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Geography and geometry of pre-Caledonian western Baltica: U-Pb geochronology and Palaeomagnetism

Rehnström, Emma LU (2003) In LITHOLUND
Abstract (Swedish)
Popular Abstract in Swedish

Den kontinent Skandinavien idag tillhör kallas Eurasia, men den kontinent vi ser idag har inte alltid funnits. Kontinenterna var inte bara sammanfogade på andra sätt, de har också rört sig över jordens yta (de långsamma med samma hastighet som naglar växer!), i platt-tektoniska processer. Innan uppkomsten av Eurasia har Skandinavien i omgångar tillhört andra konstellationer. Skandinavien är en del av en gammal kontinentkärna, vilken sträcker sig ända bort till dagens Uralberg och ner till Ukraina, som kallas Baltica och som i perioder har varit en egen kontinent. Den västra kanten av denna kontinent, dagens norska kust, var i Silurisk tid, för ca. 400 miljoner år sedan, involverad i en... (More)
Popular Abstract in Swedish

Den kontinent Skandinavien idag tillhör kallas Eurasia, men den kontinent vi ser idag har inte alltid funnits. Kontinenterna var inte bara sammanfogade på andra sätt, de har också rört sig över jordens yta (de långsamma med samma hastighet som naglar växer!), i platt-tektoniska processer. Innan uppkomsten av Eurasia har Skandinavien i omgångar tillhört andra konstellationer. Skandinavien är en del av en gammal kontinentkärna, vilken sträcker sig ända bort till dagens Uralberg och ner till Ukraina, som kallas Baltica och som i perioder har varit en egen kontinent. Den västra kanten av denna kontinent, dagens norska kust, var i Silurisk tid, för ca. 400 miljoner år sedan, involverad i en bergskedjebildande händelse. Baltica kolliderade med den Nordamerikanska kontinentalplattan och vår skandinaviska fjällkedja bildades, en händelse som kallas den Kaledonska bergskedjebildningen (Caledonia är det latinska namnet för Skottland, som också var involverat i kollisionen). Såsom skivor av snö skjuts upp över varandra på en snöskovel då man skottar, sköts skivor av berggrund, s.k. skollor, upp över kontinenten Balticas kant. Fokus i denna avhandling är att undersöka den berggrund, som nu finns i skollor i Sarek och Akkajaure, och genom att bestämma berggrundens ålder och kemiska karaktär försöka flytta tillbaka några skollor till sin ursprungliga position. Man kan på så vis få en idé om geometrin på olika geologiska bälten i prekaledonsk tid. Åldern på bergarter som saknar fossil, t.ex. de magmatiskt bildade, kan man bestämma med hjälp av radiometriska dateringsmetoder. Vissa mineral tillåter nämligen små mängder radioaktiva element i sin kristallstruktur. Efter bildningen av dessa mineral börjar de olika radioaktiva isotoperna att sönderfalla och det går sedan att mäta halten av moderisotoperna, men även dotterisotoperna, dvs de isotoper som har bildats genom sönderfallet. Man kan sedan räkna ut åldern på bergarten med hjälp av kända sönderfallskonstanter. Det element som har använts i denna studie är uran (U), som sönderfaller till bly (Pb). Felmarginalerna vid radiometriska dateringar kan tyckas väldiga i reell tid (typiskt några miljoner år), men utgör oftast endast tusen- eller hundradelar av själva åldern. Med hjälp av de åldrar jag har bestämt har korrelationer kunnat göras mellan skollorna i Sarek och Akkajaure och den berggrund som fortfarande befinner sig där den bildades vid Lofoten. Det har även framkommit data, som vittnar om andra händelser som har omvandlat och deformerat berggrunden långt före den Kaledonska bergskedjebildningen. De "nya" geometrier på geologiska bälten som har framkommit kan sedan sättas i ett globalt geografiskt perspektiv genom att positionen av olika kontinenter vid olika tid kan bestämmas genom palaeomagnetiska metoder. Man mäter en magnetisk vektor i tre dimensioner i bergartsprover och en breddgrad för den tiden då bergarten bildades erhålles. Eftersom tiden innan den Kaledonska händelsen är i fokus här, har palaeogeografiska positioner för tidsåldrarna Kambrium och Ordovicium bestämts. Baltica låg i tidig Kambrium ganska nära sydpolen och rörde sig genom Kambrium och Ordovicium norrut, samtidigt som kontinenten roterade motsols. Då kollisionen med Nordamerika var ett faktum befann sig Baltica på ekvatorn. Sedan dess har kontinenten rört sig i sakta mak längre norrut, en rörelse som forsätter än idag. (Less)
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... (More)
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. (Less)
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author
opponent
  • Dr Johansson, Åke
organization
publishing date
type
Thesis
publication status
published
subject
keywords
Physical geography, geomorphology, pedology, cartography, climatology, Fysisk geografi, geomorfologi, marklära, kartografi, klimatologi, Fennoscandia, correlations, Baltica, Early Palaeozoikum, palaeogeography, orthogneisses, nappes, palaeomagnetism, U-Pb geochronology, Caledonian orogen, Scandinavia
in
LITHOLUND
pages
29 pages
publisher
Department of Geology, Lund University
defense location
room 308, Sölvegatan 13, Lund
defense date
2003-03-28 10:15
ISSN
1651-6648
ISBN
91-86746-45-6
language
English
LU publication?
yes
id
92283be7-90fc-40fb-ba8a-caa5be3ccd76 (old id 465540)
date added to LUP
2007-09-06 14:17:37
date last changed
2018-05-29 10:23:15
@phdthesis{92283be7-90fc-40fb-ba8a-caa5be3ccd76,
  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.},
  author       = {Rehnström, Emma},
  isbn         = {91-86746-45-6},
  issn         = {1651-6648},
  keyword      = {Physical geography,geomorphology,pedology,cartography,climatology,Fysisk geografi,geomorfologi,marklära,kartografi,klimatologi,Fennoscandia,correlations,Baltica,Early Palaeozoikum,palaeogeography,orthogneisses,nappes,palaeomagnetism,U-Pb geochronology,Caledonian orogen,Scandinavia},
  language     = {eng},
  pages        = {29},
  publisher    = {Department of Geology, Lund University},
  school       = {Lund University},
  series       = {LITHOLUND},
  title        = {Geography and geometry of pre-Caledonian western Baltica: U-Pb geochronology and Palaeomagnetism},
  year         = {2003},
}