Basement gneiss domes in the Svecokarelides of eastern Finland: discussion

Abstract

A recent paper in this journal (J.-P. Brun, The cluster-ridge pattern of mantled gneiss domes in eastern Finland: evidence for large-scale gravitational instability of the Proterozoic crust, Earth Planet. Sci. Lett. 47 (1980) 441-449) resurrects Eskola's [1] concept of the mantled gneiss dome in the early Proterozoic Svecokarelian fold belt of Finland. The author cites amongst others, the Kuopio, Juoj~irvi, Maarianvaara, Kontiol~Lhti, Liperi, Oravisalo and Sotkuma domes (see Fig. 1) as examples of the diapiric upwelling of lowerdensity, granitic Archaean basement rocks, into more dense, overlying Karelian metasedimentary formations, The distribution of these basement structures is interpreted as reflecting a periodicity, which by analogy with the centrifuge experiments of Ramberg [2], is a function of the interlayer density contrast and the layer thicknesses. This interpretation takes no account of a large mass of information relating to structural characteristics of both basement and cover rocks in the vicinity of the domes listed above. Neither does it take into account the stratigraphy and sedimentation history of the cover rocks, or the isotopic systems of the Archaean rocks of the domes themselves. In addition, conclusions are drawn on the basis of assumptions relating to the gross properties of the rocks that are not only unproven, but highly improbable. The following aspects indicate the untenable nature of the interpretation. (1) Where the Archaean gneisses are exposed in the domes, such as at Sotkuma and Kuopio, they show many successively-formed structural and metamorphic features [3, pp. 485-486 and figs.; 4] and igneous intrusions that are not seen in the surrounding cover formations. Fine fracture cleavages as well as prominent composite foliations and folds of varying size and successive development, maintain consistent geometrical relationships, thus indicating the existence of corresponding structural and metamorphic histories. These Archaean features correspond in expression, orientation and sequential development to those of the main mass of gneisses in the Presvecokarelides of eastern Finland: deflection and disruption such as .would result from rheidic flow movement are not seen [35]. (2) U-Pb isotopic systems of zircons from the gneisses of the cores of some domes (e.g. Sotkuma) indicate very little disturbance during the Svecokarehan episode; they are close to those expected if samples had lost Pb continuously by diffusion. For zircons from other domes (e.g. Kuopio) a two-stage model, including disturbance at 1.9 Ga is indicated [6]. Neither the very little disturbance of some U-Pb systems, nor the marked differences in disturbance shown from dome to dome appear to be consistent with the T-P conditions implied by the diapiric model of dome formation. (3) The unconformity between the cover and basement remains intact over a considerable part of its outcrop, particularly along the eastern contact with the Presvecokarelian gneisses (Fig. 1). In places palaeosols (satrolites) are developed; representatives of A, B and C soil horizons occur with gradations into fissure-rotted gneiss, through blocky and rubbly material with extensive kaolin