S. Mertanen

Rodinia: the evidence from integrated palaeomagnetism and U-Pb geochronology

Abstract Of many hundreds of well-defined palaeomagnetic poles that have been reported from cratons around the world in the 1700–500 Ma period, only a few are precisely dated. However, such ‘key’ palaeopoles are a prerequisite for establishing rigorous palaeomagnetic reconstructions in order to chart the assembly, drift and breakup of the postulated late Precambrian supercontinent of Rodinia. Most key palaeopoles are derived from mafic dykes and sills that have been dated by U–Pb techniques. Most are from Laurentia, the largest and best studied of the continental fragments that are thought to have comprised Rodinia. Thirteen key Laurentia palaeopoles form an incomplete reference set that can be used for comparison with key palaeopoles from other cratons as they become available. Currently, there are four key palaeopoles for Baltica between 1700 and 500 Ma, although only one allows a direct comparison with a similar aged pole from Laurentia. The 1265 Ma match between Baltica and Laurentia is consistent with reconstructions in which Baltica is adjacent to present-day east Greenland, with the ca. 1700–1500 Ma Gothian and Labradorian belts aligned. Few key palaeopoles are yet available from other cratons. However, recent U–Pb dating of dykes, sills, or volcanic rocks in the Siberian, Australian and Kalahari cratons and in Coats Land of Antarctica constrains the ages of individual palaeopoles from each of these areas. Most of these are not key palaeopoles because they have not been conclusively demonstrated to be primary, or local tectonic rotations have not been ruled out. Nevertheless, they are useful in testing Rodinia reconstructions. In this paper, a U–Pb baddeleyite age is reported from the late Gardar magmatic rocks of southwest Greenland. Along with the previously published palaeopole for this unit, this age helps constrain the Mesoproterozoic location of southwest Greenland relative to North America.

Palaeomagnetism and Sm-Nd ages of the Neoproterozoic diabase dykes in Laanila and Kautokeino, northern Fennoscandia

Abstract The results of palaeomagnetic, rock magnetic and isotope data on the NE-SW trending Neoproterozoic Laanila-Ristijärvi (Finland) and Kautokeino (Norway) dyke swarms, northern Fennoscandia, are reported. Positive baked contact tests in both dyke sets indicate that the characteristic remanence is primary thermoremanence. Rock magnetic and petrological properties show similarity between the two dyke swarms. All the dykes except one (where the dominant carrier is Ti-rich titanomagnetite) have mixed multidomain and single-domain or pseudosingle-domain magnetic carriers of Ti-poor magnetite. From its remanence directions, petrography and rock magnetic properties, the easternmost of the three Laanila dykes represents a deeper crustal level than the other two dykes studied. Palaeomagnetic and aeromagnetic data suggest that the Palaeoproterozoic granulite block containing the easternmost Laanila dyke was uplifted after the dykes were emplaced. The Sm-Nd isotopic age determinations yield an age of 1042 ± 50 Ma for the Laanila dykes, 1013 ± 32 Ma for the Ristijärvi dykes and 1066 ± 34 Ma for the Kautokeino dykes indicating that the dyke swarms are of similar age within error limits. However, palaeomagnetic pole of the Laanila-Ristijärvi dykes (Plat = 2.1 °S, Plong = 212.2 °E, dp = 12.7°, dm = 21.1°, N = 3 dykes) differs significantly from the pole of the Kautokeino dykes (Plat = 35.2 °S, Plong = 236.8 °E, N = 2 dykes). The isotope and palaeomagnetic ages of the Laanila-Ristijärvi dykes are considered agreeable, but on the basis of younger precisely dated Sveconorwegian palaeopoles, a magnetization age of c. 900 Ma for the Kautokeino dykes is inferred. Three explanations for the discrepancy are suggested: (i) local tectonic movements, (ii) delayed magnetization of the Kautokeino dykes with respect to that of the Laanila-Ristijärvi dykes, (iii) the isotope ages and palaeomagnetic ages are compatible but the Fennoscandian APWP should be revised.

