Per-Gunnar Andréasson

The Baltoscandian margin in neoproterozoic-early palaeozoic times. Some constraints on terrane derivation and accretion in the Arctic Scandinavian Caledonides

Abstract Deep erosional level, rich exposure and excellent preservation of parts of the passive margin make the Arctic Scandinavian Caledonides (north of 66°N) an ideal area for attempting a reconstruction of the pre-collisional continental margin of a collisional mountain belt. Such a reconstruction is essential for the safe identification of suspect and exotic terranes, but also because “passive” margins seem to partly control tectonics and metamorphism throughout the evolution of mountain belts. The palinspastic reconstruction of the Baltoscandian Margin attempted in this paper suggests that all thrust sheets in the Arctic Scandinavian Caledonides below the nappes of ensimatic affinity could derive from a northern extension of the Caledonide foreland of southwestern Scandinavia. Thus, the paper calls in question recent interpretations of principal units of the Arctic Scandinavian Caledonides (e.g., the Kalak Nappe Complex, including the Soroy Group type stratigraphy) as exotic terranes, and the sedimentary detritus of the Soroy Group as foreign to the Baltoscandian Margin. The varied evidence of magmatism of rift affinity found in many nappes and in the southern foreland is reviewed and interpreted in terms of a coherent, though protracted evolution from the ca. 900 Ma, high-Ti-K-P-LREE continental tholeiites of the foreland aulacogen to the ca. 580 Ma T-MORB sheeted dykes of the outermost continental margin. This evolution is compared to that of modern rifts (Afro-Arabian rift system); such comparison allows interpretation of the renowned Seiland Igneous Province as magmatism in a mid-crustal section of the Baltoscandian Margin. The evidence of a strong MORB affinity of mafic dykes occurring in most thrust sheets below the exotic terranes requires extensive thinning of a wide zone of the continental margin prior to the ultimate break-up. The paper summarizes unambiguous evidence that the rifted and dyke-intruded margin was depressed to at least 50 km depth and imbricated and uplifted in Late Cambrian-Early Ordovician times, and the concept of a Finnmarkian event is retained for this early diastrophism affecting the Baltoscandian Margin. A remarkable feature of the Arctic Scandinavian Caledonides is the evidence that the sheeted-dyke complex must have been detached from the diving margin at an early stage and carried towards the foreland, thus escaping high-pressure metamorphism at 505-500 Ma. Finnmarkian cooling ages of thrusts within the Seve Nappe Complex are in conflict with popular flat-ramp models of Silurian-Early Devonian (Scandian) thrusting developed for this nappe complex. Rich evidence of metamorphic and isotopic disequilibrium in the nappe stack of the Arctic Scandinavian Caledonides suggests that the perturbations of the geotherm caused by early understacking of cold continental crust never recovered before renewed crustal thickening during Scandian accretion of exotic terranes and final assembly of the mountain belt. This raises questions about the actual significance of results obtained by P-T-t modelling in other overthrust belts, where such evidence is not preserved.

Dawn of Phanerozoic orogeny in the North Atlantic tract; Evidence from the Seve-Kalak Superterrane, Scandinavian Caledonides

Abstract Despite their Early Phanerozoic age, the Scandinavian Caledonides provide a remarkably rich record of the continental break-up and development of the continent-ocean transition as well as the ensuing subduction and imbrication of the pristine plate margin, all emplaced on land and accessible. We first organize the evidence for Baltoscandian rift basin formation and magmatism, now scattered in two major nappe complexes (and by semantics) in terms of a superterrane, the Seve-Kalak Superterrane. Extensive (1000 km) mafic dyke swarms and partly sheeted dyke complexes are interpreted as fragments of a Large Igneous Province. While attempted break-up and some tholeiitic magmatism took place already at c. 800 Ma, successful rifting occurred first in connection with intense, partly alkaline dyking and emplacement of ultramafic complexes between c. 620 and 550 Ma. This late magmatism is markedly enriched as compared to MORB, interpreted to reflect mixing between an enriched mantle source component and dep...

Geology of the protogine zone south of Lake Vättern, southern Sweden : a reinterpretation

Abstract Influential models describe the Protogine Zone (PZ) as a collisional suture and thrust front and, in particular, as the extension of the Grenvillian thrust front into Scandinavia. These models do not consider the significance of a rich variety of geological features related to Late Riphean-Vendian rifting of the craton. This paper calls attention to steep, northerly trending and anastomosing, narrow deformation zones of ductile or brittle-ductile character. The zones can be mapped out into the Vattern Graben of c. 700–800 Ma age. Uplift of southwestern Sweden after c. 910 Ma must have had a profound tectonic influence on the bedrock along the PZ. The effects of this late deformation must be identified and subtracted from the structural record before the significance of Sveconorwegian-Grenvillian deformation can be assessed. There is no other foliation south of Lake Vattern qualifying for the name “protogine zone foliation”. Rift-related magmatic and metallogenic features are remarkably restricted...

