Zones of abundant Scourie dyke fragments and their significance in the Lewisian Complex of Western Harris, Outer Hebrides

doi:10.1016/S0016-7878(71)80015-9

Proceedings of the Geologists' Association

Volume 82, Issue 3, 1971, Pages 365-377

https://doi.org/10.1016/S0016-7878(71)80015-9 Get rights and content

Abstract

In Western Harris most of the Scourie dykes occur as disrupted, usually lenticular, fragments which are individually concordant with the banding of their host gneisses. The distribution of these fragments is irregular, but prominent zones occur in which they are abundant. These zones trend across the strike of the banding of the gneisses and appear to represent a number of groups of relatively large, closely spaced sub-parallel intrusions which originally crossed the banding obliquely. The outcrop-pattern of the dyke fragments is described and discussed in relation to major Laxfordian fold structures which the zones of fragments clearly reflect.

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Cited by (4)

Origin of ultramafic–mafic bodies on the Isles of Lewis and Harris (Scotland, UK): Constraints on the Archean–Paleoproterozoic evolution of the Lewisian Gneiss Complex, North Atlantic Craton
2022, Precambrian Research
Citation Excerpt :
Based on the preserved mineral assemblages, textures and grain sizes, these ultramafic–mafic bodies have been subdivided into two groups (Dearnley, 1962; Myers and Lisle, 1971): (1) mafic rocks — comprising < 0.5 cm diameter clinopyroxene and hornblende “clots” surrounded by a plagioclase-dominated groundmass — that show relatively uniform textures and no evidence for layering or chilled margins; and (2) coarse-grained, commonly layered ultramafic–mafic rocks, comprising olivine, orthopyroxene, clinopyroxene, plagioclase and hornblende in variable proportions, alongside minor chromite and phlogopite. The latter, relatively coarse-grained and more ultramafic group are less common, but are distinctive in the field, typically displaying light brown weathered surfaces (Dearnley, 1962; Myers and Lisle, 1971). Several authors have considered all of the ultramafic–mafic rocks to represent deformed mafic dykes that can likely be correlated with the Scourie Dykes of the mainland LGC (Dearnley, 1962; Myers and Lisle, 1971; Davies et al., 1975; Fettes and Mendum, 1987; Cliff and Rex, 1989).
The Lewisian Gneiss Complex (LGC) is a tonalite-trondhjemite-granodiorite (TTG)-dominated fragment of the North Atlantic Craton (NAC) in northwest Scotland. End-member models describe the LGC as representing either a continuous piece of Archean crust or up to 12 geologically distinct Archean terranes, with interpretations sitting on a spectrum between these end-members. There is particular uncertainty over the correlations between the Archean–Paleoproterozoic magmatic and metamorphic events recorded by mainland part of the LGC and the part exposed on the Outer Hebridean islands of Lewis and Harris. In this paper, we present the results of field mapping, petrography, and major, trace and platinum-group element (PGE) bulk-rock geochemistry for four ultramafic–mafic bodies in Lewis and Harris, namely: Maaruig, Loch Mhorsgail, Coltraiseal Mor and Beinn a’ Chuailean. We consider the effects of metamorphism and element mobility, their petrogenesis, and potential correlations with ultramafic–mafic rocks elsewhere in the LGC. Our data indicate that the studied ultramafic–mafic rocks can be subdivided into two petrologically distinct groups. Metaperidotites and metapyroxenites from Maaruig and Loch Mhorsgail are interpreted as Archean (>2.8 Ga) cumulates distinct from anything currently identified in the mainland LGC, with this interpretation based on distinctive modal layering, a discordance with surrounding TTG gneiss, fractionated PGE patterns ([Pd/Ir]_N = 1.3–6.6) and negative HFSE anomalies ([Nb/La]_N = 0.2–0.8). Metagabbronorites from Coltraiseal Mor and Beinn a’ Chuailean, which also exhibit negative high field strength-element (HFSE) anomalies ([Nb/La]_N = 0.2–0.7) and show mildly fractionated ([Pd/Ir]_N = 1.2–2.8) PGE patterns, most likely represent deformed Paleoproterozoic dykes. These occurrences could be correlatives of a suite of ca. 2.4 Ga mafic dykes exposed throughout the mainland LGC (the Scourie Dykes), with the Outer Hebridean occurrences having experienced more intense Paleoproterozoic (Laxfordian) deformation/reworking. These interpretations suggest that the LGC lithologies of Lewis and Harris were proximal to the mainland LGC’s Central Region by the early Paleoproterozoic but raises the possibility that they were distinct crustal blocks in the Mesoarchean.
The tectonic evolution of the Lewisian complex in northern Lewis, Outer Hebrides
1975, Proceedings of the Geologists' Association
The earliest tectonic and metamorphic events recorded in northern Lewis, perhaps of early Scourian (c. 2800 Ma.) age, led to the formation of a gneiss complex in which large areas of banded quartzofeldspathic gneisses were interleaved with narrow belts incorporating metamorphosed supracrustal and intrusive igneous rock. The later (late Scourian and Laxfordian) stages of tectonic evolution were controlled by the development of a north-westerly and a north-easterly lineament which are followed by the present coastlines. The bulk strains recorded in these lineaments are high relative to those in the block which lies between them and a wide concentric zoning is recorded by both structural and metamorphic features. Contrasts between the style and orientation of structures in the lineaments and those of the intervening block appear to have been established at an early stage and enhanced during successive phases. Both fold-structures and fabrics appear to have undergone progressive changes and to be adjusted to the bulk strain rather than to particular strain increments.
Major early thrusting as a control on the palaeoproterozoic evolution of the Lewisian complex: Evidence from the outer Hebrides, NW Scotland
2012, Journal of the Geological Society
A Sm-Nd isotopic study of the South Harris Igneous Complex, the Outer Hebrides
1983, Contributions to Mineralogy and Petrology

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Zones of abundant Scourie dyke fragments and their significance in the Lewisian Complex of Western Harris, Outer Hebrides

Abstract

Proc. Geol. Ass.

Proc. Geol. Ass.

Scourie Dykes and Laxfordian Metamorphism

Geol. Mag.

The Structural and Metamorphic Geology of South Uist, Outer Hebrides

An Outline of the Lewisian Complex of the Outer Hebrides in Relation to that of the Scottish Mainland

Q. Jl geol. Soc. Lond.

The Lewisian Complex of South Harris

Q. Jl geol. Soc. Lond.

Metamorphosed and Deformed Pegmatites and Basic Dykes in the Lewisian Complex of the Outer Hebrides and their Geological Significance

Q. Jl geol. Soc. Lond.

A Structural Analysis of Lewisian Rocks in part of North Uist and the Sound of Harris, Outer Hebrides