Pre-1.91 Ga deformation and metamorphism in the Palaeoproterozoic Vammala Migmatite Belt, southern Finland, and implications for Svecofennian tectonics

Abstract

A metamorphic event in the Vammala Migmatite Belt (VMB) at ~1.92 Ga, revealed by SHRIMP U-Pb analyses of both zircon overgrowths and monazite,is interpreted as post-depositional and is correlated with the development of the early high-grade schistosity.Neither this Early Svecofennian deformation and metamorphism, nor the associated complex folding, is present in the overlying Ta mpere Schist Belt (TSB) sequence, consistent with the VMB being part of a pre-1.91 Ga basement complex. The ~1.92 Ga event provides a maximum deposition age for the TSB,confirming earlier age estimates. Earlier stratigraphic correlations between parts of the VMB and TSB,and associated tectonic interpretations, can no longer be sustained.The crustal thickening seen in the VMB,and previously attributed to arc accretion at ~1.89 Ga, is now attributed to accretion of a large Svionian marginal basin during the 'Early Svecofennian' orogenic phase at ~1.92 Ga.This is of similar age to the deformation and metamorphism associated with collision in the Lapland-Kola Orogen to the north of the Karelian Province. The well-known post-TSB orogenic phase was also identified in the VMB by a monazite age of 18816 Ma. A granitoid intrusion gave an emplacement age of 18885 Ma, comparable to the age of granitoid clasts in the upper part of the TSB succession.