Attempted break-up of Rodinia at 850 Ma: geochronological evidence from the Seve–Kalak Superterrane, Scandinavian Caledonides

Lower thrust sheets of the Scandinavian Caledonides derive from the margin of Baltica, which was imbricated during Early Palaeozoic closure of oceans formed during separation of the Baltica, Laurentia and Siberia cratons. At Vistas (Kebnekaise Mts), the Seve Nappe Complex preserves rare lenses of a dolerite-intruded granite formed by anatexis during emplacement of gabbro into metasedimentary rocks. Ion microprobe U–Pb dating of prismatic zircons from the granite yielded an age of 845 ± 14 Ma (mean square weighed deviation (MSWD) 1.15), interpreted to date magmatic crystallization. Cores of complex crystals indicate protolith ages of 1778 ± 11 Ma (MSWD 0.86); one core yielded an Archaean age. From narrow rims, a 605 ± 42 Ma metamorphic age is obtained, and is interpreted to reflect the emplacement of the extensive Vendian rift magmatic dolerite dyke swarm. The age pattern allows correlation with a previously dated magmatic complex within the equivalent Kalak Nappe Complex 300 km to the north. Bimodal magmatic complexes with this age pattern do not occur within the basement of the Caledonide foreland, nor elsewhere within the Baltic Shield. However, recent reinterpretations of the Knoydartian event in Scotland as rift related invites correlation with the West Highland Granite Gneiss, which intruded Moine metasedimentary rocks at c. 870 Ma. In Central Taimyr, 850–900 Ma granites of continental crustal derivation and with 1800–1900 Ma Sm–Nd model age occur associated with c. 740 Ma plagiogranites. Using recent palaeogeographical reconstructions, we interpret the evidence of 850–900 Ma magmatism in Scandinavia, Scotland and Taimyr as bimodal rift magmatism in connected arms during an attempted break-up of Rodinia.

Metamorphism in extensive nappe terrains: A study of the Central Scandinavian Caledonides

Abstract Metamorphism in the nappe terrains of the Central Scandinavian Caledonides is characterized by inverted gradients and other indications of disequilibrium caused by the superposition of nappes of various metamorphic grade and the translation of the nappe pile onto the margin of the Baltic Shield. In the lower nappes there is a gradual westward increase of grade, but some inverted gradients persist. In the upper allochthon, the variation of metamorphism is essentially vertical. It is possible to identify and evaluate parageneses formed prior to, during, and after nappe emplacement. Their interpretation in terms of separate Caledonian or Precambrian events is often ambiguous. Early Caledonian metamorphism apparently represented a range of P-T-conditions. The build-up and translation of the nappe pile was characterized by transient high pressure conditions. A metamorphic pulse, manifest particularly in the west, succeeded nappe emplacement onto the Baltic Shield. Strain-induced metamorphism in the vi...

Derivation of 500 Ma eclogites from the passive margin of Baltica and a note on the tectonometamorphic heterogeneity of eclogite-bearing crust

Several recent plate reconstructions of the Iapetus Ocean describe the margins of Baltica as passive until Silurian collision with Laurentia. Yet there is a variety of evidence to suggest that the accretion of the Scandinavian Caledonides began by latest Cambrian—early Ordovician subduction and imbrication of the passive continental margin. One such evidence is provided by eclogites occurring in the Seve Nappe Complex. Previous work by others dated the high-pressure metamorphism at 503±14 Ma (Sm—Nd garnet-omphacite age), and the uplift through the c . 500°C isotherm at 491±8 Ma ( 40 Ar/ 39 Ar hornblende plateau ages). The protolith dolerites of the eclogites have been correlated with Iapetan rift-facies dolerites of the Baltoscandian margin. If valid, such a correlation implies early Caledonian destruction of the margin, and thus modification of those plate reconstructions which require passive margins around Baltica in latest Cambrian-early Ordovician time. This paper provides a substantially improved basis for the concept that the protoliths of eclogites and their host rocks derived from Baltoscandian rift basins. The chemical similarity between coronitic dolerites and dolerites of the rift basins pertains not only to element concentrations and variations but also to the specific T-MORB signature shared by the two groups. The variation of psammitic and pelitic schists, graphitic schists, calc-silicate gneisses and marbles of the eclogite host rocks equates with sequences of sandstones, siltstones, shales, black shale, quartzite, dolomite and limestones of Baltoscandian palaeobasins. At the same time, the paper calls attention to the remarkable preservation of structural and metamorphic contrasts within the eclogite-bearing thrust sheets of the Seve Nappe Complex. Such disequilibrium is generally ascribed to the kinetics of localized deformation and fluid infiltration into dry crust. This paper presents evidence that disequilibrium is found also within inferred subducted sedimentary complexes, which are generally assumed to be pervasively flushed by fluids. Preservation of sedimentary, volcanic and magmatic structures and fabrics, and of both undeformed dolerite dykes and eclogitized dykes demonstrates that neither deformation nor high-pressure metamorphism were pervasive.

