Åke Johansson

Grenvillian basement and a major unconformity within the Caledonides of Nordaustlandet, Svalbard

Abstract Variously deformed granites and migmatites underlie the Neoproterozoic successions of Nordaustlandet, Svalbard. New U Pb zircon investigations of two types of intrusions from Lapponiahalvoya, northwestern Nordaustlandet, have yielded ages of 939 ± 8 Ma (Kontaktberget granite) and 961 ± 17 Ma (Lappoinafjellet granite), confirming previous, less precise multi-grain, Pb Pb evidence of Grenvillian igneous activity. The Kontaktberget granite cuts metasediments (Brennevinsfjorden Group), that are tightly folded and are separated from overlying volcanic and volcaniclastic rocks (Kapp Hansteen Group) by a major discordance (the Botniahalvoya Unconformity). The latter has been mapped in northwestern Nordaustlandet and is inferred to exist in central areas between Vestfonna and Austfonna. The Kapp Hansteen Group, with associated intrusive quartz porphyries, are probably a late orogenic expression of the Grenvillian igneous activity. The lack of igneous rocks in the overlying Neoproterozoic succession (Murchisonfjorden Supergroup) suggests that the base of this major unit is also a significant unconformity. This new evidence emphasizes the importance of the belt of Grenvillian tectonothermal activity, referred to here as the Nordauslandet Orogeny, that reaches from the classical provinces of North America and southern Scandinavia, via the East Greenland Caledonides, into the high Arctic.

U-Pb zircon ages of granitoids from the Småland-Värmland granite-porphyry belt, southern and central Sweden

Abstract U-Pb dating has been carried out on zircons from three Smaland-Varmland granitoids. The Filipstad granite in Varmland yields an U-Pb age of 178±10 (2σ) Ma, the Vaxjo granite west of Linkoping 1808±4 (2σ) Ma, and the Vaxjo granite east of Vaxjo 1769±9 (2σ) Ma. These results suggest that the‘postorogenic’ granites of the Smaland-Varmland belt are older than indicated by previous Rb-Sr age determinations, and that they were formed penecontemporaneous with the‘lateorogenic’ Svecokarelian granites further east.

Isotope geochemistry and age relationships of mafic intrusions along the Protogine Zone, southern Sweden

Abstract The Protogine Zone is a major tectonic lineament separating the Southwest Swedish gneiss province in the west from the Transscandinavian granite-porphyry belt and the Blekinge region in the east. Mafic intrusions occurring within the southern part of the Protogine Zone, west of it in Varmland county, and east of it in Blekinge county, have been investigated by the Sm-Nd and Rb-Sr methods. A dolerite sill from Varmland gives a Sm-Nd mineral isochron age of 1512 ± 98 Ma, confirming the middle Proterozoic age for this swarm. The dolerite dykes within the Protogine Zone proper appear to be divided into two age groups using Sm-Nd, one of around 1180 Ma in age and the other around 930 Ma. An age of around 930 Ma is also suggested for most of the Blekinge dykes, based on the Sm-Nd results. The Rb-Sr system in most dykes shows more scatter, and is less reliable for dating these rocks. None of these mafic rocks shows any strongly depleted initial Nd signature. The highest initial ϵNd value, +1.7, comes from the Olme dolerite in Varmland. Most of the 1180 Ma old Protogine Zone dolerites have initial ϵNd values around −2 and the 930 Ma old Protogine Zone and Blekinge dykes show values around 0. These values are attributed either to a near-chondritic or slightly enriched mantle source or to early large-scale mixing within the lower crust, rather than to local crustal contamination. The c. 1500 Ma Varmland dolerites may have formed during an extensional event that never led to continental breakup, while the c. 1180 Ma Protogine Zone dolerites form part of the extensional magmatism in Fennoscandia, Canada and Greenland preceding the Grenvillian-Sveconorwegian orogeny. The c. 930 Ma Protogine Zone and Blekinge dykes may have formed in response to the late Sveconorwegian uplift of the area west of the Protogine Zone. Late-stage dykes in Blekinge may alternatively be related to the opening of the Iapetus Ocean.

