The BGS Lexicon of Named Rock Units — Result Details

Sionascaig Meta-Olivine-Gabbro Subswarm

Computer Code: SMOG Preferred Map Code: symb
Status Code: Full
Age range: Palaeoproterozoic Era (AL) — Palaeoproterozoic Era (AL)
Lithological Description: The Sionascaig Meta-olivine-gabbro Subswarm comprised the Fe-rich members of the Scourie Dyke Swarm. The dykes of the Subswarm are most abundant within the Lewisian Gneiss Complex in the Assynt region, where they are found to the south of the Canisp Shear Zone. Typically they have a broadly E-W trend. The dykes of the Subswarm are typically coarse-grained and texturally heterogeneous. They have the primary mineralogy olivine - clinopyroxene - orthopyroxene - plagioclase - kaersutite - biotite - Fe-Ti oxide. The plagioclases are typically heavily altered so that they appear black in hand-specimen, and this led to these dykes originally being mapped as ultramafic. The dykes of the Sionascaig Meta-olivine-gabbro Subswarm appear to post-date all but a few late metadolerites of the Scourie Dyke Swarm.
Definition of Lower Boundary: N/a
Definition of Upper Boundary: N/a
Thickness: 1m - 100m
Geographical Limits: The dykes of the Sionascaig Meta-olivine-gabbro Subswarm are part of the Lewisian Gneiss Complex and only occur within it. They have been mapped as a separate subswarm only in the Assynt area (between Geln Canisp in the north and Loch Lurgainn in the south).
Parent Unit: Scourie Dyke Swarm (SCOUR)
Previous Name(s): Olivine Gabbro Dykes Of The Scourie Dyke Swarm (-2435)
Assynt Meta-Olivine Gabbro Subswarm [Obsolete Name And Code: Use SMOG] (AMOG)
Assynt Meta-Olivine Subswarm (-4888)
Alternative Name(s): none recorded or not applicable
Type Area  Dykes of the Sionascaig Subswarm occur in the Lewisian Gneiss Complex of Assynt, to the south of Glen Canisp. The most well studied examples are at Loch Sionascaig [NC 136 145]. Tarney, 1973; Tarney and Weaver, 1987. 
Tarney, J. and Weaver, B L. 1987. Mineralogy, petrology and geochemistry of the Scourie dykes: petrogenesis and crystallization processes in dykes intruded at depth. p. 217-233 in Evolution of the Lewisian and Comparable Precambrian High Grade Terrains. Park, R.G. and Tarney, J. (eds.) Geological Society Special Publication No. 27. 
Weaver, B L and Tarney, J. 1981. The Scourie Dyke Suite: Petrogenesis and Geotechnical Nature of the Proterozoic Sub-Continental Mantle. Contributions to Mineralogy and Petrology, Vol.78, 175-188. 
Heaman, L M and Tarney, J. 1989. U-Pb baddeleyite ages for the Scourie Dyke Swarm, Scotland; evidence for two distinct intrusion events. Nature, 340, 705-708. 
Waters, F G, Cohen, A S, O'Nions, R K and O'Hara, M J. 1990. Development of Archaean lithosphere deduced from chronology and isotope chemistry of Scourie Dykes. Earth and Planetary Science Letters, Vol.97, 241-255. 
Tarney, J. 1973. The Scourie Dyke Suite and the nature of the Inverian event in Assynt. 105-117 in Park, R G and Tarney, J (editors), The Early Precambrian of Scotland and related rocks of Greenland. [University of Keele.] 
1:50K maps on which the lithostratigraphical unit is found, and map code used: