ISOTOPE GEOCHEMISTRY AND AGE RELATIONSHIPS OF MAFIC INTRUSIONS ALONG THE PROTOGINE ZONE, SOUTHERN SWEDEN

The Protogine Zone is a major tectonic lineament separating the Southwest Swedish gneiss province in the west from the Transscandinavian granite-porphyry belt and the Blekinge region in the east. Mafic intrusions occurring within the southern part of the Protogine Zone, west of it in Varmland county, and east of it in Blekinge county, have been investigated by the Sm-Nd and Rb-Sr methods. A dolerite sill from Varmland gives a Sm-Nd mineral isochron age of 1512 +/- 98 Ma, confirming the middle Proterozoic age for this swarm. The dolerite dykes within the Protogine Zone proper appear to be divided into two age groups using Sm-Nd, one of around 1180 Ma in age and the other around 930 Ma. An age of around 930 Ma is also suggested for most of the Blekinge dykes, based on the Sm-Nd results. The Rb-Sr system in most dykes shows more scatter, and is less reliable for dating these rocks. None of these mafic rocks shows any strongly depleted initial Nd signature. The highest initial epsilon(Nd) value, + 1.7, comes from the Olme dolerite in Varmland. Most of the 1180 Ma old Protogine Zone dolerites have initial epsilon(Nd) values around -2 and the 930 Ma old Protogine Zone and Blekinge dykes show values around 0. These values are attributed either to a near-chondritic or slightly enriched mantle source or to early large-scale mixing within the lower crust, rather than to local crustal contamination. The c. 1500 Ma Varmland dolerites may have formed during an extensional event that never led to continental breakup, while the c. 1180 Ma Protogine Zone dolerites form part of the extensional magmatism in Fennoscandia, Canada and Greenland preceding the Grenvillian-Sveconorwegian orogeny. The c. 930 Ma Protogine Zone and Blekinge dykes may have formed in response to the late Sveconorwegian uplift of the area west of the Protogine Zone. Late-stage dykes in Blekinge may alternatively be related to the opening of the Iapetus Ocean.