Torbjörn Skiöld

Implications of new UPb zircon chronology to early proterozoic crustal accretion in northern Sweden

Abstract UPb ages ranging from 1.89 to 1.78 Ga are reported for zircon populations formed during major magmatic events in the Proterozoic development of northern Sweden. The studied area includes parts of the continental and the marine Svecokarelian domains of the Baltic Shield. The ages record crustal accretion and reactivation off the margin of an exposed Archaean craton and show that magmatism was broadly coeval throughout the entire north-Swedish portion of the Svecokarelian province, irrespective of regions involved. A detailed chronostratigraphy is presented for the Skellefte district of northern Sweden, and comparisons are made with the evolution in other parts of the Svecokarelian province. Two main magmatic phases commencing at 1.89 and 1.79 Ga ago are recognized. The first represents large-scale volcanism and plutonism, which in the Skellefte district was characterized by the introduction of mainly mantle-derived magmas or by magmas formed from rocks with short crustal residence time. In the vicinity of the Archaean craton some distance to the north, the crust-generating processes included the mobilization of Archaean sources and appear to have continued until c. 1.85 Ga ago. A relatively long period of consolidation and denudation separates this magmatic phase from the one at c. 1.79 Ga ago. The generation of these later, migmatite-associated granites in the Svecokarelian province coincided in time, and was possibly associated with, those processes which gave rise to the initial plutonism in the neighbouring Transscandinavian Granite-Porphyry Belt.

The interpretation of the Rb-Sr and K-Ar ages of late Precambrian rocks in south-western Sweden

Abstract Radiometric ages of some late Precambrian rocks in south-western Sweden show that the evolution of the Sveconorwegian geochronological province in this area, after the deposition of the Dal Group between 1100 and 1050 Ma (million years) ago, continued for at least about 250 Ma. The Bohus granite intruded in a late-tectonic phase, giving a Rb-Sr isochron age of 910 ± 35 Ma (λ87Rb = 1.39 × 10−11α−1) with an initial strontium-isotope ratio of 0.711. The K–Ar model ages determined on micas separated from the Bohus granite, lie in the range of 861 to 680 Ma. A Rb-Sr isochron age of 1050 ± 40 Ma with an intercept of 0.704 has been obtained for the slate of the Dal Group. The significance of this age is discussed. The K–Ar model ages, determined on whole rocks and minerals from slates and spilites of this sequence, range from 1000 to 715 Ma. The pre-Sveconorwegian granites in the Ljungbergen area yield a Rb-Sr isochron age of 1400 ± 80 Ma with an intercept of 0.704, which probably reflects the age of in...

Delineation and character of the Archaean-Proterozoic boundary in northern Sweden

Abstract Before the deposition of a Proterozoic cover and the repeated Proterozoic reworking of the older rocks, the presently exposed Archaean areas in northern Sweden formed part of a coherent craton. In the present study, we have used SmNd isotopic analyses of Proterozoic granitoids and metavolcanics to delineate the Archaean palaeoboundary. In a regional context, the transition from strongly negative ϵ Nd( t ) values in the northeast to positive values in the southwest is distinct, and approximately defines the border of the old craton. The Archaean palaeoboundary extends in a WNW direction, and is subparallel to the longitudinal axis of the Skellefte sulphide ore district but it is situated ∼ 100 km farther to the north. The ∼ 1.9 Ga old granitoids on the two sides of the palaeoboundary were all formed in compressional environments, but those situated to the north have higher contents of LILE and LREE at similar contents of Si. This indicates that they were generated in an area with thicker crust and supports the location of the Archaean-Proterozoic palaeoboundary. There is no simple correlation between the Archaean palaeoboundary, as defined by the isotopic results, and any of the major fracture systems as interpreted from regional geophysical measurements. Reflection seismic work indicates that juvenile volcanic-arc terrains to the south have been thrust onto the Archaean craton. Possible thrust faults have been identified from aeromagnetic measurements. Rifting of the Archaean craton created a passive margin ∼ 2.0 Ga ago. Spreading shifted to convergence with subduction beneath the Archaean continent ∼ 1.9 Ga ago. Subsequently, the resulting juvenile volcanic arc collided with the old continent, and the Archaean palaeoboundary as existing today was formed by a collision characterized by overthrusting. The boundary then was disturbed by later deformation predominantly along NNE-trending fracture systems.

