoproterozoic dolerite dykes and plutonic rocks including ultramafic cumulate, syenite, monzonorite and granite. Pb-Pb zircon evaporation ages are reported for seven samples of rocks along the Protogine Zone, including the Vaggeryd syenite pluton. Two samples of the main syenite facies of the Vaggeryd pluton yield ages of 1220 +/- 3 and 1219 +/- 3 Ma. Three samples of anorthosite gabbro, aplite and pegmatite, all of which form dykes in the Vaggeryd pluton, yield equivalent ages of 1220 +/- 3, 1221 +/- 3 and 1218 +/- 3 Ma. They demonstrate that the different facies recorded in the pluton belong to a single magmatic pulse at 1220 Ma, significantly older than previous estimate of 1204 +/- 4 Ma (Jarl 2002). Samples of the Gumlosa-Glimakra granite in NE Scania and the Aker metabasite, located immediately west of the Vaggeryd syenite, were dated at 1204 +/- 3 and 1567 +/- 3 Ma, respectively, hence reproducing earlier reported ages for these intrusions determined by conventional TIMS and SIMS U-Pb techniques (1204(-15)(+16) Ma, Johansson 1990 and 1562 +/- 6 Ma, Soderlund et al. 2004a). The 1575-1562 Ma suite of mafic intrusions along the Protogine Zone shows temporal correspondence with rapakivi magmatism in SW Finland and suggests that the Protogine Zone first may have originated as a zone of crustal weakness, which later was reactivated (e.g. at 1224-1215 and ca. 1204 Ma) in an extensional, possibly back-arc, tectonic setting prior to the 1.13-0.95 Ga Grenvillian-Sveconorwegian orogeny.