Catalogue of palaeomagnetic directions and poles from Fennoscandia: Archaean to tertiary

Abstract Palaeomagnetic data from Fennoscandia ranging from the Archaean to the Tertiary have been compiled into a catalogue. The data are presented in table format, listing Precambrian data according to tectonomagmatic blocks and Late Precambrian-Phanerozoic data according to geological periods. Each pole is graded with the modified Briden-Duff classification scheme. The catalogue (complete to the end of 1988) contains 350 entries from 31 tectonomagmatic blocks and/or geological periods. Normal and reversed polarity data are listed separately to allow polarity asymmetries to be studied. Each entry also has an indexed abstract summarizing relevant information, such as the age of the rock, the age of the natural remanent magnetization and the basis for the assigned reliability grade. All the data are stored in the palaeomagnetic data bank, which will be updated annually with new data. The catalogue is the basic source of data for the microcomputer-based palaeomagnetic database for Fennoscandia now being compiled.

Preliminary results of a palaeomagnetic and rock magnetic study of the Proterozoic Tsuomasvarri intrusions, northern Fennoscandia

Abstract Preliminary palaeomagnetic results are presented for the Proterozoic Tsuomasvarri central ultramafic intrusion and surrounding, older gabbro-diorite intrusion in the northeastern Fennoscandian shield. In the ultramafic intrusion, five remanence components, reflecting different geological processes, were isolated in the alternating field (AF) and thermal demagnetization of 55 samples from ten sites. A reversed polarity remanence component C (pole: Plat=26°N, Plon=245°E, dp=5°, dm=9°, n =32 samples) dominates in the ultramafic intrusion. The normal polarity pole A (Plat=44°N, Plon=232°E, dp=4°, dm=6°, n =9 samples) corresponds to an age of ∼ 1.88 Ga on the Apparent Polar Wander Path (APWP), and is probably a thermochemical remanence acquired during the Svecokarelian orogeny or in the late stages of magma cooling. Pole B (Plat=35°N, Plon=158°E, dp=9°, dm=15°, n =5 samples, normal polarity) corresponds to the ∼ 1.75 Ga old post-orogenic activity in the area, and is also seen in results of KAr (whole rock) isotopic age determinations from the vicinity. The normal polarity pole D (pole: Plat=25°S, Plon=285°E, dp=12°, dm=19°, n =11 samples) has a palaeomagnetic age of ∼ 2.44 Ga which is in contrast with the isotopic age determination of 1.93 Ga (UPb age on zircon) of the gabbro-diorite intrusion. Pole F (Plat=3°N, Plon=212°E, dp=10°, dm=18°, n =5 samples) indicates an age of 0.5-0.4 Ga on the Palaeozoic APWP segment of Fennoscandia and probably reflects a weak Caledonian overprint in northeastern Finland. In the gabbro-diorite intrusion, two remanence components were isolated in 46 samples from eight sites. The palaeomagnetic age, ∼ 1.93 Ga, of the dominant normal polarity component, A′ (pole: Plat=40°N, Plon=247°E, dp=5°, dm=8°, n =28 samples) is consistent with the isotopic age of 1.93 Ga (UPb determination on zircon). The normal polarity component D (pole: Plat=20°S, Plon=285°E, dp=14°, dm=23°, n =5 samples), which was also isolated in the ultramafic intrusion, contradicts the UPb isotopic age determination.

Palaeomagnetism of the Salla Diabase Dyke, northeastern Finland, and its implication for the Baltica-Laurentia entity during the Mesoproterozoic

