The Caledonian Nappe Complex of Arctic Norway provides rare insights into the interaction between mafic–ultramafic magmas and the deep continental crust. The Kalak Nappe Complex contains >25,000 km3 of mafic igneous rocks, mostly layered gabbros, making up the 570–560 Ma Seiland Igneous Complex. The complex has been intruded by a series of ultramafic magmatic rocks, including the Nordre Bumandsfjord pluton. Field relationships in this pluton show that extremely fluid, dry, relatively Fe-rich (Fo81) dunite magmas intruded a pile of cumulate gabbros, with block stopping and intrusive brecciation. Diking on scales from mm to metres and extensive melting and assimilation of the gabbros attest to high temperatures, consistent with a 2-km-wide granulite-facies contact aureole. Major- and trace-element trends show that the dunites were progressively contaminated by a cpx-rich partial melt of the gabbros, producing a range of lithologies from dunite through lherzolites to wehrlite. Experimental studies of natural samples at 0.8–1 GPa define the dunite solidus at 1,650–1,700 °C. In the average peridotite, contamination has produced a crystallisation interval of ca 400 °C (1,600–1,200 °C); this would provide large amounts of heat for melting and metamorphism and would maintain the fluidity of the magmas to relatively low T, consistent with field relationships. Thermochemical and dynamic modelling demonstrates that the dunitic primary magmas may represent the last melting of a rapidly ascending diapir of previously depleted subducted oceanic lithosphere. The mafic rocks of the Seiland Complex may already have been extracted from this diapir, and the late dikes of the province may reflect melting of the asthenosphere as the diapir spread out beneath the lithosphere. Ultramafic magmas, abundant in the Archean, may still be more common than usually assumed. However, they would only penetrate to the shallow crust under unusually extensional conditions, where ascent could outpace assimilation.

The results of 27 new potassium-argon age determinations on alkaline rocks from Si:iroy, Seiland, and Stjemi:iy are presented. These fall in to two major groups, i.e. 480-491 m.y. and 384-420 m.y., which are thought to correspond with two major phases of the Caledonian orogeny.

The early Proterozoic Polmak-Pasvik-Pechenga-Imandra/Varzuga-Ust'Ponoy Greenstone Belt is situated in the northeastern part of the Baltic Shield and is discontinuously developed over a length of ca. 1000km. It comprises five main zones, which are separated by longitudinal and a number of transverse syndepositional faults. Each zone contains a number of lithostratigraphic units at the rank of Groups that are subdivided into several sedimentary and volcanic formations with a total thickness over 16,000 m. The sequence comprises a series of sedimentary-volcanic rhythms usually separated by stratigraphic breaks marked by palaeo-weathering. The Groups are separated by major disconformities which may mark/record rift inversion or orogenic episodes. At least seven distinctive lithostratigraphic markers occur in the Belt including two komatiitic volcanic series separated by immature conglomerates, two red-coloured proto-evaporitic formations separated by the alkaline volcanics, and two different "black shale" formations. Perhaps the main characteristic of the belt is the apparently long history of sedimentary/volcanic development, at the same site, spanning a time-period of 2500 to 1800 Ma. A three stage rift development model is considered: (1) an intracontinental rift stage (2500-2100 Ma), separated by the major inversion (2400-2300 Ma) and comparable with present-day Afar Triangle and East African Rifts; (2) a transitional from intracontinental to to intercontinental rift stage (2100-1970 Ma) with possible short-lived spreading (about 1990-1970 Ma); (3) a collision related intercontinental rift stage (1970-1800 Ma) followed by the Svecofenian orogeny (1800-1700 Ma). The observed distribution of platinum group elements (PGE)-mineralization is related to layered gabbro-norite complexes (Fedorova and Panskie Tundry, Gora General'skaya intrusions) dated at 2470 Ma which are apparently co-magmatic and coeval with the first komatiitic volcanic series (Seidorechka Formation) found in the eastern part of the Belt only. The major chromite occurences are connected with a layered gabbro-norite intrusion (Imandra Lopoloth) which is co-magmatic and coeval between 2440-2330 Ma with the second komatiitic volcanic series (Rusinga, Polisarka, Akhmalahti and Malbekk Formations). The Ni-Cu deposits of the belt are connected with a differentiated gabbro-werhlite complex which is co-magmatic and coeval (ca. 1970 Ma) with the specific Fe- and Ti-rich ferropicrites with komatiitic affinities.

