Loch Borralan Pluton (LBRR)
|Parent:||North-west Highlands Alkaline Plutonic Suite (NWHAP)|
|Daughter(s):||Altnacealgach alkali-feldspar-syenite (AAFS), Cnoc-na-Sroine quartz-syenites (CNSQ), Ledmore nepheline-syenite (LEDS), Allt a'Mhuilinn pseudoleucite syenite and associated rocks (BORO), Bad na h-Achlaise ultramafic rocks (BAUM), Loch Urigill Carbonatite (LUCB)|
|Age (Ma):||c.430 Ma|
PARSONS, I. 1999. Late Ordovician to mid-Silurian alkaline intrusions of the North-west Highlands of Scotland. In: STEPHENSON, D, BEVINS, R E, MILWARD, D, HIGHTON, A J, PARSONS, I, STONE, P, WADSWORTH, W J. (Editors), Caledonian Igneous rocks of Great Britain. Geological Conservation Review Series, 17. Chapman & Hall, pp. 345-393.
Author(s): K M Goodenough (BGS), I Parsons (University of Edinburgh). (This edit: M Krabbendam (BGS)
- 1 Summary
- 2 Distribution
- 3 Lithological description
- 3.1 Early Suite
- 3.2 Late Suite
- 4 Field relationships
- 5 History of research
- 6 Previous names
- 7 Physical properties
- 8 See also
- 9 References (not complete)
The Loch Borralan Pluton lies in southern Assynt, and covers an area of around 26 km2, situated within the Moine Thrust Zone. It comprises mainly quartz syenite together with leucosyenite, nepheline syenite, alkali-feldspar-syenite, carbonatite and ultramafic rock. It is the only igneous intrusion composed largely of silica-undersaturated (i.e. feldspathoid-bearing) rocks in the British Isles, and many of its members are exceptionally alkaline (rich in Na2O and K2O). It provides Britain's only example of truly ultra-potassic magmatism and is associated with the only British example of carbonatite. Unfortunately the pluton is very poorly exposed, although some progress in unravelling relationships has been made through drilling and excavation work. The unusual character of the rocks was recognised in the 19th century and the intrusion has held an important place in the international development of igneous petrology.
|50K MAP EXTRACT HERE|
|Oblique aerial view taken from above Ledbeg, the dwelling in the foreground of Cnoc na Sroine part of the Loch Borralan Pluton, Sutherland. Looking south-west. P000784.|
The Loch Borralan Pluton forms an oval outcrop in southern Assynt, North-West Highlands. It occupies the hill of Cnoc na Sroine, and the low ground surrounding this hill.
The variety of rocks in the early suite is extremely large, exposure is very poor, and few internal contact relationships are seen. The most useful descriptions of field relationships are those of Woolley (1970, 1973), extended by drilling work reported by Notholt et al (1985) and Shaw et al. (1992), and by the excavations reported by Parsons and McKirdy (1983) and Young et al. (1994).
Bad na h-Achlaise ultramafic rocks (BAUM)
Biotite-magnetite pyroxenites, with and without Ti-rich melanite garnet, and hornblendites crop out only in the low ground in the SW of the intrusion. The only natural exposures of pyroxenite are in an overgrown stream at Bad na h-Achlaise [NC 2456 1152], and in the Ledmore River 200 m downstream from the ruined footbridge [NC 2470 1209] where coarse-grained, dark brown, deeply weathered biotite pyroxenite can be seen. However, a large magnetic anomaly suggests the presence of a larger area of ultramafic rocks (Parsons 1965a; Matthews and Woolley 1977) Fig. here. The magnetic evidence indicates that the main body of pyroxenite forms a thick dyke-like mass between syenite and Durness Group dolostone. The pyroxenites dip at approximately 70° to the north-east, and plunge beneath less basic rocks towards the south-east. Isolated screens of pyroxenite also occur between Bad na h-Achlaise and the Ledmore River.