Abstract New palaeomagnetic and rock magnetic results are presented for the 1122±7 Ma Salla Diabase Dyke in NE Finland. A positive baked-contact test proves that the dyke has a primary natural remanent magnetization carried by magnetite. The characteristic remanent magnetization direction (D=42.2°, I=73.9°, k=75.7°, α95=4.8°) of 13 sites along the large single dyke provides a virtual geomagnetic pole (VGP) position of Plat=71°N, Plon=113°E (A95=8.1°). Although secular variation may not have been fully averaged out, the new VGP provides an important result to define the late Mesoproterozoic position of Baltica. The VGP is not close to any known Proterozoic palaeopoles of Baltica, and therefore the pre-Sveconorwegian apparent polar wander path (APWP) of Baltica must be modified. The pre-Sveconorwegian (c. 1.3–1.0 Ga) APW swathes of Baltica, Laurentia (including the Logan Loop) and Kalahari cratons show similar shape, but new well-dated palaeomagnetic poles for c. 1.25–1.12 Ga interval from these continents are required to test the similarity. The Salla dyke VGP provides hints that the Mesoproterozoic Baltica–Laurentia connection in the Hudsonland supercontinent assembly lasted until 1.12 Ga.

The Archaean Karelia and Belomorian Provinces, Fennoscandian Shield

The Archaean bedrock of the Karelia and Belomorian Provinces is mostly composed of granitoids and volcanic rocks of greenstone belts whose ages vary from c. 3.50 to 2.66 Ga. Neoarchaean rocks are dominant, since Paleoarchaean and Mesoarchaean granitoids (> 2.9 Ga) are only locally present. The granitoid rocks can be classified, based on their major and trace element compositions and age, into four main groups: TTG (tonalite-trondhjemite-granodiorite), sanukitoid, QQ (quartz diorite-quartz monzodiorite) and GGM (granodiorite-granite-monzogranite) groups. Most ages obtained from TTGs are between 2.83–2.72 Ga, and they seem to define two age groups separated by a c. 20 m.y. time gap. TTGs are 2.83–2.78 Ga in the older group and 2.76–2.72 Ga in the younger group. Sanukitoids have been dated at 2.74–2.72 Ga, QQs at c. 2.70 Ga and GGMs at 2.73–2.66 Ga. Based on REE, the TTGs fall into two major groups: low-HREE (heavy rare earth elements) and high-HREE TTGs, which originated at various crustal depths. Sanukitoids likely formed from partial melting of subcontinental metasomatized mantle, whereas the GGM group from partial melting of pre-existing TTG crust.

Palaeomagnetism and U–Pb geochronology of c. 1570 Ma intrusives from Åland archipelago, SW Finland – implications for Nuna

Abstract We report new palaeomagnetic and isotope age data of Early Mesoproterozoic (i.e. Subjotnian) intrusions from the Åland archipelago, SW Finland. The palaeomagnetic results reveal dual-polarity magnetizations with a pronounced reversal asymmetry occurring in dykes. We explain the asymmetry by an unremoved secondary component, which is affecting more N-polarity dykes. Other explanations, such as the age difference of magnetization between normal and reversed polarity dykes, are discussed. The primary nature of magnetization in dykes for both normal (N) and reversed (R) groups is verified by positive baked contact tests. A dyke showing reversed polarity from Korsö is dated 1575.9±3.0 Ma (U–Pb) in this study. This and previous U–Pb data tighten the magmatic activity in Åland to 1580–1570 Ma. We combined new palaeomagnetic data with those from earlier studies to provide a new key-palaeomagnetic pole for Baltica. Our data positions Baltica on equatorial latitudes, supporting the NENA (North Europe–North America) connection between Baltica and Laurentia at 1.59–1.58 Ga. Palaeomagnetic data support that NENA was valid at 1.75, 1.58, 1.46, and 1.26 Ga, forming the core of Mesoproterozoic Nuna (a.k.a. Columbia) supercontinent.

