Late Caledonian plutonic rocks, Southern Uplands
|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.|
The major granitic plutons in south-west Scotland — Loch Doon, Cairnsmore of Fleet, Criffel–Dalbeattie and Bengairn (P912339) — are less dense than their Ordovician–Silurian host strata and so cause pronounced gravity lows (CF, LD and CD respectively in P912340). It is possible to model the subsurface forms of the plutons from their gravity anomalies and so establish that they are steep sided and extend down to depths of 8–12 km (P912341). Farther to the north-east, a gravity low in the Tweeddale area (TW in P912340) has been ascribed to a large, concealed Caledonian granitic pluton.
The isotope ratios of the igneous rocks, in particular the isotopic ratios of lead and strontium, provide insight into the origin of the magmas and their source rocks. The granitic plutons in the north of the Southern Uplands (such as Loch Doon) show similarities to igneous rocks in the Midland Valley whilst those in the south (such as Criffel–Dalbeattie) have characteristics more similar to igneous rocks in the English Lake District. This implies that the southern plutons were generated in response to collisional melting of underthrust Avalonian lithosphere, whilst the northern plutons were sourced in Laurentian lithosphere. At the very least, the contrast requires different crustal properties beneath the northern and southern sectors of the Southern Uplands (P912318), though as a complicating factor there is no absolute requirement for any one pluton to have been derived from a single source. For example, the isotopic evidence shows that the parental magma of the Criffel–Dalbeattie pluton contained a significant mantle component, although its central part, and the whole of the Cairnsmore of Fleet pluton, has the characteristics of crustal-derived granites. All of the plutons were probably formed in response to partial melting of lower crustal rocks initiated by underplating and intrusion of basic, mantle-derived melts. They are all compositionally zoned and so are likely to have been formed by several successive intrusive pulses, all derived from a single parental magma which was evolving at depth.
Loch Doon Pluton
The Loch Doon Pluton (406 ± 2 Ma, 410 ± 1 Ma, U-Pb, zircon) is an hour-glass-shaped body, elongated north–south, which cuts across the Leadhills and Fardingmullach tract-bounding faults (P912325 and P912342) and intrudes the sedimentary rocks of the Kirkcolm, Portpatrick and Shinnel formations. Small satellite bodies occur to the east and south-west of the main mass, which the gravity model shows to be steep sided down to about 10 km, with a shallow subsurface plateau on the eastern side linking the main mass to at least one of the satellites. The steep sided nature of the pluton is also indicated by the relatively narrow thermal aureole of biotite hornfels. A steeply dipping igneous foliation is developed parallel to the margin of the pluton and the common xenoliths and autoliths are aligned parallel to the fabric. The presence of this planar fabric within the pluton, coupled with the marked deflection of the regional strike across its central ‘waist’, argues for forceful emplacement (P912342). However, there is less evidence of distortion of the regional strike at the northern and southern ends of the pluton suggesting that there the intrusion was, at least in part, permissive. The large number of xenoliths in those parts of the pluton suggests that stoping was an important process. Overall, intrusion of the Loch Doon Pluton may have been partially facilitated by a transtensional stress regime related to strike-slip movements along the tract-bounding faults.
The main mass of the pluton is compositionally zoned from diorite at the margin, through granodiorite and into granite at the centre (P912342). The boundaries between the various phases are gradational. The fine- to coarse-grained diorites and quartz-diorites are mainly composed of plagioclase and pyroxene (including hypersthene) with minor amounts of biotite, K-feldspar and quartz. Relict olivine and actinolitic hornblende after pyroxene are locally present. Passing through a transitional tonalite facies into the granodiorite, plagioclase becomes more calcic and pyroxene decreases, whilst quartz and K-feldspar increase towards the granitic centre of the pluton. The central granite is mainly composed of K-feldspar, plagioclase and quartz, with minor biotite, and locally contains large (up to 25 mm) K-feldspar phenocrysts. The main phases of the pluton are cut by a series of aplite, microgranite, porphyritic granite and pegmatite veins. Petrogenesis is likely to have been complex and polyphase, involving partial melting of basic, lower crustal rock, with variable contamination by assimilated sedimentary material.
