A re-evaluation of a Laxfordian terrane boundary in the Lewisian Complex of south Harris, NW Scotland

• Published in: Journal of the Geological Society Vol. 162; no. 2; pp. 401 - 407
• Main Authors: Mason, A. J, Brewer, T. S
• Format: Journal Article
• Language: English
• Published: London Geological Society of London 01-03-2005; The Geological Society of London; Geological Society; Geological Society Publishing House
• Subjects: absolute age; Crystalline rocks; dates; Earth sciences; Earth, ocean, space; Europe; Exact sciences and technology; faults; Geochronology; Geological time; Geology; granites; Great Britain; Harris; Hebrides; Igneous and metamorphic rocks petrology, volcanic processes, magmas; Igneous rocks; Isotope geochemistry. Geochronology; Laxfordian; Lewisian Complex; Mesoproterozoic; nesosilicates; orthosilicates; Outer Hebrides; Paleoproterozoic; pegmatite; plutonic rocks; Precambrian; Proterozoic; Rock deformation; Scotland; shear zones; silicates; Structural geology; Tectonics. Structural geology. Plate tectonics; terranes; U/Pb; United Kingdom; upper Precambrian; Western Europe; zircon; zircon group; Outer Hebrides; Great Britain; Scotland; United Kingdom; Hebrides; Harris; Western Europe; absolute age; U/Pb; Palaeoproterozoic; terranes; Lewisian Complex; tectonic controls; lower Proterozoic; greenschist facies; Europe; reactivation; igneous rocks; Precambrian; Lewisian; granites; plutonic rocks; Terranes; shear zones; Proterozoic; deformation; dating; pegmatite; tectonite; middle Proterozoic; Laxfordian; concordia diagram
• ISSN: 0016-7649; 2041-479X
• DOI: 10.1144/0016-764904-050

The recent reinterpretation of the Lewisian Complex as a series of amalgamated terranes represents a significant advance in our understanding of these rocks. However, on the Outer Hebrides, a key observation that a supposed post-1675 Ma terrane boundary is based on is flawed. Specifically, on South Harris a group of c. 1675 Ma granite pegmatites has previously been described as occurring only to the NE of, and terminating abruptly within, a supposed terrane-bounding shear zone (the Langavat Shear Zone), which thus post-dates the pegmatites. New mapping indicates that granite pegmatites occur on both sides of, and throughout the Langavat Shear Zone, and that within most of this shear zone the pegmatites are not strongly deformed, inconsistent with them predating this shear zone. Furthermore, new U-Pb geochronological data suggest that the pegmatites SW of the supposed boundary were also intruded around 1675 Ma, similar to those in the NE, and supporting the field evidence that the pegmatites continue through the Langavat Shear Zone without interruption. Two phases of post-pegmatite deformation are identified, but these relate to relatively minor reactivation of the Langavat Shear Zone, not the initiation of this major shear zone, which predates the pegmatites.