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Petrogenesis of Mesoproterozoic (Subjotnian) rapakivi complexes of central Sweden: implications from U–Pb zircon ages, Nd, Sr and Pb isotopes

Published online by Cambridge University Press:  26 July 2007

U. B. Andersson ,
L. A. Neymark  and
K. Billström
U. B. Andersson
Affiliation:
GeoForschungsZentrum, P.B. 4.1., Telegrafenberg, D-14473 Potsdam, Germany.
L. A. Neymark
Affiliation:
United States Geological Survey, Denver Federal Center, M.S. 963W, Denver, CO 80225, U.S.A.
K. Billström
Affiliation:
Laboratory for Isotope Geology, Swedish Museum of Natural History, Box 50007, S-104 05, Stockholm, Sweden.
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Abstract

U-Pb zircon geochronology of Mesoproterozoic (Subjotnian) rapakivi complexes in central Sweden yields: 1526 ± 3 Ma (Mullnäset), 1524 ± 3 Ma (Mårdsjö), 1520 ± 3 Ma (Nordsjö) and 1497 ± 6 Ma (Rödön). Together with complexes further S in Sweden, they constitute the westernmost, youngest (1·53−1·47 Ga) belt of rapakivi magmatism in the Fennoscandian shield.

The low initial εNd values (−8·9 to −4·8) of all studied Subjotnian basic, intermediate and silicic rocks, require an input from an old (Archaean) low-radiogenic source component, as evidence for Palaeoproterozoic protoliths in the age range 2·5−2·1 Ga is lacking in this region. Crustal, early Svecofennian + Archaean (roughly 30−40%) sources are suggested for the Subjotnian A-type granites and syenites, where the granites derive from undepleted, granodioritic, and the syenites from monzodioritic (±depleted crustal) protoliths. The basic rocks originate from a depleted mantle acquiring the enriched Nd isotopic signatures during interaction with an Archaean lower crust (20−40%), largely depleted after rapakivi melt extraction. Pb isotope data from feldspars (207Pb/204Pb to 15·018−15·542) support the presence of Archaean components in the magmas.

The results indicate that an Archaean basement is underlying relatively wide areas of Svecofennian formations in central Sweden. This old basement section was most likely rifted off the Archaean craton in the NE in Palaeoproterozoic times.

Keywords

Archaean basementA-Typeassimilationcontaminationinitial εNd ratiosLREE enrichment/depletionmixing
Type
Research Article
Information
Earth and Environmental Science Transactions of The Royal Society of Edinburgh , Volume 92 , Issue 3 , September 2001 , pp. 201 - 228
Copyright
Royal Society of Edinburgh 2001

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