Despite the large amount of paleomagnetic, paleoclimatic, biogeographic, and tectonic data available, uncertainty still surrounds the paleotectonic evolution of Siberia and its proximity to other continents during the Ordovician to Silurian periods. Much of this uncertainty arises from the fact that paleomagnetic data cannot be adequately assessed.However, recent Ordovician‐Early Silurian paleomagnetic poles from the southern Siberian Platform (Lena River) show that Siberia was geographically inverted at low southerly latitudes during the Early Ordovician (Figure 1a). The Siberian plate then drifted slowly northward and across the equator at an average paleolatitudinal velocity of ˜5–8 cm yr−1. During Late Ordovician‐Early Silurian time, its velocity increased northward.

Fission-track dating of apatite, zircon and sphene, and K-Ar dating of K-feldspars in samples taken from a transect across the More-Trondelag Fault Zone in Trondelag confirm the long-lived nature claimed for this complex fault zone. Apatite, zircon and sphene, which have blocking temperatures of 125-degrees +/- 25-degrees-C, 200-degrees +/- 50-degrees-C and 250-degrees +/- 50-degrees-C, respectively, together indicate a complex history of post-Caledonian tectonism. Uplift and cooling are indicated in Early Palaeozoic (Late Ordovician-Early Carboniferous) and Triassic-Jurassic times, with erosion of at least 3 km of Early and possibly Late Palaeozoic supracrustal cover since the Late Palaeozoic. Late Jurassic activity is indicated along the Verran Fault.

The Scandinavian Caledonides represents a now linear montage of nappes and thrust-sheets derived by imbrication of the Baltoscandian continental rise prism and miogeocline and from the accretion of far-travelled, outboard, oceanic terranes. The originally near-continuous pattern of such geotectonic features was destroyed during the episodic Caledonian, contractional and strike-slip, orogenic processes that eventually led to juxtaposition of tectonic units which, in many cases, represent terranes of considerable, though unknown, geographical separation.

Summary Deformation isotopically dated at 1100–1000 Ma has been recorded in the Moine rocks of N Scotland and may be related to the latest stages of Grenville orogenesis of N America. Late Precambrian orogenesis is widely recorded in the dispersed fragments of Avalonia particularly in the eastern Appalachians, eastern Newfoundland and southern Britain. Localized evidence for late Cambrian-early Ordovician orogenesis is forthcoming in the Appalachians (Penobscot) in Scandinavia (Finnmarkian) and in Scotland and Ireland (Grampian).

Summary Dismembered and fragmented ophiolite assemblages constitute important elements of the metamorphic allochthon of the Scandinavian Caledonides. Ophiolites recognized to date range in age from probable Vendian to Middle Ordovician and occur at two principal tectonostratigraphic levels. Field criteria, with support from geochemical data, permit a classification of many of the ophiolite complexes into two fundamental groups. Group I is characterized by a generally well-developed pseudostratigraphy, MORB as well as IAT and WPB petrochemistry, plagiogranites at higher levels, and a conformable cap of oceanic sediments with local ocean or immature arc volcanism. The group is representative of either a major ocean or evolved marginal-basin regime. Evidence points to their obduction, and initial internal deformation, in the Finnmarkian orogenic event in pre-Middle Arenig time. Group II complexes are younger, of late Arenig to Llanvirn and possibly Llandeilo age, and show a poorer-defined pseudostratigraphy. They are laterally and vertically intercalated with either siliciclastic sediments from a contemporary magmatic arc or arcs, and are considered as having been generated in a restricted marginal-basin setting. Their prime tectonic deformation, and translation within nappes, is essentially of Silurian age. Group I ophiolites were also further dissected and deformed during this main Caledonian event.