Cairnsmore of Carsphairn Pluton
The roughly triangular Cairnsmore of Carsphairn Pluton (410 ± 4 Ma, Rb-Sr, mineral/ whole-rock) lies to the north-east of the Loch Doon Pluton, intruded into Kirkcolm Formation strata. The pluton has a well-developed thermal aureole and cross-cuts the strike of the host rocks, although this is locally deflected around the western and northern margins of the intrusion. Though relatively small, the pluton is zoned with a central granite phase enclosed within microgranite, granodiorite and a marginal quartz diorite with lenses of diorite. The boundaries between these zones are gradational, though chilled contacts have been recorded between the microgranite and granodiorite, and between the two granite phases. The diorites and granodiorites at the margin of the pluton contain xenoliths that show degrees of melting and assimilation. The pluton was emplaced as a series of magma pulses, ranging from dioritic to granitic in composition and derived from a single parental basaltic magma which fractionated at depth, though it is likely that the diorite melt fractionated in situ to quartz-diorite.
Cairnsmore of Fleet Pluton
The Cairnsmore of Fleet Pluton (397 ± 3 Ma, U-Pb, zircon) is the most evolved of the major granitic plutons and may also be the youngest. It is ovoid in form with a long axis extending east-north-east to west-south-west and is intruded into Gala Group host strata. The gravity model shows steeply dipping margins down to a depth of about 10 km. The pluton lies entirely within the Moniaive Shear Zone, but its intrusion postdates the main phase of deformation within the shear zone. As a result, it distorts the regional strike of the shear zone fabric and has produced a thermal metamorphic aureole of biotite-hornfels, up to several kilometres in width, which overprints the shear fabric. However, garnet and cordierite porphyroblasts, which developed during the earliest stages of thermal metamorphism, are deformed by the shear fabric. This suggests that initial emplacement of the pluton was facilitated by sinistral shear during the transtensional stress regime of the Early to Mid Devonian.
The pluton is compositionally zoned with a broad margin of coarse-grained, commonly microcline-phyric granite concentric to a core of fine- to medium-grained aphyric granite. Blocks of the coarse-grained, marginal granite are present within the aphyric granite, which is therefore likely to be a later intrusive phase. A weak foliation, defined by aligned feldspar and mica crystals, is present within both granite phases and occurs parallel to the margins of the pluton. The overall intensity and dip of this foliation increases towards the margins and are consistent with the fabric having developed during forceful emplacement. The local presence of screens of country rock within the southern part of the pluton suggest that it was, at least in part, emplaced as thick sheets. The absence of veining and only rare occurrence of country rock xenoliths, particularly in the roof zone, argues against significant stoping. Two pulses of intrusion are likely, the parental magmas having originated as partial melts of deep-seated basic rocks that began to fractionate during ascent, whilst at the same time becoming increasingly contaminated by assimilated sedimentary material.
As described above, in the thermal metamorphic aureole around the pluton, early garnet and cordierite porphyroblasts are contained in a broad zone of later biotite hornfels. In addition, skarn alteration occurs at several places within the aureole as calc-silicate lenses up to several metres in length and composed of epidote, amphibole and rare diopsidic pyroxene. These skarn minerals overgrow and replace the earlier-developed biotite.
The Criffel–Dalbeattie Pluton (397 ± 2 Ma, Rb-Sr, whole-rock) is the largest of the major granitic plutons within southern Scotland. It is a composite, concentrically zoned granitic body with an ovoid shape, elongated north-east to south-west and intruded into Hawick Group strata. The gravity model shows steeply dipping margins down to a depth of at least 9 km, but the pluton was probably emplaced at a high level in the crust (and was rapidly unroofed) as boulders of granodiorite are found in Upper Devonian to lower Carboniferous conglomerate exposed a few kilometres to the south.
The pluton has an incomplete outer shell of foliated granodiorite containing aligned xenoliths of diorite, which surrounds a core of unfoliated porphyritic granite (P912343). The foliation is defined by the preferred orientation of feldspar, hornblende, biotite and quartz crystals. The development of the foliation and preferred alignment of the xenoliths within the granodiorite shell probably occurred during the forceful emplacement of the later granite core, possibly as a result of oblique, north-east-directed diapirism. Thermal metamorphism associated with the emplacement of the pluton resulted in a 2 to 3 km wide zone of biotitehornfels and hornblende-diopside-hornfels.
