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

• Published in: Precambrian research 2008-06, Vol.164 (1), p.86-109
• Main Authors: Skyttä, Pietari, Mänttäri, Irmeli
• Format: Article
• Language: English
• Subjects: Extension; Fennoscandian Shield; Ion microprobe; Structural geology; Tectonics; U–Pb geochronology; Zircon
• ISSN: 0301-9268; 1872-7433; 1872-7433
• DOI: 10.1016/j.precamres.2008.04.001

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