Titanite-rutile thermochronometry across the boundary between the Archaean Craton in Karelia and the Belomorian Mobile Belt, eastern Baltic Shield

Abstract U–Pb isotopic dating has been carried out on titanites and rutiles from the Karelian Protocraton, the Belomorian Mobile Belt and the intervening junction zone. These are some of the principal Archaean crustal units in the Baltic Shield which have undergone regeneration to various degrees during the Palaeoproterozoic. Palaeoproterozoic resetting of U–Pb titanite ages was complete in the Belomorian Belt and almost complete in the junction zone, while it hardly affected the Karelian Protocraton. In the latter, major crustal cooling occurred at 2.71–2.69 Ga after a major igneous event at 2.74–2.72 Ga, but a tectonothermal event at 2.65–2.64 Ga was less comprehensive. In the Belomorian Belt, a northeastern marginal zone immediately underlying the collisional-thrusting suture of the Lapland-Kola orogen has somewhat higher titanite ages of ca. 1.94–1.87 Ga than the central zone where these ages range between 1.87 and 1.82 Ga. Comparison between the titanite and rutile U–Pb ages suggests a postorogenic cooling rate between 2 and 4°/Ma in these parts of the Belt. The Neoarchaean junction zone between the Karelian and Belomorian provinces was a zone of particularly intense tectonic, magmatic and hydrothermal activity during or after the Palaeoproterozoic Lapland-Kola orogeny. Dominant, newly grown titanites in that zone have ages as young as 1.78–1.75 Ga, and the age differences between the titanite and rutile U–Pb ages are substantially smaller than elsewhere.

Westward accretion of the Baltic Shield: implications from the 1.6 GaÅmål-Horred Belt, SW Sweden

Abstract Three new U-Pb zircon age determinations are reported from the Horred region, south-southeast of Goteborg, SW Sweden. This is a region of the Southwest Scandinavian Domain, within which a major N S trending shear zone and tectonic boundary, the Mylonite Zone, juxtaposes comparatively weakly migmatised lithologies in the west against more intensely migmatised gneisses in the east. West of the Mylonite Zone, a metavolcanic rock (the Mjosjodacite) yields an age of 1643 ± 29 Ma, whereas a cross-cutting plutonic rock (the Idala tonalite) has an age of 1584 ± 15 Ma. Together with a recent age for a volcanic rock from theAmal region farther north (1.61 Ga, Lundqvist and Skiold, 1992 ), these ages help to establish the existence of a coherent calc-alkaline igneous belt of 1.6 Ga age for which the nameAmal-Horred Belt is proposed. East of the Mylonite Zone, a presumably metavolcanic rock (the Grimmared gneiss) yields an age of ∼ 1.61 Ga. The obtained age and the compositional similarity of rocks on each side of the Mylonite Zone indicate that more deformed and more strongly metamorphosed equivalents of the rocks in theAmal-Horred Belt may occur also to the east of the Mylonite Zone in what is termed the Eastern Segment of the Southwest Scandinavian Domain. The new results establish theAmal-Horred Belt as a major geological unit younger than most other crustal components in southern Sweden such as theOstfold-Marstrand Belt (∼ 1.76 Ga), the Eastern Segment gneisses (> 1.66 Ga) and the three age groups of the Transscandinavian Igneous Belt (∼ 1.81 – 1.65 Ga; Larson and Berglund, 1992 ). The configuration of the crustal units in SW Sweden appears to necessitate more complex Proterozoic models than those with a persistent younging from the present east to the west. The present concept of the “Gothian orogeny” must be revised since at least two different orogenic episodes at ∼ 1.7 and 1.6 Ga can now be distinguished.

