Geology and Geochronology of the Suomujärvi Complex: a new Archaean gneiss region in the NE Baltic Shield, Finland

• Published in: Precambrian research 2002-06, Vol.116 (3), p.285-306
• Main Authors: Evins, Paul M, Mansfeld, Joakim, Laajoki, Kauko
• Format: Article
• Language: English
• Subjects: Archaean; Belomorian; Finland; SIMS; U–Pb zircon geochronology
• ISSN: 0301-9268; 1872-7433
• DOI: 10.1016/S0301-9268(02)00025-6

Recent mapping in the eastern Central Lapland Granitoid Complex and dating of zircons at the NORDSIM facility was performed in order to highlight the existence of an 892-km 2 area of Archaean gneisses not previously identified in southeast Lapland, Finland. This Archaean enclave is composed of northeast-trending, km-scale bands of orthogneisses with varying mafic content and is named the Suomujärvi Complex. Along its southeastern boundary lies the Aholanvaara volcano-sedimentary package. The evolution of the Suomujärvi Complex began at ca. 2810 Ma with intrusion of the homogeneous Jumisko biotite tonalite–granodiorite in the west. This lithology becomes increasingly heterogeneous to the east where muscovite and an inherited ca. 2870 Ma old rock component represented by zircon cores are present. These orthogneisses were later intruded by the 2755±18 Ma (1 σ) protolith of the Miehinkävaara amphibole and biotite bearing diorite–tonalite gneisses producing diffuse contacts characterized by gradual increases in mafic minerals. All of the orthogneisses were then migmatized/metamorphosed. After ca. 2730 Ma the Aholanvaara volcano-sedimentary package was deposited. Its main quartzite unit was dominantly derived from an unknown 2731±17 Ma (1 σ) source but it also contains a ca. 3.4 Ga detrital component. The absence of 2800–2820 Ma detrital material in these sediments indicates they were not derived locally from the Suomujärvi Complex. Instead, they share affinities with supracrustal packages further south in Posio, east in the Belomorian Province and north in the Central Lapland Greenstone belt. Widespread, voluminous intrusion of granites occurred in the area surrounding the Suomujärvi gneisses and left a marked metamorphic overprint on the gneisses during the ca. 1.8 extensional collapse of the Svecofennian and Lapland-Kola orogens. Multi-scale folding produced subhorizontal, southwest-plunging, tight to isoclinal, upright folds and the dominant axial planer, northeast-trending fabric in the area. This fabric is cut by a northwest-trending fabric in the southwestern part of the area. Geological characteristics of the Suomujärvi Complex and ages of the orthogneisses within it indicate that it most likely represents the southwestern margin of the Archaean Belomorian block.