| 1. |
- Rehnström, Emma, et al.
(författare)
-
Cambrian sediments and Proterozoic granites in the Dividalen-Tometrdsk area, northern Scandinavia: Palaeomagnetism and U-Pb geochronology
- 2003
-
Ingår i: GFF. - 2000-0863. ; 125:3, s. 131-138
-
Tidskriftsartikel (refereegranskat)abstract
- New palaeomagnetic data from the autochthonous Early Cambrian Dividal Group (northern Scandinavia) confirm earlier findings, and a refined palaeomagnetic pole of 58.4degreesN and 122.5degreesE, places Baltica at intermediate southerly latitudes at similar to535 Ma. Palaeomagnetic data from the immediately underlying granitic basement (pole: 9.8degreesN, 226.7degreesE) differ markedly from the Dividal sediments. The result of this fieldtest increases the palaeomagnetic reliability of the Dividal Group results. We dated the granitic basement in the eastern part of the Tornetrask area (corresponding to our palaeomagnetic site) to 1786+/-4 Ma (U-Pb zircon and titanite), whilst deformed allochthonous granites west of lake Tornetrask are slightly older (1800+/-4 Ma; U-Pb zircon). These ages are compatible with autochthonous basement ages to the east of the study area, but also with ages from basement windows to the west. Preliminary palaeomagnetic data from the 1786+/-4 Ma granitic basement are clearly pre-Phanerozoic in origin, and comparable and concordant zircon and titanite ages may hint that the magnetisation could be primary. However, the palaeomagnetic pole does not match contemporaneous poles from Baltica, which suggest a tectonic explanation (no palaeohorizontal control), a problem of secular variation (only one site) or a younger but pre-Phanerozoic remagnetisation event, which did not affect the U-Pb system in zircon and titanite.
|
|
| 2. |
- Rehnström, Emma, et al.
(författare)
-
Evidence of a Late Precambrian (637 Ma) deformational event in the Caledonides of northern Sweden
- 2002
-
Ingår i: Journal of Geology. - 0022-1376. ; 110:5, s. 591-601
-
Tidskriftsartikel (refereegranskat)abstract
- The Caledonian nappes in Scandinavia record two main phases of early Paleozoic metamorphism, but their pre-Caledonian tectonothermal history and paleogeographic position are largely unknown. Here we present a U-Pb age of 637+/-3 Ma for metamorphic titanite in the 1776+/-4 Ma (zircon age) Skarja granitic gneiss in northern Sweden. The titanite age is interpreted to represent a Neoproterozoic tectonometamorphic overprint. Geochronologic and paleogeographic considerations suggest that the gneiss was located at the outermost margin of pre-Caledonian (northwest) Baltica and was affected by Neoproterozoic tectonic activity related to terrane accretion, the Baikalian (or Timanian) orogeny, coincident with Cadomian terrane accretion along the Gondwanan margin of northern South America and northwest Africa.
|
|
| 3. |
- Rehnström, Emma
(författare)
-
Geochronology and petrology of the Tielma Magmatic Complex, northern Swedish caledonides - Results and tectonic implications
- 2003
-
Ingår i: Norsk Geologisk Tidsskrift. - 1502-5322. ; 83:3, s. 243-257
-
Tidskriftsartikel (refereegranskat)abstract
- In the Caledonides of Scandinavia a lower part of the nappe pile, the Middle Allochthon, consists of metasedimentary rocks and orthogneisses, transported in a southeastern direction and presumably derived from the rifted margin of pre-orogenic cratonic Baltica. In the Sarek-Kvikkjokk area in northern Sweden, telescoped orthogneisses are composed of lithologies that share characteristics with the autochthonous AMCG (anorthosite-mangerite-charnockite-granite) -suite of the Lofoten Complex in northern Norway. The allochthonous Tielma Magmatic Complex (TMC) consists of syenitic and gabbroic rocks, gabbro dykes and ferrogabbros and contains other noteworthy lithologies, such as anorthosites and Fe-Ti ores, which also occur in the Lofoten Complex. All rock types of this study show strong similarities in their trace element and rare earth element patterns with corresponding lithologies in Lofoten, which could indicate a common source and/or process of formation. U-Pb age determinations on zircons constrain the timing of the magmatic activity in the TMC to between 1776 +/- 7 Ma and 1761 +/- 9 Ma. Subsequent felsic dykes were dated to 1731 +/- 5 Ma. The new geochronological, petrological and structural data from the TMC constrain the geological evolution of these crustal elements, which were probably derived from the outermost cratonic Baltoscandian margin and strengthen a correlation with the Lofoten Complex. This correlation together with structural data on Caledonian transport directions suggest orthogonal transport directions towards ESE during the final phase of nappe emplacement.
