Igneous rocks in the Southern Uplands

Map showing the igneous rocks of the Southern Uplands and Ballantrae area

Igneous rocks in the Southern Uplands and the Ballantrae area
Key to the map of Igneous rocks in the Southern Uplands and the Ballantrae area

Igneous rocks the Ordovician Period in the Ballantrae Complex and the Southern Uplands

The Ballantrae Complex crops out over 75 km2 between Girven and Ballantrae, north of the Southern Uplands Fault but connected to the terrain to the south on account of its origins. Since it is north of the Southern Upland Fault it is not, strictly speaking, a part of the Southern Uplands but its geology is strongly associated with that of of the Southern Uplands and so is included here.
The Ballantrae Complex is an assemblage of Early Ordovician, mostly igneous ultramafic and mafic rocks that originated as ocean crust and mantle in a variety of oceanic regimes. Fragments exist here from island-arc, ocean-island and mid-ocean-ridge environments. The main elements of the complex are pillow lavas and olivine-rich, ultramafic rock that is now altered to serpentinite.

Quite how these rocks come to be here in this arrangement is still the subject of study and speculation although there is general agreement that they were generated in late Cambrian to Early Ordovician times in one or more island-arc zones with back-arc spreading. These environments could have been created by a Laurentian-connected ocean plate being subducted under the Iapetus Ocean plate and then drawn under the Laurentian plate following a reversal in the polarity of subduction (see animation).
The obduction process, subduction in reverse, that returned these rocks from depth onto what was then the leading edge of the Laurentian continent, appears to be less well understood. It is supported by details relating to some of the more minor elements of the complex: the gabbro and trondhjemite intrusions, the small portion of a sheeted dyke assemblage, and the sheared sedimentary melange together ‘take on a disproportionate significance in the interpretation of the complex as an ophiolite’. (Stone P. 2012)

Map: Ordovician igneous rocks in the Southern Uplands and Ballantrae area

Ordovician igneous rocks in the Southern Uplands and Ballantrae
Key to map of Ordovician igneous rocks in the Southern Uplands and Ballantrae

Silurian and Devonian igneous rocks in the Southern Uplands

There are four large outcropping plutons in the western Southern Uplands and one very large but unexposed pluton that gravity anomalies suggest is underlying a large portion of the Tweeddale area in the east. These are accompanied by a number of smaller plutons and many minor intrusions.
Loch Doon, Cairnsmore of Fleet, Criffel–Dalbeattie and Bengairn plutons are the major ones. They are all compositionally zoned and were probably formed by mafic mantle-derived magma underplaying, intruding, and melting crustal rocks to form a single parent magma that arrived into their magma chambers in successive pulses.
Igneous rocks found at Loch Doon and other minor plutons to the north of the Southern Uplands are similar to Midland Valley rocks while rocks from the three more southerly plutons have more in common with those of the Lake District – a situation that suggests the influence of different underlying lithospheric and/or crustal material.

To the east of the Southern Uplands, the smaller Ordovician and Devonian plutons range from dioritic to granodioritic.
They include:
Culvennan (diorite)
Glenluce (diorite)
Cairngarroch (microtonalite and granodiorite)
Portencorkrie (diorite and granodiorite)
Spango (granodiorite)

Minor intrusions include:
Carsluith intrusion (diorite)
Kirkmabreck (or Creetown) (granodiorite)
The Knipe (diorite and granodiorite)

To the west, there are only a few outcropping intrusions that tend to be more felsic.
They include:
Priestlaw (granodiorite, diorite and olivinenorite)
Cockburn Law (granodiorite and quartz-diorite)
Broad Law (granodiorite)

Map: Silurian and Devonian igneous rocks in the Southern Uplands and Ballantrae area

Silurian and Devonian igneous rocks in the Southern Uplands and Ballantrae
KeY to map of Silurian and Devonian igneous rocks in the Southern Uplands and Ballantrae

Beginning in the late Silurian and continuing until the Early Devonian, two sets of dykes belonging to the Northern Britain Calc-alkaline Dyke Suite were intruded throughout the deformed accretionary strata of the Southern Uplands; the lamprophyre dykes and the microdiorite to microgranite dykes. More dykes were intruded into the north than the south and more to the west than the east, fewest dykes being toward the centre of the region. The lamprophyre dykes are concentrated in the south.

