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 occurr ing close to the edge of the Archaean craton in northern Sweden have been s tudied. 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 val ues. 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-N d isotope characteristics as the 1.8 Ga granitoids to the southwest. Our in terpretation 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 obd ucted onto the reworked Archaean continent. The 1.8 Ga granitoids were form ed by remobilisation of continental crust. Partial melting at 1.8 Ga result ed in the intrusion of granitoids carrying the Sm-Nd isotopic signature of the Archaean continent into juvenile rocks. It is probable that the collisi on discussed here was part of a major accretion and amalgamation to the Arc haean craton after the formation of the calc-alkaline 1.9 Ga granitoids, bu t before the formation of the 1.8 Ga granitoids. (C) 1999 Elsevier Science B.V. All rights reserved.