Age of deformation episodes in the Palaeoproterozoic domain of northern Sweden, and evidence for a pre-1.9 Ga crustal layer

It is proposed in a companion paper that the volcanic rocks of the Skellefte Group in northern Sweden were deposited on a pre-1.9 Ga basement, rather than formed as a juvenile volcanic arc and subsequently accreted to the continent. It is suggested that the proposed basement, the Robertsfors Group to the south of Skelleftea, was already deformed during an early episode of deformation (D1), prior to the deposition of the Skellefte stratigraphic sequence during an extensional tectonic episode. A younger episode of deformation, D2, then affected both areas. This hypothesis has important implications for the evolution of the Svecofennian Province. To test it, by constraining the ages of the principal deformation episodes, we have undertaken both isotope dilution and ion microprobe studies of zircon and monazite from three localities in the Robertsfors Group. A phase of migmatisation directly related to D2 shear zones allows the dating of D2 at ≈1860 Ma. Other planar granitoid bodies, which cut the D1 structures, confirm the age of this event. In the context of earlier published data, we conclude that the correlation of D2 in the Robertsfors Group with the main folding in the Skellefte Group is valid. Another granitoid body, folded by D2 but emplaced after D1, contains a suite of zircons of apparent magmatic aspect, but with a spread of ages from 1870 to 1900 Ma. Because of the high MSWD, the suite was divided into two groups with ages of ≈1896 and 1874 Ma. Recognising the possibilities of inheritance and of updating by younger events, we have preferred to interpret the younger age as a minimum age for magmatic intrusion. This is consistent with the basement hypothesis but is our only definite constraint on the age of D1. However, we infer, using other evidence, that D1 is probably older than 1900 Ma. Xenocrysts, interpreted as detrital zircons derived from the associated metasediments, give ages between 2716 and 1941±20 Ma. The latter age provides a maximum age of deposition for part of the Robertsfors Group and a maximum age for D1. Other sequences, to the south and west, considered broadly equivalent to the Robertsfors Group, are intruded by granitoids dated between ≈1920 and 1960 Ma, providing local minimum ages of deposition for the sequences involved. Regional correlation with respect to age and deformation style can be made with the pre-1.9 Ga Kalevian rocks to the NE, deposited on Archaean basement on the SW margin of the Karelian Province. We suggest that pre-1.9 Ga complexes, equivalent to the Robertsfors Group, are widespread in the Svecofennian Province, and were deposited in a marginal basin which may have been initiated as early as 2.2 Ga ago. This hypothesis of an extensive basement crustal layer evidently requires that a substantial volume of crustal growth occurred prior to 1.9 Ga, in contrast to previous hypotheses. Erosion of the pre-1.9 Ga basin rocks after D1 is also the likely source of the 2.1–1.9 Ga detrital zircons in younger sediments associated with the overlying post-1.9 Ga volcanics. The source of these detrital zircons has previously been problematical.

Chemistry of Proterozoic orogenic processes at a continental margin in northern Sweden

Abstract Sm-Nd isotopic, major-, trace- and rare-earth-element data for two separate age groups of plutonic rocks have been used to characterize early Proterozoic crust formation mechanisms in the vicinity of the exposed Archaean of northern Sweden. The granitoid rocks of the older (1.89–1.87 Ga) group show calc-alkaline trends (diorite-quartz monzonite-adamellite) and have negative ϵ Nd -values ranging from about −8 to −5. They represent variable contamination of isotopically juvenile mantle melts with Archaean crustal material. The younger (1.80 Ga) group of plutonic rocks are of true granitic composition. They show a restricted range of ϵ Nd -values which is encompassed by those of the older group. Trace- and major-element data are consistent with the derivation of the younger group of rocks by remobilization of the earlier formed Proterozoic continental crust. This is in contrast to the temporally similar but isotopically more juvenile granitoids of the Skellefte district to the south.