|
|
| 4. |
- Rehnström, Emma
(författare)
-
Geography and geometry of pre-Caledonian western Baltica: U-Pb geochronology and Palaeomagnetism
- 2003
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In an attempt to reconstruct the pre-Caledonian cratonic margin of Fennoscandia, U-Pb geochronological and geochemical investigations have been performed on allochthonous orthogneisses in the northern Swedish Caledonides. The crystalline basement of the nappes have also been examined to further constrain the continuation of various chronologically defined orogenic belts. To constrain the global palaeogeographic setting for Baltica prior to the Silurian Caledonian collision, palaeomagnetic data from Cambrian and Ordovician sedimentary rocks were recorded. The basement in the Torneträsk region in northern Sweden consists of 1785-1800 Ma granites, whereas the 1871 Ma Ritsem Granite in the Akkajaure area further south is much older. The Torneträsk rocks can be correlated with rocks of similar age both to the west, north and east. The Ritsem Granite provides an important link between the Perthite Monzonite Suite, of Caledonide foreland in the east and the Hopen and Lødingen intrusions in the Lofoten area, in the west. Ages of the allochthonous orthogneisses from the Akkajaure-Sarek-Kvikkjokk areas fall into two groups. The older group (1779-1800 Ma) consists of granites from the Akkajaure area and these are correlated with similarly aged felsic magmatism in the Lofoten region. The younger group consists of the Tielma Magmatic Complex (TMC), which is a disrupted and telescoped AMCG-suite. The age of the TMC is bracketed between 1776 and 1761 Ma, however, with felsic dyke magmatism as late as 1731 Ma. The TMC is closely correlated with the Lofoten AMCG-complex on petrological grounds, but the bulk magmatism is younger. The complex could however, represent a younger phase of magmatism not represented onshore present-day Lofoten. No Archaean or older Palaeoproterozoic (e.g. 1860-70 Ma) ages were indicated in the nappes and it is concluded that these domains probably did not extend very much further than today, in pre-Caledonian times. The only metamorphic overprint that was recorded comes from 637 Ma old titanites from a granitic gneiss tectonostratigraphically overlying the TMC. The Cambrian palaeomagnetic data, obtained in this study, place Baltica in intermediate southerly latitudes, in an up-side-down position. Data from the early and late Cambrian show small differences, which has implications for the current discussions on a true polar wander event in the Cambrian. The results also suggest a likely palaeogeographic setting, with subduction under the Kara Block in the Ægir Sea realm for the formation of the Cambrian- Early Ordovician eclogites now present in the Caledonian nappes. Palaeomagnetic data from Early Ordovician limestones in southern Sweden give temporal constraints on the Baltica- Avalonia docking. A partial remagnetisation in the Late Ordovician is attributed to thermochemical resetting by orogenic fluids.
|
|
| 5. |
|
|
| 6. |
- Rehnström, Emma, et al.
(författare)
-
Palaeoproterozoic U-Pb ages of autochthonous and allochthonous granites from the northern Swedish Caledonides - regional and palaeogeographic implications
- 2004
-
Ingår i: Precambrian Research. - : Elsevier BV. - 0301-9268. ; 132:4, s. 363-378
-
Tidskriftsartikel (refereegranskat)abstract
- In the Silurian, Baltica and Laurentia collided to form the Caledonian orogenic belt. During this process, nappes derived from the pre-collisional Baltic margin, from outboard terranes of Baltica, and from exotic terranes of probable Laurentian origin, respectively, were thrust upon the indenting Baltica. The Middle Allochthon is a tectonostratigraphic level inferred to represent the rifted Neoproterozoic margin of Baltica and is heterogeneously composed of telescoped fault-controlled sedimentary basins and subjacent basement plinths. In the Akkajaure area of northern Sweden, the Middle Allochthon consists mainly of granites (Akkajaure Nappe Complex). The Ritsem Granite, a part of the (para-) authochthonous basement below the nappe complexes, yields a U-Pb zircon and titanite age of 1871 +/- 11 Ma. Granite and felsic dykes in the lowermost thrust-sheet in the Middle Allochthon yield ages of 1800 +/- 2 Ma and 1795 +/- 4 Ma, respectively. The granite of the overlying thrust-sheet is 1781 +/- 2 Ma old and crosscutting dykes are 1779 +/- 7 Ma. These ages constrain the pre-Caledonian position of the nappe complex to the general region west of Lofoten, northwestern Norway, substantiating a link with the Transscandinavian Igneous Belt (TIB) of the Fennoscandian Shield. (C) 2004 Elsevier B.V. All rights reserved.