The compositional boundary between the foliated shell and inner core is gradational and characterised by steep geochemical gradients. The outer shell comprises unzoned metaluminous hornblende-biotite-granodiorite, locally pyroxene-bearing at the outer margin. This passes progressively inwards through a zone of locally porphyritic biotite-granite, which contains small enclaves of hornblende-biotite-granodiorite, and then into muscovite-biotitegranite towards the core of the pluton. The outer granodiorite was probably derived from a mantle or lower crustal source, whereas the more evolved granitic core is more likely to have been generated by partial melting of middle and upper crustal, immature metasedimentary rocks.
The Bengairn Complex
The Bengairn Complex is a composite pluton lying between the Black Stockarton Moor subvolcanic complex, to the north-west, and the Criffle–Dalbeattie Pluton, to the north-east (P912343). Its emplacement into the sedimentary rocks of the Hawick Group postdated the first phase of activity within the Black Stockarton Moor complex (P912385) but the Bengairn Complex was intruded by a suite of north-west-trending lamprophyre and microdiorite dykes during a later phase of subvolcanic activity. Although no intrusive contacts between the Bengairn Complex and the Criffel–Dalbeattie Pluton have been recognised, it is likely that the Bengairn Complex was emplaced prior to, and is intruded by, the Criffel–Dalbeattie Pluton.
The Bengairn Complex is a zoned pluton with an incomplete outer rim of quartz-diorite intruded by a granodiorite core. The medium- to coarse-grained quartz diorite consists of plagioclase, hornblende, biotite, quartz and K-feldspar. It possesses a weakly developed foliation and contains xenoliths of partially assimilated, hornfelsed country rock and basic igneous material. The granodiorite is a fine- to medium-grained rock composed of zoned plagioclase, quartz, K-feldspar, biotite and hornblende. Xenoliths are uncommon within the granodiorite, but where present are aligned parallel to a more widely developed igneous foliation.
Minor intrusive bodies
A number of small intrusive masses cut the Ordovician and Silurian strata in the far west of the Southern Uplands (P912339). The Culvennan diorite (to the west of Newton Stewart) has an outcrop area of about 1.5 km2 and is mainly composed of pyroxene-biotite-diorite with some tonalite and granite. The surrounding thermal aureole encloses several satellite pods of diorite, which suggests that its subsurface form may be significantly larger than the outcrop. The Glenluce diorite (exposed at the head of Luce Bay, on the eastern side) is mainly composed of a coarse-grained plagioclase-phyric pyroxene-diorite with minor plagioclase-pyroxenebiotite-phyric andesite, quartz diorite and pyroxene-rich melanocratic diorite phases. On the opposite (western) side of Luce Bay, an approximately 2 km2 area of hornfelsed sandstone at Sandhead coincides with a magnetic anomaly indicative of a probably dioritic body rising to within 50 m of the ground surface. This feature — the sandhead anomaly — straddles the Orlock Bridge Fault, which may well have influenced the site of intrusion. The small Cairngarroch Complex (on the west coast of the Rhins of Galloway, south of Portpatrick) lies immediately to the north of the Orlock Bridge Fault, which forms the southern margin of this polyphase, dioritic to granodioritic body. The complex must postdate movement on the fault since the southern part of its metamorphic aureole, which extends across the fault, is not displaced. Nevertheless, it is probable that the Orlock Bridge Fault had a major influence on the site and shape of the intrusion. The older part of the Cairngarroch Complex is composed of a plagioclase-phyric biotite-hornblende-microtonalite which was intruded by a later phase of plagioclase-phyric biotite-hornblende-granodiorite. The southernmost part of the intrusion contains numerous screens of sandstone as well as brecciated, quartz-porphyritic granite, the latter occurring as xenoliths and enclaves within the quartz-microdiorite. Pervasive hydrothermal alteration and widespread Fe-As-Cu-Mo mineralisation are associated with the Cairngarroch Complex. Farther south, the roughly rectangular Portencorkrie Pluton crops out over about 5 km2 on the Rhins of Galloway between Laggantalluch Head and Crammag Head. It is a compound dioritic to granodioritic stock with an outer zone of locally foliated pyroxene-mica-diorite on its northern side and hornblende-mica-diorite to the south. The centre of the intrusion is composed of medium-grained granodiorite, locally containing phenocrysts of K-feldspar. Regular, continuous geochemical trends between the diorites and granodiorites have been used to suggest that the Portencorkrie Complex developed in response to the multiple intrusion of magma that was fractionated at depth.