Parsons (in Johnson and Parsons, 1979) suggested that the pyroxenites are an in situ metasomatic assemblage (a skarn) formed at the contact between the syenites and dolostones. However, the excavations near Bad na h-Achlaise (Parsons and McKirdy, 1983) clearly showed the pyroxenites to be intrusive into quartzites of the Cam Loch klippe. Matthews and Woolley (1977) suggested that the pyroxenites are cumulate rocks from the base of the sheet forming the 'early suite', and that they have been brought to their present attitude by faulting or by squeezing of a partly consolidated layered sequence.
Further information has come from an extensive drilling programme in the south-western part of the pluton (Matthews and Woolley, 1977; Notholt et al., 1985; Shaw et al., 1992). Drilling was undertaken initially because of a high content of vanadium in magnetite in the pyroxenites, and then because of their high apatite content. While not currently economic, the ultramafic body constitutes the most significant phosphate resource yet found in the United Kingdom. The drilling proved the existence of a large skarn between the intrusive biotite-pyroxenites and Durness Group dolostones (Fig. [7.2]). It is thus clear that there has indeed been some reaction between the igneous rocks and the host dolostones.
Ledmore melanite-pyroxene nepheline-syenites (LEDS)
The less mafic part of the early suite comprises mesocratic melanite-pyroxene nepheline-syenites, termed 'ledmorites' by Shand (1910). Good exposures of these rocks can be seen on the A837 just north of Ledmore Junction [NC 2439 1317 to NC 2475 1258], in the Ledbeg River at [NC 2427 1318], and in the Ledmore River at Ledmore [NC 2467 1207]. They also occur, chiefly as veins cutting pyroxenites, in the excavations near Bad na h-Achlaise [NC 244 115]. They are coarse-grained syenites, varying in colour from red to green with differing proportions of alkali feldspar and pyroxene, and they are generally heavily weathered and quite rotten. At some localities (e.g. in the Ledbeg River) they show an igneous lamination defined by aligned laths of alkali feldspar.
|Melanite-pyroxene syenite. Photomicrograph. Light: PPL. Objective: X2.5. Sample: ED6783. Ledmore, Sutherland. P054742.|
|Melanite-pyroxene-nepheline-syenite, Borolan Complex, Aultivullin Quarry. P527532.|
Nepheline-syenites do not crop out in the poorly exposed area around the south side of Loch Borralan, but drilling (Notholt and Highley, 1981) revealed that nepheline-syenites extend over at least 3 km south and southeast from Ledmore. The drill cores showed that the syenites are intrusive into the pyroxenites, and also showed the existence of several different types of syenite, including some with up to 50% melanite and others with none.
Allt a’Mhuilinn pseudoleucite syenite and associated rocks (AMMU)
These rocks include the Loch Borralan Pluton's best known rock type: a pyroxene-melanite-nepheline-pseudoleucite syenite originally termed 'borolanite' by Horne and Teall (1892). Their main exposures are in the eastern part of the pluton, in the Allt a'Mhuillin [NC 289 104], and in the Aultivullin quarry to the east [NC 287 097] (Woolley, 1973). They also occur as sheets cutting dolostone in the quarry at Ledbeg, where spectacular patterns can be seen in the metasomatised dolostone around the intrusions [NC 253 137].
The ‘borolanites’ are coarse-grained, dark grey syenites containing black melanite garnets up to 5 mm across, and conspicuous white spots up to 2.5 cm across (Woolley, 1973). These spots are intergrowths of nepheline and potassium-rich alkali feldspar, generally believed to be pseudomorphs after leucite. These pseudoleucites have varying degrees of ellipticity within the Aultivullin quarry, and in the lower part of the Allt a'Mhuillin gorge they are flattened into white streaks that give the rock a foliated appearance. In contrast, the pseudoleucites in the Ledbeg quarry exposures do not have a flattened appearance. The origin of the flattening is controversial: it could either be tectonic in origin, or it could have formed through deformation of a crystal mush in the magma chamber, as suggested by Elliot and Johnson (1980). It is therefore unclear whether the early suite of the Loch Borralan Pluton was emplaced during or after movement of the thrusts in the area.
