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U-Pb age determinations on zircon from granitoids in the Archean of eastern Finland were conducted using the SIMS, LA-ICP-MS and TIMS techniques, with an emphasis on low-HREE granitoids. The oldest rocks in the Fennoscandian Shield are 3.4-3.5 Ga. Several samples were collected close to these rocks, but none of the samples were as old, indicating that the oldest rocks are only small, possibly allochthonous fragments in the Neoarchean bedrock. Some tonalite-trondhjemite-granodiorite (TTG) samples yielded homogeneous 2.72-2.73 Ga zircon populations, and in these samples, the initial εNd was also close to the depleted mantle (DM) values. However, several granitoid samples with a main zircon population of 2.7-2.8 Ga had 2.9-3.2 Ga grains or inherited cores, and in some samples, all grains were of 2.9-3.0 Ga. In these samples, the εNd value was also close to zero or slightly negative. These features suggest that apart from the juvenile Neoarchean magmas, the abundance of reworked 2.9 Ga material is considerable in the Archean crust, which developed during successive juvenile magmatic inputs that melted and assimilated the older sialic crust. The low-HREE geochemical character of granitoids has no correlation with their age, with the low-HREE granitoids yielding an age span of 2.72-2.98 Ga. [ABSTRACT FROM AUTHOR]

We have identified two contrasting styles of Paleoproterozoic metamorphism in the northern part of the Fennoscandian Shield. The Karelia and Lapland-Kola Provinces, comprising Archean and overlying Paleoproterozoic supracrustal rocks, show a typical medium pressure Barrovian-style metamorphism with commonly found kyanite-bearing mineral assemblages and ITD (isothermal decompression) PT paths. In the juxtaposed Svecofennia Province metamorphism represents low pressure-high temperature Buchan style with garnet-cordierite migmatites and intercalated andalusite-cordierite and andalusite-staurolite schists and sillimanite-muscovite gneisses. The retrograde PT paths show only a moderate uplift during cooling. U-Pb age determinations on monazite were made using the LA-ICP-MS from more than 80 samples from metasedimentary rocks. The sampling covered most parts of the Paleoproterozoic bedrock in Finland. The analyses reveal three peaks at c. 1.91 Ga, 1.86–1.88 Ga and at 1.79–1.81 Ga. The oldest, c. 1.91 Ga monazites are mostly found in the Lapland-Kola Province which is located in the northernmost Finland. In the Karelia Province where the Paleoproterozoic is underlain by Archean bedrock monazite yielded ages of 1.76‒1.81 Ga with only a few older exceptions in samples showing a spread of 207Pb/206Pb ages from c. 1.92–1.81 Ga. The Karelia Province underwent tectonic thickening, where monazite ages of around 1.80 Ga mostly represent exhumation near the temperature maximum. In the Svecofennia Province monazite ages vary from c. 1.89 to 1.78 Ga. In the Western Finland Subprovince the monazite ages in high-grade migmatites are mostly 1.86‒1.88 Ga but within the older migmatite areas there are lower grade zones where monazite yields ages of c. 1.80 Ga. Some samples also show a spread of 207Pb/206Pb ages from 1.89‒1.86 Ga to c. 1.78 Ga. In the Southern Finland Subprovince most ages are either 1.80‒1.78 Ga, especially in the andalusite grade schists, or the sample shows a spread of 207Pb/206Pb ages from c. 1.88 to 1.78 Ga. Only in the eastern part of the Southern Finland Subprovince there are rocks which yield merely 1.86‒1.89 Ga ages. Low pressure-high temperature metamorphism and lack of high or medium P/T rocks in the Svecofennia Province refers rather to accretionary than collisional processes. [ABSTRACT FROM AUTHOR]

U–Pb age data from Archean lower crustal granulites in the Varpaisja¨rvi area, central Finland, indicate several metamorphic events that took place in Mesoarchean–Neoarchean and Paleoproterozoic time. Zircons separated from the granulites fall into two distinct age groups. In the older group, the protolith age is 3.2 Ga and many zircons indicate metamorphism at ca. 3.1 Ga. In contrast, the oldest zircons in the younger group are ca. 2.73 Ga. Both terranes were migmatized at ca. 2.70 Ga with the emplacement and crystallization of igneous enderbites. The enderbites and the leucosomes of migmatites in both granulite terranes have 2.7 Ga zircons. High grade metamorphic conditions prevailed from 2.70 to 2.63 Ga which is the age range of most zircons in the eastern terrane. Also the enderbites show metamorphic zircon growth at ca. 2.64 Ga. Two distinct age groups are interpreted to be the result of juxtaposition of two terranes of different ages. This indicates that collisional and accretionary processes operated during Neoarchean time. The area east from the eastern granulite terrane underwent almost pervasive deformation and amphibolite facies metamorphism during the Svecofennian orogeny at ca. 1.9 Ga. However, the U–Pb ages of zircons in the retrograde area are similar to those of the eastern terrane, thus indicating that these rocks were metamorphosed under granulite facies conditions (2.70–2.63 Ga). Although the ca. 1.9 Ga Svecofennian metamorphism is not evident in the ion microprobe results, some Paleoproterozoic Pb-loss is indicated by TIMS U–Pb analyses of zircons. In addition, monazite age of 1.89 Ga from the retrograde area reflects Svecofennian metamorphism. [Copyright &y& Elsevier]

Zircons from six samples collected from igneous and metamorphic rocks were dated using the NORDSIM ion microprobe, in order to investigate the tectonic evolution of the Palaeoproterozoic Svecofennian Orogen in southwestern Finland. These rocks represent pre-collisional, collisional and post-collisional stages of the orogeny. The ion microprobe results reveal two age groups of granodioritic–tonalitic rocks. The intrusions have different tectonic settings: the Orijärvi granodiorite represents pre-collisional 1.91–1.88 Ga island-arc-related magmatism and yielded an age of 1898±9 Ma, whereas the collision-related Masku tonalite was dated at 1854±18 Ma. The latter age accords with more accurate previous conventional zircon age data and constrains the emplacement age of collisional granitoids to ≈1.87 Ga. This is interpreted to reflect the collision between the Southern Svecofennian Arc Complex with the Central Svecofennian Arc complex and the formation of a suture zone between them during D2 deformation. Granulite facies metamorphism in the Turku area was dated at 1824±5 Ma using zircons from leucosome in the Lemu metapelite. This age constrains D3 folding related to post-collisional crustal shortening in this area. Crustal melting continued until ≈1.81 Ga, as indicated by the youngest leucosome zircons and metamorphic rims of enderbite zircons. New metamorphic zircon growth took place in older granitoids at granulite facies, but not at amphibolite facies. Detrital zircons with ages between 2.91 and 1.97 Ga were found in the mesosome of the Lemu metapelite and 2.64–1.93 Ga inherited cores were found in the 1.87 Ga Masku tonalite. [Copyright &y& Elsevier]