Palaeomagnetism of Middle Ordovician Carbonate Sequence, Vaivara Sinimäed Area, Northeast Estonia, Baltica

The hill range of Vaivara Sinimäed in northeast Estonia consists of several narrow east- to northeast-trending glaciotectonic fold structures. The folds include tilted (dips 4–75°) Middle Ordovician (early Darriwilian) layered carbonate strata that were studied by mineralogical, palaeomagnetic, and rock magnetic methods in order to specify the postsedimentational history of the area and to obtain a better control over the palaeogeographic position of Baltica during the Ordovician. Mineralogical studies revealed that (titano)magnetite, hematite, and goethite are carriers of magnetization. Based on data from 5 sites that positively passed a DC tilt test, a south-easterly downward directed component A (Dref = 154.6°± 15.3°, Iref = 60.9°± 9.7°) was identified. The component is carried by (titano)magnetite, dates to the Middle Ordovician (Plat = 17.9°, Plon = 47.3°, K = 46.7, A95 = 11.3°), and places Baltica at mid-southerly latitudes. Observations suggest that in sites that do not pass the tilt test, the glaciotectonic event has caused some rotation of blocks around their vertical axis.

Mesoproterozoic Nuna Supercontinent and the Geomagnetic Field in Light of Recent Paleomagnetic Data from Diabase Dykes of Finland

Mesoproterozoic supercontinent Nuna (e.g. Columbia, Hudsonland) has increased in recent years enabling more reliable global continental reconstructions (e.g Hoffman 1997; Evans and Mitchell 2011; Zhang et al. 2012; Pisarevsky et al. 2014). Supercontinent Nuna included Baltica, Laurentia, Siberia, proto-Australia and Antarctica, Amazonia and West Africa, Congo-São Francisco, North China, Kalahari and India cratons. Baltica and Laurentia are thought to represent two of the most important building blocks of this supercontinent in a single geologically valid NENA (North EuropeNorth America) juxtaposition between ca. 1.75-1.27 Ga forming the core of Nuna with Siberia (e.g. Gower et al. 1990; Evans and Mitchell 2011). Recent high quality, precisely dated Mesoproterozoic paleomagnetic poles of Baltica support the NENA connection. These include the pole from Åland (1575.9 ± 3.0 Ma; U-Pb) diabase dykes (Salminen et al. 2015) and coeval pole from Satakunta diabase dykes (Salminen et al. 2014) in Finland; a pole for the Mesoproterozoic Satakunta sandstones in Finland (Klein et al. 2014); and poles for Lake Ladoga basalts and intrusives (1459 ± 3, 1457 ± 2 Ma; U-Pb) in Russia (Salminen and Pesonen 2007; Lubnina et al. 2010). One striking feature of the 1.576 Ga high quality paleomagnetic data for Åland and Satakunta is the asymmetry of polarity, i.e. the mean directions of normal (N) and reversed (R) polarities are not antiparallel at 95% confidence level and do not pass the reversal test (McFadden and McElhinny 1990). One possible reason for such an asymmetry could be an unusual behavior of the geomagnetic field at the Mesoproterozoic, which would hamper the paleomagnetic reconstructions. Antipodality of N and R directions is expected in the case where the geomagnetic field is represented by the geocentric axial dipole (GAD), whereas steepening or shallowing of inclinations can result from the contamination of GAD by zonal multipolar fields. We used 26 global dual-polarity paleomagnetic results from PALEOMAGIA database (Veikkolainen et al. 2014a) to detect possible deviations from the GAD hypothesis (Hospers 1954) applying the quantity called inclination asymmetry (Veikkolainen et al. 2014b). The asymmetry tests indicate that GAD is a relatively good fit at the Mesoproterozoic (1.7-1.4 Ga) and therefore zonal multipolar fields do not explain the observed asymmetry. One other possible reason for asymmetry is an unremoved secondary component, which could explain the asymmetry for Åland and Satakunta data. Additional support for component mixing comes from the secondary component distribution, which is streaked in part toward the N-polarity direction. A third reason can be a small but significant age difference between N and R magnetized dykes which could explain the asymmetry. However, the actual age span for the Mesoproterozoic dykes for Baltica awaits further precise age dating. In addition to results from Åland, Satakunta and Lake Ladoga we present here new high quality Mesoproterozoic paleomagnetic and geochronological results from the Häme dykes (1642 ± 2 Ma, 1647 ± 14 Ma; U-Pb) in Finland that do not show asymmetry. These results also support the NENA connection placing Baltica on equatorial latitudes at 1.64 Ga.