The pseudoleucite syenites in the Aultivullin quarry are cut by a set of undeformed pegmatite veins containing an assemblage unique in Britain: feldspar, nepheline, biotite, melanite, magnetite, sphene, allanite, zeolites and a blue, sulphatic cancrinite (vishnevite) (Stewart, 1941).
Other undersaturated rocks in the eastern part of the Borralan complex form an extremely diverse suite, and can be grouped into three types that are arranged in eastward-dipping sheets (Woolley,
The body known as the Loyne mass, at the NW extremity of the pluton [NC 255 143], has also been attributed to the Allt a’Mhuilinn suite of rocks (Woolley, 1970). However, outcrops here have a rather different appearance to those of the classic ‘borolanites’. The syenites of the Loyne mass are reddish in colour, coarse grained, with large (up to 5 mm) crystals of alkali feldspar. They lack the characteristic white spots, and generally contain rather less garnet than the pseudoleucite syenites from the Allt a’Mhuilinn area.
Melanite syenites with white spots resembling pseudoleucite occur in a borehole (Notholt and Highley,1981) near dolostone at [NC 256 098], but these, and the phlogopite- and serpentine-carbonate rocks that they cut, are not exposed at the surface.
Loch Urigill Carbonatite (LUCB)
Carbonatite (igneous rock composed largely of carbonate minerals), was first discovered as loose blocks of orange-brown carbonate rock on the beach at Loch Urigill, some 400 m outside the contacts of the Loch Borralan Pluton [NC 2470 1050] (Young et al., 1994). The outcrops were improved by excavations (Threadgould et al., 1994) and a total of four varieties of carbonatite have been found: porphyritic sövite (coarse-grained carbonatite, composed almost entirely of calcite), phlogopite sövite, sövite breccia and foliated silicocarbonatite (Young et al. 1994). The first three types have been found in situ, the latter only as a 30 cm loose block 30 cm across. The carbonatite body contains numerous xenoliths of nepheline syenite and biotite pyroxenite, which can be correlated with units from the Loch Borralan Pluton, and also xenoliths of Durness Group dolostone that have been rotated during the emplacement of the carbonatite magma.
|Carbonatite, Loch Borralan Pluton, Loch Urigill. P617989.|
The most striking part of the carbonatite is the phlogopite sövite, which owes its orange colour to myriads of small phlogopite plates included in a matrix of large calcite crystals. The rock also contains rosettes of apatite. The porphyritic sövite is almost wholly composed of coarse white calcite crystals and is locally layered, with 2 cm thick bands of the relatively rare mineral chondrodite (a hydrated magnesium silicate) separated by 25 cm layers of normal sövite. The sövite breccia is a matrix-supported breccia of brown carbonatite fragments in a coarsely crystalline, brown sövite matrix resembling the phlogopite sövite. The carbonatite thus shows considerable internal heterogeneity, involving several generations of brecciation followed by incorporation into later phases of magma injection. Similar features have been recorded in carbonatites elsewhere (REF)
The Loch Urigill intrusion is the only known British example of a carbonatite. Its distinctive trace element, carbon and oxygen isotope signatures show that the intrusion is of mantle origin, and does not represent locally mobilised Durness Group carbonates (Young et al., 1994). The surrounding area is poorly exposed, and so the overall extent of the carbonatite remains uncertain, but carbonatites commonly form subcircular stocks tens to hundreds of metres across (Le Bas, 1987).
The silica-saturated and oversaturated alkali-feldspar-syenites of the late suite are relatively well exposed on the slopes of Cnoc-na-Sroine. The rocks are less exotic than those of the earlier suite, and are similar to the ‘perthosites’ and melanite syenites in the Loch Ailsh intrusion.
The Cnoc-na-Sroine quartz-syenites form the upper slopes of Cnoc-na-Sroine itself. They are coarse-grained, red syenites and contain around 12 vol.% quartz, more than the quartz-syenites at Loch Ailsh. The perthitic alkali feldspars have bulk albite:orthoclase ratios (Ab60Or40) similar to Loch Ailsh and to silica saturated syenites in general.
