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