Zircon ages from an Archaean gneiss province in northern Sweden

Abstract U-Pb isotopic analyses for zircons from an Archaean granitoid gneiss west of the Tjarro supracrustal belt in northern Sweden are presented. Four size fractions showing different degrees of overgrowth on eu- to subhedral zircons are interpreted as reflecting a U-Pb isotopic disturbance during a metamorphic event between 2740 and 2694 Ma ago. Some indication of the primary zircon crystallization is given by the mean diffusion age (2834 ± 40 Ma, 18) of the core-dominated coarser fractions. Subsequent Proterozoic metamorphism, between 1900 and 1750 Ma ago, up to amphibolite facies has caused no significant imprint on the analysed zircons. Together with previous geochronological work, the result indicate the existence of large areas of Archaean crust between the Tjarro belt and the Caledonides.

Radiometric ages of plutonic and hypabyssal rocks from the Vittangi-Karesuando area, northern Sweden

Abstract Some of the intrusion and metamorphic episodes in the Svecokarelian (early Proterozoic) orogenic development of northern Norrbotten County are discussed on the basis of U-Pb zircon and Rb-Sr whole-rock analyses. Prior to the main Svecokarelian folding, the mafic volcanic rocks of the Vittangi-Soppero-Karesuando belt were penetrated by hypabyssal sills, of which an albite-diabase has yielded a zircon age of about 1874 Ma. According to zircon data, intermediate to acid plutonic rocks were emplaced in close connection with the hypabyssal sills and in an early-orogenic stage at about 1880–1860 Ma. Corresponding Rb-Sr whole-rock systems have been more or less disturbed, however, indicating metamorphic-metasomatic conditions until at least 1700 Ma ago. A large granite massif south of Vittangi was emplaced, according to zircon data, at about 1795 Ma, while the 40 Ma younger Rb-Sr whole-rock isochron age is due to uplift or a delayed cooling process. Another granite dome, outcropping to the west of the a...

Diversity of 1.8 Ga potassic granitoids along the edge of the Archaean craton in northern Scandinavia: a result of melt formation at various depths and from various sources

The edge of the Archaean craton in northern Scandinavia had been intensively reworked during the Svecofennian orogeny 1.93-1.86 Ga ago and was subsequently intruded by potassic granitoids of 1.79–1.80 Ga age. Despite similar or even identical ages and overlapping areas of occurrence, these rocks belong to two different groups, the Edefors and Lina granitoids, which have contrasting geochemistries and SmNd isotopic characteristics. The Edefors granitoids range from syenites to granites, and are alkali-rich and distinctly metaluminous. They crystallized from dry magmas. This is indicated by the scarcity of pegmatites and aplites. The contacts to older rocks are often distinct, but gradual transitions to Lina-type granitoids are common. The Edefors granitoids have high contents of Zr but not of elements such as Y, REE, Ta and Nb, and have low MgMg+Fe ratios. They also frequently have positive Eu anomalies, even in the quartz rich varieties. Initial ϵNd values range from −2.1 to +1.4, indicating that the Edefors granitoids were formed by the mixing of mantle-derived magmas and continental crustal materials. The amount of crustal component was probably less than 35% in most cases. The Lina granitoids are accompanied by abundant pegmatites and aplites. Ghost structures and remnants of country rock are common. True granites predominate, but also quartz monzonites occur. The content of HFS elements is low and the MgMg+Fe ratios are higher than in the Edefors granitoids. Initial ϵNd values range from −9.3 to −3.7, reflecting a significant portion of Archaean Nd in the source materials. The Lina granitoids are largely the result of remobilisation of continental crust with a small input of juvenile material. However, the dominant source for these crustally derived granitoids are c. 1.9 Ga old granitoids. These carry a large proportion of Archaean Nd. The most probable environment of the formation of potassic migmatite granitoids, such as the Lina type, is a collision zone between two masses of felsic crust (e.g. arc-continent or continent-continent), but the details of such a collision in the Baltic Shield remain to evaluated. The formation of the Edefors granitoids could have been associated with an extensional zone developed due to delamination caused by separation of the down-dip oceanic lithosphere from the continental lithosphere.