|
|
| 7. |
- Roberts, RJ, et al.
(författare)
-
The Early Carboniferous Mageroy dykes, northern Norway: palaeomagnetism and palaeogeography
- 2003
-
Ingår i: Geological Magazine. - 0016-7568. ; 140:4, s. 443-451
-
Tidskriftsartikel (refereegranskat)abstract
- Palaeomagnetic data from the 337 Ma Mageroy dykes (northern Norway) are of exceptionally high quality, and a positive contact test along with an existing regional result from the Silurian Honningsvag Igneous Suite attests to a primary Early Carboniferous magnetic signature. The palaeomagnetic pole (S 14.8degrees, E320.1degrees, dp/dm = 4.4/8.6degrees) is the first Early Carboniferous pole from Baltica, and implies that northernmost Norway-Greenland, the Barents Sea and Svalbard were located at tropical to low northerly latitudes at this time. Northward drift during Carboniferous times (5-6 cm/yr) as demonstrated from palaeomagnetic data is also reflected in the sedimentary facies in the Barents Sea realm, that is, a change from tropical (Early Carboniferous) to subtropical (20-30degrees N) carbonates and evaporites in the Late Carboniferous. The Mageroy dykes are continental tholeiites which intruded into a set of NW-SE-trending normal faults parallel to the Trollfjorden-Komagelva Fault Zone and the Mageroysundet Fault immediately to the north and south of Mageroya, respectively. These, and many other NW-SE-trending faults (onshore and offshore), were active during Late Palaeozoic extension, and the dykes were probably contemporaneous with the earliest syn-rift sedimentation in the Barents Sea (for example, the Nordkapp Basin).
|
|
| 8. |
- Torsvik, Trond H, et al.
(författare)
-
The Tomquist Sea and Baltica-Avalonia docking
- 2003
-
Ingår i: Tectonophysics. - 0040-1951. ; 362:1-4, s. 67-82
-
Tidskriftsartikel (refereegranskat)abstract
- Early Ordovician (Late Arenig) limestones from the SW margin of Baltica (Scania-Bomholm) have multicomponent magnetic signatures, but high unblocking components predating folding, and the corresponding palaeomagnetic pole (latitude = 19degreesN, longitude = 051degreesE) compares well with Arenig reference poles from Baltica. Collectively, the Arenig poles demonstrate a midsoutherly latitudinal position for Baltica, then separated from Avalonia by the Tomquist Sea. Tornquist Sea closure and the Baltica-Avalonia convergence history are evidenced from faunal mixing and increased resemblance in palaeomagnetically determined palaeolatitudes for Avalonia and Baltica during the Mid-Late Ordovician. By the Caradoc, Avalonia had drifted to palaeolatitudes compatible with those of SW Baltica, and subduction beneath Eastern Avalonia was taking place. We propose that explosive vents associated with this subduction and related to Andean-type magmatism in Avalonia were the source for the gigantic Mid-Caradoc (c. 455 Ma) ash fall in Baltica (i.e. the Kinnekulle bentonite). Avalonia was located south of the subtropical high during most of the Ordovician, and this would have provided an optimum palaeoposition to supply Baltica with large ash falls governed by westerly winds. In Scama, we observe a persistent palaeomagnetic overprint of Late Ordovician (Ashgill) age (pole: latitude-4degreesS, longitude= 012degreesE). The remagnetisation was probably spurred by tectonic-derived fluids since burial alone is inadequate to explain this remagnetisation event. This is the first record of a Late Ordovician event in Scania, but it is comparable with the Shelveian event in Avalonia, low-grade metamorphism in the North Sea basement of NE Germany (440-450 Ma), and sheds new light on the Baltica-Avalonia docking. (C) 2002 Elsevier Science B.V. All rights reserved.
|
|