Altnacealgach alkali-feldspar-syenites (AAFS)
The Cnoc-na-Sroine quartz-syenites pass down into the Altnacealgach alkali-feldspar-syenites, which Woolley (1970) described as ‘perthosites’. These rocks are coarse grained, red or grey in weathering colour, and consist almost entirely of crystals of perthitic alkali feldspar, with melanite garnet present in some areas. At some localities, a good igneous lamination is defined by aligned laths of alkali feldspar. The alkali-feldspar-syenites occur on the lower slopes of Cnoc na Sroine and in the low ground west of the Allt a'Mhuillin gorge.
Because of its position within the Assynt Culimnation of the Moine thrust Zone, the Loch Borralan Pluton forms a potential for constraining the age of movement of thrusts within this tectonic zone. These constraints critically depend on the contact relationhips with the country rock and the thrusts it contains.
Contacts between early and late suites
The most important contact between the early and late suites is exposed in the lower part of the Allt a'Bhrisdidh [NC 253 119]. Near the Ledmore River this stream flows in medium grained, brownish nepheline-syenite, but upstream the rock darkens and becomes veined and speckled by pink feldspar (Woolley, 1970). Upstream, about 50 m north of the A837, foliated nepheline-syenite has a sharp, irregular but overall sub-vertical contact with pink, leucocratic, alkali-feldspar-syenite. In streams draining Cnoc-na-Sroine nearer to Ledmore, pink leucocratic syenite veins are seen to cross-cut nepheline-syenite, but the correlation of these syenite veins with the main mass of Cnoc-na-Sroine is not certain.
A second critical contact is in poorly exposed ground about 1 km NE of the Allt a'Mhuilinn gorge [NC 290 130]. Here, a tongue of quartz-syenite extends into the pseudoleucite syenites. The intrusive contacts are sharp, and the later syenite becomes finer grained towards the contact (Woolley, 1970). When these detailed field relationships are considered together with the larger-scale, mapped relationships that show the late suite cross-cutting contacts in the early suite, it seems certain that the late suite represents a separate magmatic phase. Woolley (1970) suggested that the early suite represents a sheet-like, composite intrusion, whilst the rocks of the late suite form a plug-like body, with steep to vertical contacts, which has intruded through the early suite.
Contacts of the Loch Borralan Pluton against country rocks are in general very poorly exposed. On the A837 near Ledbeg [NC 2439 1317], red early suite melanite-pyroxene nepheline-syenites are seen enclosing xenoliths of recrystallised Durness Group dolostone with pyroxene selvages, the syenite being strongly deformed at the margins of the xenoliths. The syenite at this location is in faulted contact with Basal Quartzite of the Ledbeg Thrust sheet. Just west of Ledbeg, outcrops of nepheline-syenite are seen in close proximity to metamorphosed Durness Group dolostones in the Sole Thrust sheet.
The Loyne mass, composed of pyroxene-melanite-nepheline pseudoleucite syenite, is separate from the main pluton, and both roof and floor are exposed. Woolley (1970) stated that the Loyne intrusion was emplaced entirely within Durness Group dolostones, but noted that dolostones to the east of the intrusion are not recrystallised. This was taken as evidence that intrusion of the Loyne mass pre-dated thrusting. However, recent BGS mapping has shown that the western end of the Loyne mass is in contact with rocks of the An-t-Sron Formation, which are clearly thrust over the Durness dolostones in the Ledbeg River. It is thus more likely that the Loyne mass was emplaced following thrusting.