Metabasalts from the sheeted dyke complex and pillow lavas in the Karmøy ophiolite, southwest Norwegian Caledonides, show large ranges of both trace element concentrations and incompatible element ratios. These features are best explained by batch melting and to some extent by dynamic melting of a depleted, heterogeneous mantle, with some subsequent modification of the melts by fractional crystallization. The geochemistry may reflect a more complicated tectonic environment of basalt generation than for other, geochemically more uniform ophiolitic sequences further to the north in the Norwegian Caledonides. Geological features of the Karmøy ophiolite and another nearby ophiolitic sequence (the Lykling ophiolite) indicate the presence of an oceanic fracture system. It is speculated if a relationship may exist between the varied geochemistry and the oceanic fracture system.

Summary The status of the major horizon of gneisses (Cowhythe, Donside, Ellon and Inzie Head gneisses) in NE Scotland has been reinterpreted, on the basis of geochronological and fabric evidence, as the basal element of a major nappe, the Banff nappe. It is suggested that these gneisses represent a pre-Caledonian basement gneiss-complex, thrust into its present structural position during the Grampian phase of the Caledonian orogeny, after the formation of the Tay nappe. The basal zone of this unit is characterised by the development of distinctive mylonitic assemblages. In this interpretation, the rocks of the overlying Banff division form part of the allochthonous assemblage, as a Caledonian cover sequence, uncoupled from the basement along the Boyne line during emplacement.

The Kalak nappe complex of N Norway involves late Precambrian to Middle Cambrian sediments and a Precambrian gneiss basement on which the sediments were deposited. While the uppermost nappe in Finnmark was emplaced during the Silurian the members of the Kalak nappe complex were emplaced in late Cambrian/early Ordovician times during the Finnmarkian orogenic stage—probably the analogue of the Grampian stage of the British Caledonian. The tectonic-metamorphic events of the Finnmarkian stage were broadly coeval with the emplacement of basic and alkaline igneous bodies of the Seiland Igneous Province which were introduced from 552 ± 17 Ma (syn-D1) until 501 ± 27 Ma (late D2) and which themselves reflect magmatic evolution from tholeiitic to alkaline types.

I n this note we draw attention to data which may require major revision of Dalradian stratigraphy and structure in northeast Scotland. The Central Highland sequence of the Dalradian and the Keith Division ( Read 1955 ) of Banffshire are separated from the metasediments of the Banff Division by gneisses. These are referred to as the Cowhythe Gneiss ( Read 1923 ), Queens Hill Gneiss (Harris & Pitcher 1975), Donside and Ellon Gneiss ( Read 1955 ) and Inzie Head Gneiss ( Read & Farquhar 1956 ) after the main areas of outcrop. The reader is referred to Read (1955 , pl. 1) where the general distribution of the gneisses is shown. Their relationships to the Central Highland succession are often complicated by the large granitic intrusions of the post-tectonic Younger Granite suite. The generally accepted model for the region interprets the gneisses as migmatites developed from the Ben Lui and Ben Lawers schists (e.g. Read 1955 , Harris & Pitcher 1975). Further they are considered to represent the migmatitic core of the Tay Nappe (e.g. Ashworth 1975 ). Read (1955) originally considered that the rocks of his Banff Division lay on a lag-surface (the Boyne Line) above the gneisses. The great recumbent fold of which the Banff Division formed the upper limb was called the Banff Nappe, and was considered to be a continuation of the Tay Nappe ( Read 1955 ). Recent investigations by the authors reveal a sharp tectonic break between the Dalradian rocks of the Central Highlands/Keith Division and the gneisses. This break is marked by discordance and a major

On the basis of reconnaissance geo chronological investigations, and the preliminary results of the reinvestigation of the tectono/metamorphic patterns of the Bergen Arc System it is possible to present a series of results which help to clarify the relationships of the rocks units involved in this system. These results fundamentally affect the interpretation of the Bergen Arc System in Caledonian reconstructions of the geology of Western Norway.