Two small but significant intrusive bodies crop out on the east side of Wigtown Bay (P912339). The Carsluith intrusion is a near-vertical body (up to 150 m wide at outcrop) lying parallel to bedding in the steeply dipping country rocks. It is a composite intrusion with an outer carapace of plagioclase-phyric microdiorite enclosing a microgranodiorite core. The presence of screens of microdiorite within the microgranodiorite core, and xenoliths of mudstone that are aligned parallel to bedding within the adjacent country rocks, suggest that the Carsluith intrusion was emplaced as a series of sheets. An irregular dolerite dyke that cuts both phases is most probably of Palaeogene age. The Kirkmabreck (or Creetown) intrusion is a relatively small, subvertical and sheet-like microgranodiorite body (up to 200 m wide at outcrop) emplaced along the Laurieston Fault. The intrusion has a complex emplacement history with an early phase, represented by a swarm of closely spaced microdiorite dykes, cut by the main microgranodiorite body, which contains xenoliths of the dyke microdiorite (P008440). The intrusion is enclosed by a well-developed thermal aureole (up to 100 m wide) of biotite-hornfels with subsequent skarn alteration (grossular + diopside + clinozoisite + actinolite), though these effects appear to have been generated by the early dyke swarm and predate the main intrusion.
A number of small intrusive bodies crop out in the central, northern part of the Southern Uplands (P912339). None are well exposed and the Ballancleuch granodiorite is known only from a boulder field, the shape of which suggests a dyke-like body adjacent and parallel to the Leadhills Fault; though no more than 300 m wide it extends for about 4 km along strike. The most substantial of these bodies are those at Spango and The Knipe. The spango pluton has an outcrop of about 10 km2. It is a hornblende-biotite-granodiorite that cuts across the tract-bounding Carcow Fault but is itself cut by the Southern Upland Fault along its north-western flank. The granodiorite becomes more melanocratic towards its margin and contains small xenoliths of comagmatic microdioritic rocks as well as rarer xenoliths of hornfelsed sedimentary rocks. The knipe intrusion is a composite body with a core of hornblende-biotite-granodiorite or quartz-diorite passing outwards into pyroxene-bearing dioritic rocks. The main part of the intrusion has an outcrop area of about 1.5 km2, but a linear offshoot to the north forms a separate, narrow outcrop about 2 km long. To the south of The Knipe intrusion, several small, dioritic bodies lie in a linear zone between it and the Cairnsmore of Carsphairn Pluton; examples include the intrusion at Cannock Hill. In the same generalised zone, areas of hornfelsed Ordovician sandstone indicate the presence of additional small intrusions, concealed but close to surface.
In the north-eastern part of the Southern Uplands, only a few granitic/granodioritic intrusions crop out, the largest of which are those at Priestlaw, Cockburn Law and Broad Law (P912339). The Priestlaw intrusion in the Lammermuir Hills is an irregularly shaped pluton with an outcrop area of about 3.5 km2, which is mainly composed of a variably porphyritic biotite- or hornblende-biotite-granodiorite. The granodiorite is enclosed within a marginal phase of slightly porphyritic quartz-augite-biotite-diorite or microdiorite; olivinenorite forms the north-west corner of the intrusion. To the south-east of the Priestlaw intrusion, the Cockburn Law intrusion (c. 1.5 km2) is zoned from an outer shell of quartz-diorite into a granodiorite core. For its size, this relatively small intrusion is enclosed within a remarkably wide thermal aureole, locally extending up to 1 km from the intrusion. The small intrusive mass at Broad Law, in the Moorfoot Hills, is largely made up of biotite-granodiorite with phenocrysts of plagioclase.
- Obduction of the Ballantrae Complex, Southern Uplands
- Deformation of the Girvan succession, Southern Uplands
- Southern Uplands accretionary complex
- Late Caledonian dyke swarms, Southern Uplands
- Late Caledonian plutonic rocks, Southern Uplands
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