Sm-Nd isotope evidence of a collisional event in the Precambrian of northern Sweden

The Sm-Nd isotope characteristics of c. 1.9 and c. 1.8 Ga granitoids occurring close to the edge of the Archaean craton in northern Sweden have been studied. Strongly negative epsilon(Nd(t)) values occur farther southwest in the potassic 1.8 Ga granitoids than in the calc-alkaline 1.9 Ga granitoids. Intrusions of the younger granitoids with distinctly negative epsilon(Nd(t)) values are surrounded by granitoids of the older group with positive values. Northeast of the 1.8 Ga granitoids discussed here, the c. 1.9 Ga calc-alkaline intrusions formed within the Archaean continent have the same Sm-Nd isotope characteristics as the 1.8 Ga granitoids to the southwest. Our interpretation is that a juvenile volcanic-arc terrane to the south collided with and was thrust onto the Archaean continent after the formation of the 1.9 Ga granitoids. The outcome was that these juvenile rock masses were obducted onto the reworked Archaean continent. The 1.8 Ga granitoids were formed by remobilisation of continental crust. Partial melting at 1.8 Ga resulted in the intrusion of granitoids carrying the Sm-Nd isotopic signature of the Archaean continent into juvenile rocks. It is probable that the collision discussed here was part of a major accretion and amalgamation to the Archaean craton after the formation of the calc-alkaline 1.9 Ga granitoids, but before the formation of the 1.8 Ga granitoids. (C) 1999 Elsevier Science B.V. All rights reserved.