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Journal of the Geological Society; 2009; v. 166; issue.6; p. 1033-1047;
DOI: 10.1144/0016-76492009-006
© 2009 Geological Society of London

Research Article

Evidence for 930 Ma metamorphism in the Shetland Islands, Scottish Caledonides: implications for Neoproterozoic tectonics in the Laurentia–Baltica sector of Rodinia

K.A. CUTTS1, M. HAND1, D.E. KELSEY1, B. WADE1, R.A. STRACHAN2, C. CLARK3 & A. NETTING4

1 Centre for Tectonics, Resources and Exploration, School of Earth and Environmental Sciences, University of Adelaide, Adelaide, SA 5005, Australia
2 School of Earth & Environmental Sciences, University of Portsmouth, Burnaby Road, Portsmouth PO1 3QL, UK
3 The Institute for Geoscience Research (TIGeR), Department of Applied Geology, Curtin University of Technology, GPO Box 1987, Perth, WA 6845, Australia
4 Adelaide Microscopy, The University of Adelaide, Adelaide, SA 5005, Australia

*Corresponding author (e-mail: kathryn.cutts{at}adelaide.edu.au)

Zircon and monazite laser-ablation inductively coupled plasma mass spectrometry U–Pb geochronological data for two metasediment samples from the Westing Group, northern Shetland Islands, Scottish Caledonides yield ages between 938 ± 8 and 925 ± 10 Ma (Tonian) for upper amphibolites-facies metamorphism. Texturally early metamorphism is recorded by a migmatitic garnet + sillimanite + plagioclase + muscovite + biotite assemblage, which formed at c. 650–700 °C and 7 kbar. Subsequent reworking resulted in the growth of a secondary garnet + kyanite + plagioclase + muscovite + biotite assemblage at c. 650 °C and 8–9 kbar. In situ electron probe microanalysis (EPMA) U–Th–Pb chemical dating of monazite hosted within garnet grains and the matrix of one sample also give Tonian ages, apparently indicating that all the metamorphism occurred during the Neoproterozoic. However, the dominant structural fabrics appear to have formed during the Ordovician–Silurian Caledonian orogeny, suggesting that the reworking was substantially younger despite the apparent absence of Caledonian monazite or zircon ages. Detrital zircons are consistent with Laurentia–Baltica provenance. Deposition of the Westing Group is constrained to between c. 1030 and 930 Ma. The timing of Tonian metamorphism suggests possible correlations with sequences elsewhere in the northern Caledonides, including the Krummedal Succession of East Greenland and Laurentian-derived successions in Svalbard and northern Norway.

Supplementary material: U–Pb LA-ICPMS and EPMA data are available at http://www.geolsoc.org/SUP18379.





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P. A. Cawood, R. Strachan, K. Cutts, P. D. Kinny, M. Hand, and S. Pisarevsky
Neoproterozoic orogeny along the margin of Rodinia: Valhalla orogen, North Atlantic
Geology, 2010; 38: 99 - 102.
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