|View along 'borolanite' (melanite-pseudoleucite-nepheline syenite) sheet cutting metamorphosed limestone, Ledmore Marble Quarry, Assynt. P531476.|
|Cut slabs of foresterite/diopside marble, Ledbeg/Ledmore Marble Quarry. P527533.|
Complex intrusive contacts can be seen in the active marble quarry near Ledbeg, in which Basal Quartzite of the Ledbeg Thrust sheet is thrust over metamorphosed Durness Group dolostones. The precise features of exposures in the quarry clearly vary, as quarrying operations continue. The intrusion of massive sheets of pseudoleucite syenite into the dolostones has led to the development of beautiful serpentine marbles with conspicuously banded reaction zones. Evidence can also be seen for the contemporaneous presence of both borolanite and mobilised carbonate liquids (Thow, 2000). The mobilised carbonate material forms sheets and dykes that may superficially resemble carbonatites, but they have trace element and isotopic compositions similar to marbles and limestones (Thow 2000), and distinct from the Loch Urigill carbonatite (Young et al. 1994). Unfortunately, at the time of writing, no exposures in the quarry provide conclusive evidence for the relationship between the pseudoleucite syenite sheets and the Ledbeg Thrust. However, the borolanites in the quarry are relatively undeformed, suggesting that they post-date thrusting.
The northern contact of the Loch Borralan Pluton is poorly exposed, but this contact is perpendicular to the north-south trending traces of a number of thrust planes of the Breabag Dome (see section X.Y), and these are presumably truncated by the pluton. Igneous sheets cutting dolostones are exposed in the area around the Four Burns in the NE of the complex [NC 295 135], but again relationships to thrusts are not clearly seen. Contacts between igneous rocks and the envelope are nowhere exposed along the SW and eastern edges of the intrusion. Only drilling work (Matthews and Woolley, 1977; Notholt et al., 1985; Shaw et al., 1992) has revealed the metamorphic calc-silicate rocks that form the contact of the igneous pyroxenite bodies along the SW margin.
The most important contacts of the Loch Borralan Pluton occur on its western side, around Bad na h-Achlaise; outcrops in this critical area have been excavated to clarify the field relationships (Parsons and McKirdy, 1983). At this locality [NC 2442 1152] Basal Quartzite of the Cam Loch Thrust sheet is cut by nepheline-syenite and pyroxenite belonging to the early suite of the Loch Borralan Pluton. This provides clear evidence that the Loch Borralan Pluton does indeed cut the lower brittle thrusts in the Moine Thrust Zone.
History of research
The first specific account of the intrusion was by Horne and Teall (1892) who described the pyroxene-melanite-nepheline-pseudoleucite syenite which they called 'borolanite'.
The detailed work of Shand (1906, 1909, 1910 and 1939) maintained the Assynt area at the forefront of the developing science of igneous petrology, by his introduction of the important concept of 'silica saturation'. He believed that the silica-undersaturated character of some alkaline rocks (the Loch Borralan Pluton being a good example) was a result of the extraction of silica from a magma of broadly granitic composition by reactions with limestones, precipitating calcium silicate minerals and releasing carbon dioxide. The idea was taken up by Daly (1914) and became known as the ‘Daly-Shand hypothesis’, developed because of a common association observed between nepheline-bearing rocks and 'limestone'. However, it is now known that the carbonate rocks which often occur in association with nepheline syenites are igneous carbonatites, and an example was discovered in Assynt as recently as 1988 (Young et al. 1994).
Shand (1910) postulated that the Loch Borralan Pluton has the form of a gradationally stratified laccolith, a model accepted by Bowen (1928) who reproduced Shand's section in his seminal book ‘The evolution of the igneous rocks’. Bowen argued that pseudoleucite-bearing rocks at the base of the laccolith formed as a result of crystal settling, leaving a silica-oversaturated liquid that crystallised to produce quartz-syenites. He justified this interpretation on the basis of insights gained from his experimental petrological studies, but later work (eg. REF) has demonstrated the impossibility of passing continuously from under- to over-saturated magma compositions by crystal fractionation alone.
