Structural setting of late Svecofennian granites and pegmatites in Uusimaa Belt, SW Finland: Age constraints and implications for crustal evolution

Abstract

Complex late Svecofennian deformation patterns and voluminous granitic magmatism at ∼1.85–1.80 Ga characterise the Palaeoproterozoic Uusimaa Belt in SW Finland. Detailed structural analysis and ion microprobe U–Pb zircon geochronology were used to (i) evaluate the evidence for crustal extension that was assumed to pre-date the predominant contractional deformation, and (ii) integrate tectonic evolution with granitic magmatism. The early Svecofennian structures (D 1 and D 2) are only locally preserved in specific parts of the Uusimaa Belt, while the late Svecofennian structures occur throughout the area. Coaxially deformed granite dykes and weak S 3 foliations in granite most likely indicate that crustal extension (D 3) took place concurrent with the initial stages of crustal melting during ∼1835–1825 Ma. Subsequent evolution involved continued granitic magmatism and a shift to contraction at ∼1820 Ma. Contraction started under horizontal ∼N–S bulk compression and resulted in intense upright as well as N–NW vergent asymmetric folds (D 4). Continued deformation with bulk compression shifting towards E–W at <1820 Ma caused strain localisation into sub-vertical transpressive shear zones with east-side-up shear sense (D 5). Another expression of sub-vertical stretching during D 5 is the generation of sheath and doubly plunging F 5 folds. The volume of granite magmatism decreased from D 3 to D 5. Also the mode of granite emplacement shifted from large flat-lying sheets during D 3 towards thinner migmatising sheets and veins during D 4, while D 5 involved intrusion of cross-cutting pegmatite dykes along sub-vertical crustal weakness zones.