Most probably, the lamprophyre dykes were the earlier set to be intruded, many following the NE to SW bedding of the accretionary prism but some follow the region’s cross faults. Their magma had its source deep in the mantle and was modified by interaction with higher mantle and crustal rocks resulting in a range of compositions including: mica-bearing kersantite, hornblende-bearing spessartite, and hornblende-rich appinite. Most of them are kersantites that are similar in composition to the coeval lamprophyre dykes of the Lake District; a fact that is seen to support the view that Avalonian mantle was underthrust beneath the leading edge of Laurentia and was therefore the same source for the two geographically distinct sets.

The microdiorite to microgranite set of dykes are characterised by high plagioclase content and a low proportion of mafic minerals.They range from andesite to rhyolite in composition and bear feldspar, amphibole, biotite and quartz phenocrysts in a fine-grained quartzofeldspathic groundmass and occasionally clinopyroxene phenocrysts in the microdiorites. The larger dioritic intrusions are coarser grained with an upper mantle/crustal magma source.
Probably, most of the more felsic dykes had their origins in the same magma chambers as formed their adjacent plutons but differentiated lamprophyric magma may have been the source for the rhyolitic dykes.

Volcanic vents and breccia pipes are occasionally associated with the dykes as at Shoulder O’Craig.

Late Devonian and Carboniferous igneous rocks in the Scottish Uplands

Following the erosion of the high land created by igneous and accretionary processes associated with the closure of the Iapetus Ocean, extensional forces dominated in the Southern Upland region. We believe this because much of the igneous material erupted and intruded across the region in the Late Devonian and Lower Carboniferous is alkaline in nature, high in sodium and potassium and low in silica, and rocks of this type are closely associated with magmatic activity in present day areas of extension and rifting, as well as with hot spots created by mantle plumes.
Alkaline magma is far from common and the question of why this type of magma is high in alkaline metals is still under discussion.
One answer is that the source of this magma lies at a much deeper level than magmas associated with subduction events and that sodium and potassium ions are more concentrated at depth. This could be because deep lying magma is not subject to repeated recycling by the constant creation, subduction and destruction of oceanic plates, as is magma at higher levels. Recycling causes higher-level magmas to lose some of their sodium and potassium to the continental crust. Another suggested explanation is that the alkali metals, K and Na, are not compatible with the characteristic minerals formed at depth in the mantle. Low silica minerals such as Olivine are dominant at depth so that the melt is often silica-undersaturated.
Regardless of cause, much of the mafic rock here is alkaline and feldspathoids like nepheline and occasionally leucite, as well as analcime appear in its groundmass.
The magmatic episode that formed these alkali rocks began in the Late Devonian, reached a maximum in the early Carboniferous and then drew to a close in the Upper Carboniferous, roughly 330 million years ago. In the landscape, it is evidenced by numerous agglomerate and olivine microdiorite and microgabbro filled volcanic vents that are associated with extensive basalt flows.
The Minto Hills are an example of the agglomerate/tuff- filled volcanic vents while Dunion Hill and Peniel Heugh serve as examples of the volcanic plugs.
Along with the mafic rocks, there are some substantial trachytic intrusions in the north-east of the region accompanied by a profusion of felsic dykes.
In the Border region, the hard trachytes, agglomerates and basalts have resisted the effects of erosion better than the softer accretionary complex greywackes, shales and mudstones and Old Red Sandstones that underlie and surround them. Consequently they stand proud of the sedimentary areas and form a plethora of rounded hills and dark outcrops. The larger igneous intrusions form the more prominent features like the Eildon Hills and The Dirrington Laws near Duns.

The olivine-rich basalt/microgabbro lavas, plugs, dykes and sills of the region were classified in the early surveys into two main categories in respect of large or small sized phenocrysts, both of which are sub-divided into three sub- groups in respect of the mineralogy of the phenocrysts. The system continues to be used on BGS mapping.

Macroporphyritic olivine basalt/microgabbro
Markle
type with large labradorite feldspar and olivine phenocrysts
Dunsapie type with large labradorite feldspar, olivine and augite phenocrysts
Craiglockhart type with large olivine and augite but no labradorite phenocrysts

Microporphyritic olivine basalt/microgabbro
Jedburgh type with small labradorite feldspar and olivine phenocrysts
Dalmeny type with small olivine phenocrysts and some sparse, small feldspar and augite phenocrysts
Hillhouse type with small olivine phenocrysts and some sparse small augite phenocrysts

The groundmass of both categories is mainly labradorite, augite and opaque iron oxides except for the Hillhouse type that has augite and iron oxide with some analcime and/ or glass.
Intermediate types, particularly between the Markle and Jedburgh types are common in the Border region an example occurring at Dunion Hill.
The Jedburgh type is interesting in that the volcanic plugs of the region tend to be of this type and the numerous feldspar laths in its groundmass tend to be aligned in parallel, frozen in their direction of flow.

