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Archaean supracrustal complexes, known in the Fennoscandian (Baltic) Shield, are described and discussed by analysing the time sections 3.1-2.9, 2.9-2.75 and 2.75-2.65 Ga. Data on granitoid complexes, interrelated in time and space, and evidence for Archaean metamorphic events are classified and presented briefly. Fragments of ophiolitic and eclogitic associations have been found in Archaean rocks in the Shield. The first evidence of continental crust in the Shield is from Meso-Archaean time (3.5-3.1 Ga); isolated microcontinents, such as Vodlozero, lisalmi and North Finland, have been identified. New continental crust was mainly generated in the 2.9-2.65 Ga interval. The geodynamic settings in which the continental crust was formed in the Meso-and Neoarchaean included subduction (ensialic and ensimatic), accretion and collisional mechanisms. The continental and oceanic crust were affected by mantle plumes.

The North Atlantic craton in southern West Greenland mainly consists of a tectonic collage of Mesoarchean continental crustal terranes, which were amalgamated at c. 2.7 Ga and are currently exposed at mid-crustal amphibolite to granulite facies levels. Tonalitic orthogneisses predominate, intercalated with slightly older tholeiitic to andesitic metavolcanic rocks and associated gabbro-anorthosite intrusive complexes. The North Atlantic craton also contains enclaves of Eoarchean, c. 3.86-3.6 Ga orthogneisses and supracrustal rocks including the Isua greenstone (or supracrustal) belt. This is the oldest known assemblage of rocks deposited at the surface of the Earth, comprising mafic pillow lavas, banded iron formations and metasedimentary schists with local disseminated graphite of possible biogenic origin. Eoarchean rocks have not been found in Kola and Karelia in Fennoscandia where most rocks are 2.9-2.7 Ga tonalitic-trondhjemitic-granodioritic orthogneisses with intercalated coeval greenstone belts and amphibolites. Mesoarchean 3.0-3.2 Ga rocks are found in the eastern and western parts of the Karelian province. Subduction-related rocks like the Iringora supra-subduction type ophiolite and basalt-andesite-dacite-rhyolite series volcanic rocks in many greenstone belts, as well as eclogites are found in the Archean of Fennoscandia. A clear distinction between Greenland and Fennoscandia is the abundance of 2.75-2.65 Ga igneous rocks in Fennoscandia which indicates that these two cratons had a separate evolution during the Neoarchean.

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We assess the proposal of Hendriks & Redfield (Earth and Planetary Science Letters, 236, 443-458, 2005) that cross-over of the predicted apatite fission track (AFT) . (U-Th-Sm)/ He (AHe) age relationship in the southeastern Fennoscandian shield in southern Finland reflects a-radiation-enhanced annealing (REA) of fission tracks at low temperatures and that more robust estimates of the denudation history are recorded through reproducible AHe data. New AHe results from southern Finland showing variable dispersion of single-grain ages may be biased by different factors operating within grains, which tend to give a greater weighting towards older age outliers. AHe ages from mafic rocks show the least dispersion and tend to be consistently lower than their coexisting AFT ages. In general, it is at the younger end of the single-grain variation range from such lithologies where most meaningful AHe ages can be found. AHe data from multigrain aliquots are, therefore, of limited value for evaluating thermal histories in southern Finland, especially when compared against coexisting AFT data as supporting evidence for REA.New, large datasets from the southern Canadian and Western Australian shields show the relationship between AFT age, single-grain age or mean track length as a function of U content (determined by the external detector method). These do not display the moderately strong inverse correlations previously reported from southern Finland in support of REA. Rather, the trends are inconsistent and generally exhibit weak positive or negative correlations. This is also the case for plots from both shields, as well as those from southern Finland, where AFT parameters are plotted against effective U concentration [eU] [based on U and Th content determined by inductively coupled plasma-mass spectroscopy (ICP-MS)], which weights decay of the parents more accurately in terms of their a-productivity. Further, samples from southern Finland yield values of chi-square x 2 .5%, indicating that there is no significant effect of the range of uranium content between grains within samples on the AFT ages, and that they are all consistent with a single population.The oldest AFT ages in southern Finland apatites (amongst the oldest recorded from anywhere) are found in gabbros, which also have the highest Cl content of all samples studied. We suggest, that it is Cl content rather than REA that has influenced the annealing history of the apatites, which have experienced a history including reburial into the partial annealing zone by Caledonian Foreland basin sedimentation. The study of apatite from low U and Th rocks, with relatively low levels of a-radiation damage may provide the most practical approach for producing reliable results for AFT and AHe thermochronometry studies in cratonic environments.

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