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Geochronological constraints on granitic magmatism, deformation, cooling and uplift on Bornholm, Denmark

Publikation: Forskning - fagfællebedømtTidsskriftartikel

Tod Earle Waight, Dirk Frei, Michael Storey

U-Pb ages on zircon from 11 samples of granitoid and gneiss from the Danish island of Bornholm have been obtained using laser ablation - inductively coupled plasma mass spectrometry. These ages indicate that the felsic basement rocks were generated over a restricted period in the Mesoproterozoic at 1455 ± 10 Ma. No evidence has been found for the presence of 1.8 Ga basement gneisses as observed to the north in southern Sweden and as inferred in previous studies. No distinction in age can be made between relatively undeformed granitic lithologies and gneissic lithologies within the errors of the technique. This indicates that granitic magmatism, deformation and metamorphism all occurred within a relatively restricted and contemporaneous period. The granitic magmatism on Bornholm can thus be correlated to similar events at the same time in southern Sweden, Lithuania, and elsewhere in Baltica, and is therefore part of a larger magmatic event affecting the region. Argon and Rb-Sr ages on various minerals from a single sample of the Rønne Granite provide constraints
on the cooling and uplift history of the basement in the region. Using recently published closure temperatures for each isotopic system a cooling curve is generated that illustrates a period of rapid cooling immediately after and/or during crystallisation. This likely represents the period of emplacement, crystallisation, and deformation of the felsic basement. The modelled rate of post-emplacement cooling is highly dependent on the choice of closure temperature for Ar isotopes in biotite. Use of recently published values of around 450¢ªC defines a prolonged period of slower cooling (c. 4¢ªC per million years) over nearly 100 million years down to c. 300¢ªC and the closure temperature of Sr isotopes in biotite. Use of older and lower closure temperatures defines curves that are more consistent with theoretical models. The low closure temperature of Sr isotopes in biotite explains much of the wide variation in previous age determinations using various techniques on Bornholm. There is no evidence in the geochronological data for disturbance during later tectonic events in the region.
OriginalsprogEngelsk
TidsskriftBulletin of the Geological Society of Denmark
Vol/bind60
Sider (fra-til)23-46
Antal sider24
ISSN0011-6297
StatusUdgivet - 2012

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