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Journal of the Geological Society; 2004; v. 161; issue.5; p. 837-848;
DOI: 10.1144/0016-764903-132
© 2004 Geological Society of London

Original Article

New light on the construction, evolution and correlation of the Langavat Belt (Lewisian Complex), Outer Hebrides, Scotland: field, petrographic and geochronological evidence for an early Proterozoic imbricate zone

A.J. Mason2, S. Temperley1 & R.R. Parrish1,2

1 1Department of Geology, University of Leicester, Leicester LE1 7RH, UK (e-mail: ajm47@le.ac.uk)
2 2NERC Isotope Geoscience Laboratory (NIGL), Nottingham NG12 5GG, UK

The South Harris Complex is a domain of largely Palaeoproterozoic rocks within the late Archaean, tonalite–trondhjemite–granodiorite (TTG)-dominated Lewisian gneisses of the Outer Hebrides, NW Scotland. The complex is distinguished by a high proportion of metasedimentary rocks and distinctive meta-igneous units, in part representing the remnants of a continental volcanic arc. The Langavat Belt defines the NE border zone of the South Harris Complex, separating the latter from the Archaean gneisses further NE, and has been repeatedly interpreted as a discrete supracrustal unit. However, detailed mapping and petrographic analysis reveals that up to 60% of the belt may be composed of highly deformed felsic orthogneiss. The metasedimentary rocks that are present have a late Archaean zircon provenance signature, implying that they are younger than, and possibly derived from the TTG gneisses to the NE. Previous work on zircon populations in the Leverburgh metasediments, towards the southern flank of the South Harris Complex, indicates very different provenance and we repudiate correlation of the two sequences. The disposition of units of metasediment, amphibolite and felsic orthogneiss is not a primary, pre-tectonic feature and is very unlikely to be the result of folding. Although direct evidence in the form of early fabrics has been wiped out by later, penetrative ductile shearing and metamorphic annealing, we propose that the Langavat Belt was assembled as a zone of thrust imbrication during early Proterozoic contraction. After imbrication, ductile shearing occurred in two distinct periods, separated by intrusion of granite pegmatites dated at c. 1660 Ma.


Keywords: Lewisian Complex, Palaeoproterozoic, U–Pb, imbricate tectonics, terranes.




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N.M. KELLY, R.W. HINTON, S.L. HARLEY, and S.K. APPLEBY
New SIMS U-Pb zircon ages from the Langavat Belt, South Harris, NW Scotland: implications for the Lewisian terrane model
Journal of the Geological Society, 2008; 165: 967 - 981.
[Abstract] [Full Text] [PDF]

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A.J. Mason and T.S. Brewer
A re-evaluation of a Laxfordian terrane boundary in the Lewisian Complex of South Harris, NW Scotland
Journal of the Geological Society, 2005; 162: 401 - 407.
[Abstract] [Full Text] [PDF]