Tectonic implications of an 1846±1 Ma old migmatitic granite in south‐central Sweden

Abstract

Abstract The age of migmatitic granites that form in response to crustal thickening after large collision events provides a minimum estimate for the end of subduction‐related processes and the beginning of crustal thickening. Migmatitic granites in south‐central Sweden are bracketed in age by two major phases of granitoid magmatism: 1.89–1.85 Ga arc‐related magmatism and 1.80–1.77 Ga post‐kinematic and post‐metamorphic magmatism. The migmatitic granite at Köping yields a U‐Pb monazite age of 1846±1 Ma and is the oldest so far identified anatectic granite in south‐central Sweden. Its age demonstrates that (1) subduction processes ceased before ca. 1.85 Ga, (2) migmatite granites in south‐central Sweden have the same age as corresponding rocks in southern Finland, and (3) this type of granite seems to be distinctly older in south‐central Sweden than corresponding rock in the Bothnian Basin. The 1846±1 Ma of the migmatitic granite at Köping extends the previously recognized age zonation in geochemically related pegmatites to comprise also migmatitic granites. This age zonation indicates for corresponding rocks older ages to the south, and may be interpreted to be indicative for earlier collision and crustal thickening of the Svecofennian region in the south. Romer, R.L. & Öhlander, B., 1995: Tectonic implications of an 1846±1 Ma old migmatitic granite in south‐central Sweden. GFF, Vol. 117 (Pt. 2, June), pp. 69–74. Stockholm. ISSN 1103–5897.