The early evolution of the Southwest Swedish Gneiss Province: geochronological and isotopic evidence from southernmost Sweden

Abstract The continental crust in southwestern Sweden has a complicated history of formation, apparently commencing ∼ 1.75 Ga ago and continuing until the end of the Sveconorwegian orogeny ∼ 0.9 Ga ago. Most of the rocks are polymetamorphic gneisses, but massifs of intrusive rocks can commonly be recognized. This study reports new UPb zircon ages from the province of Skane in the southernmost part of the region. Two gneiss samples and four samples of granitoid rocks were dated by the UPb zircon method. In the two gneiss samples, selected single zircon crystals were analyzed in addition to the standard multi-grain size fractions. The multigrain zircon analyses yielded an upper intercept age of 1613 ± 6 Ma for a grey gneiss from the central part of the province of Skane, and 1557−27+32 Ma for a red aplitic gneiss from southern Skane. Individual zircon crystals from these samples gave 207Pb 206Pb -ages between 1335 and 1669 Ma. For the grey gneiss, the individual zircons define a separate discordia with an upper-intercept age of 1640±16 Ma; in the red gneiss sample they scatter around a line with an upper intercept at 1675±25 Ma. Core-bearing zircons from a red gneissic granite in central Skane yielded an age of 1575−61+77 Ma, while similar zircons from a strongly foliated red granite from the Kullaberg horst in northwestern Skane gave an age of 1497−34+47 Ma. Core-free zircons from the Beden granodiorite in southern Skane have an age of 1449−11+23 Ma, and core-bearing zircons from a gneissic charnockite near Orkelljunga in northern Skane yielded an age of 1452−50+350 Ma. Although the latter age is poorly defined, it allows for similar intrusion ages of the Orkelljunga charnockite and the Beden granodiorite. SmNd isotopic analyses yield TDM ages of 1.84 o 1.99 Ga, and TCHUR ages of 1.48 to 1.60 Ga for these rocks, while RbSr analyses give TUR ages between 1.46 and 1.71 Ga. The combined evidence suggests that the main crust-forming episode in this part of southwestern Sweden occurred 1.6 to 1.7 Ga ago, with presumably subduction-related magmatism forming the protoliths of the red and grey gneisses. Around 1.5 to 1.6 Ga ago red anatectic granites were generated. The Beden granodiorite and the Orkelljunga charnockite, as well as the Varberg charnockite farther north, were all formed ∼ 1.45 Ga ago, possibly by mantle-generated anorogenic magmatism. Some 0.9 Ga ago, after the Sveconorwegian crustal thickening and high-pressure metamorphism, uplift within the Southwest Swedish Gneiss Province exposed lower- to mid-crustal granulitic and charnockitic rocks in its southern part.

Characterization of the Paleoproterozoic Mantle beneath the Fennoscandian Shield: Geochemistry and Isotope Geology (Nd, Sr) of ~ 1.8 Ga Mafic Plutonic Rocks from the Transscandinavian Igneous Belt in Southeast Sweden

Geochemistry and Sr-Nd isotope geochemistry of ca. 1.8 Ga, mafic intrusions of the Transscandinavian Igneous Belt (TIB-1) in the Fennoscandian shield were studied in southeastern Sweden. These rocks show LILE-LREE-enriched, HFSE-depleted, calc-alkaline, continental arc signatures in the north, grading to slightly less enriched, oceanic affinities southward. εNd(1.80) values range from +2.0 to +0.7 and 87Sr/86Sr(1.80) from 0.7022 to 0.7029 (with one outlier at 0 and 0.7033), without correlation to fractionation (e.g., Mg#) or crustal contamination, indicating sources that are mildly depleted. The most depleted ratios occur in the south, trending with the geochemistry toward more enriched compositions northward. The sources represent depleted mantle wedge material that was subjected to enrichment not long before (TDM ca. 2.0 Ga), i.e., during the preceding arc subduction (2.1-1.82 Ga), and/or during the TIB-1 magmatism itself, by hydrous fluids with a sediment and/or melt input increasing northward. The TIB-1 magmatism occurred above a south(west)ward-retreating subduction zone along the continental margin of the juvenile Svecofennian continent at 1.81-1.76 Ga.

The age and geotectonic setting of the Småland-Värmland granite-porphyry belt

Abstract Recent U-Pb zircon dating on the Smaland-Varmland granites and porphyries suggests that these rocks formed 1760–1840 Ma ago, about 50 to 100 Ma earlier than indicated by previous Rb-Sr age determinations. This would make the ‘postorogenic’ granites of the Smaland-Varmland belt similar in age to the ‘lateorogenic’ granites of the Svecokarelian province. A tectonic model is proposed where the Smaland-Varmland belt originated in a tensional regime, possibly an ensialic continental rift, penecontemporaneous with folding, migmatitization and formation of ‘lateorogenic’ anatectic granites in a compressional regime further to the northeast. The relationship between the Smaland-Varmland belt and the Southwest Swedish gneiss province remains uncertain, and may partly have been obscured by movements along the Protogine zone.