Subsequent interpretations of the internal structure of the intrusion have questioned the gradational character of the boundaries within the Loch Borralan pluton. Woolley (1970) divided the complex into an earlier suite of relatively mafic silica-undersaturated rocks, and a relatively leucocratic later suite of silica-saturated or oversaturated syenites. Woolley accepted the views of Harker (in Tilley, 1957) and MacGregor and Phemister (1937) that the boundary between the two suites, exposed around the Allt a’Bhrisdidh at the foot of Cnoc na Sroine, is an intrusive one. He showed that the early suite has a laccolithic form, and that the later suite has the form of a thick plug-like body, punching through the earlier units. The pluton has been dated by van Breemen et al. (1979b), who showed that the whole sequence of intrusion occurred in the interval 430 ± 4 Ma. However, they were unable to resolve the age differences between the early and late suites.
The Loch Borralan Pluton has been variously named:
- Loch Borralan Complex
- Loch Borolan Laccolith (note spelling Borralan vs. Borolan)
- Loch Borralan Intrusion
A number of uncommon, now obsolete, terms have been used in the past for the lithologies present within the Loch Borralan Pluton:
|Rock name||Used by||Modern equivalent|
|Assyntite||Shand (1910)||Sodalite nepheline-syenite|
|Borolanite||Horne & Teall (1892) – still occasionally used informally||Melanite-biotite (pseudoleucite) nepheline-syenite|
|Cromaltite||Shand (1910)||Melanite-biotite pyroxenite|
|Ledmorite||Shand (1910) – still occasionally used informally||Melanite–augite nepheline syenite|
|Perthosite||Phemister (1926)||Alkali-feldspar syenite|
|Sovite||Young et al. (1994) – still a valid name||Calcium cabonatite|
Tectonic setting Geochemistry Economic geology – mineralisation Age dating
The Loch Borralan Pluton was first dated at 430 +/- 4 Ma using U-Pb zircon TIMS dating techniques (van Breemen et al. 1979a). This date was based on a number of samples derived from different intrusive phases.
Intact rock strength / Hardness: Aggregate Impact Value (British Standards Institution, 1975): Hydrogeology Porosity: Permeability: Transmissivity: Specific capacity: Weathering and mass movement Type and reference sections
|Grid reference||Name and description||Access and preservation|
|NC 2442 1152||Bad na h-Achlaise
Syenites of the Loch Borralan Pluton cut quartzites of the Cam Loch Klippe at Bad na h-Achlaise, providing evidence that the syenites post-date thrusting.
|Excavations, rapidly becoming weathered and overgrown, accessed from forest track that is closed to vehicles.|
|NC 2530 1370||Ledmore Marble Quarry
Sheets of Allt a’Mhuillin pseudoleucite-syenite (‘borolanite’) cut Ghrudaidh Formation limestones in Ledmore Marble Quarry. Spectacular metasomatic effects are seen in the limestones close to these sheets. Quartzites of the Ledbeg Thrust sheet can also be seen in the quarry.
|For access to quarry contact Ledmore Marble Ltd|
|NC 2870 0970||Aultivullin Quarry
Good exposures of Allt a’Mhuillin pseudoleucite-syenite (‘borolanite’) with strongly-flattened pseudoleucites in abandoned quarry at Aultivullin. Undeformed pegmatites cut the earlier syenites high in the quarry faces.
|Easily accessible from A837. Quarry has steep and in places rather loose faces.|
|NC 2520 1195||Allt a’Bhrisdidh
Pink-weathering Altnacealgach alkali feldspar-syenite, of the late suite of the Loch Borralan Pluton, has a steep intrusive contact with brown-weathering Ledmore melanite-pyroxene nepheline-syenite of the early suite. Lower part of the Allt a’Bhrisdidh above Loch Borralan.
|Easily accessible from A837.|
|NC 2469 1039||'Loch' Urigill
Outcrop of carbonatite on shore of Loch Urigill
|Accessed from forest track that is closed to vehicles|
- Loch Borralan
References (not complete)
Goodenough, K.M., Millar, I., Strachan, R.A., Krabbendam, M., Evans, J.A., 2011. Timing of regional deformation and development of the Moine Thrust Zone in the Scottish Caledonides: constraints from the U-Pb geochronology of alkaline intrusions. Journal of the Geological Society of London, 168: 99-114.