Carboniferous magmatism of the Southern Uplands

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Stone, P, McMillan, A A, Floyd, J D, Barnes, R P, and Phillips, E R. 2012. British regional geology: South of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.

Magmatism[edit]

Eildon Hills near Melrose, formed by a Carboniferous igneous intrusion (a composite laccolith). P219630.
Vertical dyke of Carboniferous olivine-dolerite intruded into Devonian at Fairy Castle. P000941.

The Birrenswark and Kelso volcanic formations (Inverclyde Group) are the extrusive manifestations of Late Devonian to early Carboniferous magmatism, which also gave rise to a range of minor intrusions across the south of Scotland. The presence higher in the succession of the Kershopefoot Basalt (Fell Sandstone Formation) and the Glencartholm Volcanic Member (Tyne Limestone Formation) demonstrates that magmatism continued well into the Visean. Although there are many small intrusions and agglomerate-filled vents scattered across the Scottish Borders between Langholm and Duns, and though most are regarded as of Carboniferous age, it is not possible to identify any of them as a specific centre from which the lavas and tuffs were erupted. Nevertheless, a general association seems highly likely. Several of the minor intrusions cut Border Group strata and these can therefore be tentatively related to the younger volcanic units.

Most of the regional swarm of early Carboniferous minor intrusions — vents, plugs, dykes and sills — are, in compositional terms, alkali basalts, with some of the larger and coarser- grained intrusions, which cooled relatively slowly, being texturally doleritic. This assemblage is accompanied by a range of more evolved trachytic intrusions, some of which are formed by silica-undersaturated phonolites whilst others consist of more silica-rich rhyolitic or micro-granitic rocks. The relatively silica-rich trachytic magmas were more viscous than the basaltic types and their intrusion commonly resulted in sills and laccoliths rather than in extrusive rocks. The trachytic rocks are also particularly tough and resistant to erosion and so tend to give rise to prominent landscape features. The best-known of the evolved, silica-undersaturated intrusions now forms the Eildon Hills (P219630) (NT 550 325), near Melrose, but other examples in that general vicinity occur at Skelfhill Pen (phonolite), Millstone Edge (phonolitic trachyte), Linhope Burn (phonolitic trachyte), Tudhope Hill (phonolitic trachyte) and Pikethaw Hill (phonolite). Farther east, a cluster of trachytic and rhyolitic sills and laccoliths intruded into Upper Devonian strata of the Stratheden Group crops out to the west of Duns around Dirrington Great Law (NT 698 549).

The Eildon Hills intrusion (P219630) is a composite laccolith made up from several thick sheets of locally columnar jointed, trachytic riebeckite-microgranite and phonolite. It cuts Silurian strata and an outlier of the Upper Devonian Stratheden Group. A radiometric date of about 352 Ma suggests that intrusion occurred quite early in Carboniferous times. A scattering of dykes and small irregular intrusions of similar composition to the main Eildon Hills body is seen within a few kilometres of it, and suggests that the original intrusive mass was much larger than the preserved remnant. To the north-west of the Eildons, a large volcanic vent (Chiefswood, approximately 2 km2) of rhyolitic tuff is thought to be sightly younger than the laccolith intrusion, whilst the latter is itself intruded by a small basaltic vent. The Eildon Hills intrusion straddles one of the major strike faults in the Lower Palaeozoic Southern Uplands terrane and it is tempting to think that intrusion was localised by that pre-existing structure.

Throughout the region, there are minor intrusions, mostly mafic, which are of uncertain age and association but which are assigned a general Carboniferous age. An example is the sparse swarm of broadly north-west-trending olivine-dolerite dykes that cut the Lower Devonian Great Conglomerate Formation in Lauderdale (P000941). Whilst a Carboniferous age for these dykes seems most likely, a Mid Devonian age has also been proposed and cannot be discounted. Analcime-bearing dolerite intrusions thought to be of early Carboniferous age occur around Duns, where they have been quarried for aggregate.

Late Carboniferous to early Permian mafic minor intrusions are widely distributed throughout the Midland Valley of Scotland, with major sill complexes developed both there and in northern England. However, in the Southern Uplands minor intrusions of this age are rare and are restricted to scattered alkali-dolerite and quartz-dolerite sills and dykes; the quartz-dolerite dykes are late Carboniferous in age (about 300 Ma) but the alkali-dolerites may be older. Together, the dolerite dykes form a sparse, east–west-trending swarm that is most extensive in the north-eastern part of the Southern Uplands, though possibly the best exposed examples are to be seen in the River Esk and the Liddel Water north of Canonbie.

Rather more unusual igneous rocks of late Carboniferous to early Permian age are ‘camptonite’ or ‘monchiquite’ dykes, lamprophyric rocks respectively with plagioclase or feldspar-free with analcite. Although they are rare, they are fairly widespread, with two small intrusions of ‘essexite’, a type of nepheline-gabbro, near Wanlockhead and Abington. In the Sanquhar Basin, the Westphalian Coal Measures are cut by a number of highly altered analcime dolerite sills and a few north-west-trending ‘monchiquitic’ or ‘camptonitic dolerite’ dykes. The dykes are generally decomposed and have fed sheets intruded along the coal seams, the sheets being converted to ‘white trap’ in the process and rendering the coal unworkable locally. Both the dykes and sills are thought to be related to thin (<50 m) lower Permian lava sequences locally present at Sanquhar and in the nearby Thornhill Basin: the lower Permian lavas are described in Chapter 7. The north-west-trending, upper Carboniferous or lower Permian dyke-like ‘Essexite’ intrusion exposed in Craighead Quarry near Abington (formerly known as the Crawfordjohn Essexite) is now classified as a nepheline-gabbro. This distinctive rock is characterised by the presence of large phenocrysts of euhedral Ti-rich augite set in a finer-grained groundmass of plagioclase, clinopyroxene, olivine, nepheline and analcime. It has become well known as the raw material for high-quality curling stones, rivalling the better-known Palaeogene microgranite lithology from Ailsa Craig, the small island off Girvan in the Firth of Clyde.


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