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Cleaning up the record – revised U-Pb zircon ages and new Hf isotope data from southern Sweden
Swedish Museum of Natural History, Department of Geology. Swedish Museum of Natural History.ORCID iD: 0000-0002-1598-5231
2021 (English)In: GFF, ISSN 1103-5897, E-ISSN 2000-0863, Vol. 143, no 4, p. 328-359Article in journal (Refereed) Published
Abstract [en]

Ten samples of orthogneisses and granitoids from the Eastern Segment of the Sveconorwegian Orogen in southern Sweden, previously dated by ID-TIMS on zircon, have been dated anew using SIMS spot analysis of individual zircon grains, leading to more reliable and in most cases also more precise revised magmatic crystallization ages. A gneissic monzonite within the Protogine Zone south of Alvesta in Småland yields a revised U-Pb age of ca. 1725 Ma, four samples of orthogneiss from Skåne all yield revised ages between 1690 and 1700 Ma, while two samples of coarse-grained granitoid gneiss in the same region yield ages between 1680 and 1690 Ma. These revised ages are between 15 and 250 m.y. older than previously obtained TIMS ages. Two samples of the Gumlösa-Glimåkra granite along the Protogine Zone in northern Skåne and one sample of related syenite yield ages around 1220 Ma, similar but more precise compared to the previous ages. The U-Pb zircon data have been complemented by Hf isotope analysis by LA-ICP-MS on the same zircon grains, and previously obtained initial Sr and Nd whole-rock isotope data have been recalculated to the revised crystallization ages. The Sr isotope data scatter, while the revised initial εNd values fall between +1 and +2 for the older granitoids and orthogneisses, and close to 0 for the 1220 Ma intrusives along the Protogine Zone. Initial εHf in magmatic undisturbed zircons shows relatively little spread within each sample, between 2 and 4 Epsilon units, disregarding occasional outliers, with average values for the 1725 - 1680 Ma rocks falling between +3and +5.5, and at ca. +1.5 in the 1220 Ma rocks. In spite of the limited variation, there is a covariation between initial εNd and initial εHf in the older rocks, suggesting either mixing between two isotopically distinct magma sources, or one magma source which is isotopically heterogeneous. The isotopic signatures of the 1220 Ma intrusives along the Protogine Zone is indicative of juvenile mantle input totheir magmas, rather than pure crustal melting.

Place, publisher, year, edition, pages
2021. Vol. 143, no 4, p. 328-359
Keywords [en]
U-Pb zircon dating, Hf isotopes, granitoids, Eastern Segment, Protogine Zone, Sveconorwegian Orogen, southern Sweden.
National Category
Geology
Research subject
The changing Earth
Identifiers
URN: urn:nbn:se:nrm:diva-4410DOI: 10.1080/11035897.2021.1939777OAI: oai:DiVA.org:nrm-4410DiVA, id: diva2:1617542
Funder
The Geological Survey of Sweden (SGU)Available from: 2021-12-07 Created: 2021-12-07 Last updated: 2022-02-14

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