Geochemistry of amphibolites from Mt. Sylarna, Central Scandinavian Caledonides

Abstract A geochemical study of the low-grade Seve amphibolites from Mt. Sylarna indicates that they have an affinity to abyssal, tholeiitic magma. The spilitic character of the amphibolites is emphasized in the interpretation of their magma-tectonic implications. A proposed pre-orogenic relationship between, the underlying Sarv tholeiitic dykes and the Seve amphibolites in the Sylarna-Helags area is discussed. It is concluded that a magma-genetic relation may exist, which, however, does not place constraints on the translation distance of the Seve Nappe. A hypothetical model for a comagmatic origin of the Sarv dolerites and the Mt. Sylarna amphibolites is proposed, involving early basaltic volcanism in the Iapetus Ocean.

Mafic dyke swarms of the Baltica-Iapetus transition, Seve Nappe Complex of the Sarek Mts., Swedish Caledonides

Abstract Mafic dyke swarms of the Baltica-Iapetus transition, Seve Nappe Complex of the Sarek Mts., Swedish Caledonides With respect to petrography and chemistry, the Sarek dolerites compare excellently to T-MORB as far as REE abundance patterns and some trace element ratios are concerned: chondrite normalized (La/Sm) = 0.77–1.04 and (La/Yb) = 1.2–2.9; Zr/Nb = 8.3–11.6; Zr/Y = 2.8–4.8; Y/Nb = 1.9–4.3. However, high abundances of LIL elements and straddling of boundaries to fields of destructive margins in Nb-Y-Zr and Hf-Ta-Th discrimination diagrams indicate a mixed magmatectonic signature. In this respect, the Sarek dolerites compare to basalts of ensialic back-arc basins e.g. the Gulf of California. However, such a setting is incompatible with our present knowledge about the Baltoscandian scenario 500–600 Ma ago, unless the Sarek sheeted dyke complex was part of the Laurentian passive margin. The favoured interpretation is a situation similar to the present-day opening of the South Atlantic Ocean, where...

Metamorphism in the Tømmerås area, western Scandinavian Caledonides

Abstract Structural and metamorphic discontinuities in the Tommeras basement-cover sequence confirm the allochthonous character of the Lower Snasa Group, the Leksdalsvann Group, and the upper basement of the Tommeras Window. Three major deformational events are recognized. During an early event the upper unit (the Snasa Group amphibolites and gneisses) was metamorphosed under amphibolite facies conditions and thereafter became emplaced above the unmetamorphosed Leksdalsvann sediments. The interkinematic (D1-D2) parageneses of these sediments indicate a high P H2O and T ca. 450°C. D2 recrystallization, corresponding to the transitional greenschist-amphibolite facies (500–550°C, P > 600 MPa), is largely controlled by the distribution of shear strain. Calcium-rich mantles of late D2 garnets in the same sediments indicate a pressure peak before the main thrusting (late D2). D3 folded the thrusts and the isograd pattern established during D2 around the NE-SW trending axis of the Tommeras Antiform. Various feat...

Baltoscandian margin, Sveconorwegian crust lost by subduction during Caledonian collisional orogeny

Abstract Underthrusting of Laurentia by the continental margin of Baltica during Caledonian orogeny resulted in the lateral emplacement of Iapetus Ocean-related terranes of the Upper Allochthon at least 500 km onto Baltica. The underlying Lower and Middle allochthons of the Baltoscandian margin mostly comprise Cryogenian, Ediacaran and Cambro-Silurian sedimentary successions; basement to these formations are present only as minor, isolated fragments, except at the base of the Middle Allochthon and within the underlying windows. The upper parts of the Middle Allochthon are notable for the presence of early Ediacaran dyke-swarms and other components of the Baltoscandian continent–ocean transition zone (COT). New data are presented here on the c. 610 Ma age of the COT-related dolerites in the Kalak Nappe Complex in Northern Norway and also on detrital zircons in the underlying Laksefjord and Gaissa nappes. The former confirms that the Baltoscandian COT has a similar age along the length of the orogen; the latter shows that the detrital zircon signatures in the Lower and Middle allochthons are comparable throughout the orogen. These sedimentary rocks have dominating populations of Mesoproterozoic to latest Palaeoproterozoic zircons similar to those from southern parts of the orogen, where Sveconorwegian complexes comprise the basement to the Caledonides. Thus, they help define the probable character and age of the crystalline basement that existed along this outer margin of Baltica during the Neoproterozoic, continental lower crust that was partly subducted during Ordovician continent-arc collision and subsequently lost beneath Laurentia during the 50 million years of Scandian collisional orogeny.