Grenvillian and Caledonian tectono-magmatic activity in northeasternmost Svalbard

Abstract The Nordaustlandet Terrane of NE Svalbard forms an exposed part of the Barentsia microcontinent. Augen gneisses, migmatites, granites and gabbros dominate the scattered outcrops along the northeastern coast of Nordaustlandet, and on the smaller islands to the north and east, as far as Kvitøya. These outcrops probably represent the deepest exposed crustal levels within the folded Caledonian basement of the Nordaustlandet Terrane. In the present study, a variety of rock types have been analysed by different U-Pb dating techniques (conventional, Pb-evaporation and ion microprobe) on zircon, titanite and monazite The major and trace element compositions and Sm-Nd isotope geochemistry of these rocks have also been investigated. The augen gneisses yield U-Pb ages of c. 950 Ma, indicating that they are deformed late Grenvillian granites, similar to the Grenville-age granites and augen gneisses of northwestern and central Nordaustlandet. Migmatites, grey granites, aplitic dykes and a syenite (boulder) yield U-Pb ages mainly falling in the 430–450 Ma range, slightly older than the 410–420 Ma late-tectonic Caledonian granites further west. Both the Grenvillian and Caledonian granites are of crustal anatectic origin, and the Caledonian granites and migmatites may have formed largely by remelting of Grenvillian crust. The ages of the mafic rocks are uncertain, but Sm-Nd data indicate a possible emplacement age of c. 700 Ma for two of the gabbros, suggesting that they may be the result of rift-related magmatism in connection with the opening of the Iapetus Ocean. A few enigmatic inherited zircons of similar late Neoproterozoic age found in younger granites may possibly be related to this event. No evidence for late Neoproterozoic orogenic activity, similar to that in the Timanides of northern Russia, is seen in eastern Svalbard. At this time, eastern Svalbard (Barentsia) was probably part of the Laurentian margin, and probably located far away from northern Baltica.

U-Pb age of titanite in the Mylonite Zone, Southwestern Sweden

Abstract U–Pb dating of titanite from a mafic lens in the Mylonite Zone in southwestern Sweden yields an almost concordant age of c. 920 Ma. The titanite formed during retrogression of granulite-to amphibolite-facies rocks. Sources for Ti and Ca in titanite were Fe-Ti oxides and relatively grossular-rich garnets, respectively. Deformation is contemporaneous with or post-dates the growth of titanite, which took place at temperatures above c. 550AC. The obtained U–Pb age of titanite is close to the Sm-Nd isochron ages of garnet-bearing granulite-facies mineral assemblages in the Varberg area just south of the Mylonite Zone. Since the blocking temperature is higher for Nd diffusion in garnet than for the Pb diffusion in titanite it is suggested that the rocks of the Mylonite Zone were undergoing amphibolite-facies deformation, uplift and cooling while the granulites of the Varberg area were still at a considerably higher temperature.

ISOTOPE STUDIES OF GRANITOIDS FROM THE BANGENHUK FORMATION, NY-FRIESLAND CALEDONIDES, SVALBARD

The Caledonian Hecla Hoek succession in Ny Friesland, eastern Svalbard has been interpreted, for many decades, to be a continuous stratigraphic sequence. Early Palaeozoic and Neoproterozoic strata ...

A revised geochronology for the Blekinge Province, southern Sweden

Abstract Zircon grains from nine samples of metavolcanic rocks, gneisses, granitoids and migmatites from the Blekinge Province in southeasternmost Sweden have been dated by U–Pb using ion microprobe. The results suggest that most of the Blekinge bedrock was formed within a narrow time interval of 1.77–1.75 Ga, including the Västanå supracrustal formation and ‘coastal gneisses’ previously dated to c. 1.70 Ga. One sample, the 1.81 Ga Nättraby gneissic granite, appears to represent a sliver of slightly older proto-crust. An age of c. 1.75 Ga for an aplitic granite crosscutting the deformed Tving granitoids in eastern Blekinge brackets their deformation to between 1.77 and 1.75 Ga, whereas zircon grains of similar age from a migmatite neosome at Lindö may be inherited, making the result inconclusive. Thin metamorphic zircon overgrowths and resetting of the U–Pb system in titanite indicate a regional tectonothermal event at 1.45 to 1.40 Ga, accompanying the intrusion of the Karlshamn-type granites. The crust of the Blekinge Province thus was formed at 1.77–1.75 Ga from relatively juvenile sources in a subduction-related environment along the southern edge of Fennoscandia. It was deformed and partly uplifted relative to the undeformed TIB-1 granitoids of the Småland block further north, prior to the intrusion of the Karlshamn-type granites at 1.45 Ga.