Map: Late Devonian to Carboniferous igneous rocks in the Southern Uplands
Late Devonian to Carboniferous igneous rocks in the Southern Uplands
Key to Late Devonian and Carboniferous igneous rocks in the Southern Uplands and Ballantrae area

The Birrenswark Formation and the Kelso Volcanic Formation (the Kelso Traps) are extrusive, basalt lava type of these magmas. The formation extends from Duns in the north east of the region to Kirkbean in the southwest. The vast quantities of lava must have flowed from multiple vents but none of the lavas can be directly associated with any particular volcanic vent. However the general association with the current vents and plugs is presumed.
Amongst the more unusual intrusions of this period in the region, one at Southdean Law, close to the England-Scotland border is an olivine-basanite in which we have found a small amount of what looks like leucite. The mineral has low birefringence and exhibits tartan twinning, both indicative of leucite, although it appears that analcime can sometimes exhibit these properties.
A similar, coarser-grained nepheline-gabbro occurs as the late Carboniferous or early Permian ‘Crawfordjohn’ dyke that is exposed in Craighead Quarry near Abington.

Whilst there are numerous Late Devonian – Early Carboniferous mafic volcanic plugs in the region, there are only few alkaline mafic dykes of the same age. Two examples can be found to the south-east of Selkirk. In contrast, there are many felsic dykes trending NE-SW parallel to the host rock strata. A swarm of these dykes lie between Selkirk and Melrose.

Close by, the Eildon Hills group make a prominent landmark. The hills are the remains of an eroded laccolith intruded later in the Carboniferous, c.352 million years ago. It is composed of several thick sheets of sanidine- and riebeckite-trachyte as well as riebeckite-microgranite along with smaller areas of basalt and agglomerate on the western side of Eildon Mid Hill. The Mid Hill also contains a small area of unusual olivine-augite-trachyte. A series of similarly silica-poor dykes of the same age lie in its vicinity.
Other silica-undersaturated trachytic intrusions close-by are:
Skelfhill Pen (phonolite),
Grey Pen (trachyte)
Millstone Edge (phonolitic trachyte),
and Linhope Burn (phonolitic trachyte).

Permian igneous rocks in the Southern Uplands

The scattered remnants of Permian olivine basalt lavas through the Thornhill and Lochmaben basins testify to its original extent. Present exposures amount to a few, thin layers often interbedded with basalt-dominated breccias.

Map: Late Carboniferous to Permian igneous rocks in the Southern Uplands

Late Carboniferous to Permian igneous rocks in the Southern Uplands
Ket to the map of Late Carboniferous to Permian igneous rocks in the Southern Uplands

Palaeogene dykes in the Southern Uplands and Ballantrae

The final magmatic activity in the Southern Uplands took place in the Palaeogene between 55 and 60 million years ago. It was associated with thermal uplift caused by a proto-Icelandic mantle plume and the consequent opening of the North Atlantic Ocean. The volcanic activity and the eruption of basaltic lavas that ensued generated a swarm of dykes across Scotland, south-westwards into England.
Dykes range in thickness from less than 2m up to 23m wide and are said to be ‘tholeiite basalt’ or microgabbro, basalt to basaltic andesite often comprising plagioclase, clinopyroxene, iron oxides and interstitial quartz or glass. Chlorite and serpentine tend to replace olivine as alteration minerals.

Map: Palaeogene dykes in the Southern Uplands and Ballantrae area

Palaeogene dykes in the Southern Uplands and Ballantrae
Key to map of Palaeogene dykes in the Southern Uplands and Ballantrae

It was generally believed that the Palaeogene dykes were intruded laterally from a common source near Mull. However, recently doubts have been cast on this because of compositional variations along the length of dykes, considerable lateral offsets and trend differences between same-dyke segments, and difference in composition between sub-parallel adjacent dyke segments.
Current thinking proposes a regional, low level reservoir of magma generated by magmatic underplating of the lithosphere by the proto-Icelandic mantle plume giving rise to smaller, high-level magma chambers that produced risers that intruded host rocks vertically.

Timeline of magmatism in the Ballantrae Complex and the Southern Uplands

Timeline of magmatism in the Ballantrae Complex and the Southern Uplands

References

MacAdam A.D., Clarkson E.N.K., Stone, P. (editors) 1993 .Scottish Borders geology: an excursion guide. Edinburgh, Scottish Academic Press.

Stone, P. (editor) 1996.Geology in south-west Scotland: an excursion guide. Keyworth, Nottingham: British Geological Survey.

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

BGS online Geology of Britain http://mapapps.bgs.ac.uk/geologyofbritain/home.html

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