Late Caledonian dyke swarms, 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.|
Towards the end of accretionary deformation, the Ordovician and Silurian strata of southern Scotland were intruded by two coeval suites of calc-alkaline dykes: one microdiorite to microgranite in composition and the other comprising lamprophyres. The overall abundance of dykes increases southward and though both suites range across the full extent of the Southern Uplands, there are proportionately more microdiorite and microgranite dykes in the north, and more lamprophyre dykes in the south. Both suites of dykes appear to have been intruded over a considerable interval of time, from the late Silurian to the Early Devonian, and in structural terms they are pre-D2 to post-tectonic. The two suites are broadly coeval but it is likely that the earliest intrusions were mica-bearing lamprophyres, whereas the last dykes emplaced were probably microdiorites.
Most dykes postdate the D1 deformation (P104245), but in the south of the Southern Uplands a few lamprophyre dykes seem to have been folded by the youngest phase of the diachronous D1 episode. Rather more dykes, though still only a minority and including some from both suites, carry the S1 cleavage fabric, particularly at their margins. Some dykes, mostly lamprophyres, are demonstrably folded (or otherwise affected) by D2 structures (P008561). These early dykes were mostly intruded parallel to the bedding of the host strata and so have a general north-east to south-west orientation. Other dykes, that appear to be entirely post-folding, may also lie north-east to south-west, parallel to the host bedding, but have also been widely intruded into the north-west- and north-north-east-trending cross faults, where the foliated margins of some show their emplacement to have been synfaulting. In a few cases, for example within the Moniaive Shear Zone, north-east-trending dykes have been foliated by sinistral strike-slip shear, either as a D3 effect, or during subsequent ‘Acadian’ deformation. Overall, there is no preferential pattern of orientation in either dyke suite relative to the other.
Successful radiometric dating has been restricted to undeformed lamprophyre dykes, from which has been obtained a range of ages from 400 ± 9 Ma to 418 ± 10 Ma (P912335). This provides a minimum age for the end of penetrative deformation, but equally suggests that dyke intrusion was protracted and probably polyphase. The youngest dykes are likely to include the microdiorite intrusions into the younger of the granitic plutons of south-west Scotland (described below), which have themselves been dated at about 397 Ma. In contrast, some lamprophyre dykes are metamorphosed by the thermal aureoles surrounding the plutons.
In petrographical terms, each of the two dyke suites contains a range of rock types:
- (i) The mafic, lamprophyre suite is the more distinctive of the two, comprising mica-bearing kersantite and hornblende-bearing spessartite as well as coarse-grained hornblende-rich lamprophyric rock informally known as ‘appinite’.
- (ii) The microdiorite–microgranodiorite–microgranite suite, originally termed the ‘porphyrite–porphyry series’, is more variable and comprises a broad range of intermediate to felsic rock types.
The lamprophyre dykes consist of fine-grained, highly altered, feldspathic rock that is characterised by the presence of large hornblende or mica phenocrysts within a finer grained groundmass of plagioclase, amphibole and biotite with minor quartz and K-feldspar. Clinopyroxene is present in some rocks, with pseudomorphs after olivine being more common in mica-lamprophyres. The lamprophyres may contain felsic segregations and veinlets, as well as locally abundant quartz xenoliths. A dyke exposed near Garheugh (NX 269 501) contains xenoliths of variably foliated and metamorphosed diorite and/or tonalite which have been interpreted as representing either the basement to the Southern Uplands or intrusions emplaced at depth within the sedimentary cover. In general, however, xenoliths are uncommon. The calc-alkaline lamprophyric magmas originated deep within the mantle, with the kersantites and spessartites probably resulting from slight variations in melting conditions and differing degrees of modification in the upper mantle and crust. The lamprophyre dykes in the southern part of the Southern Uplands are mostly mica-bearing kersantites similar in composition to dykes of the same age found to the south in the English Lake District. This similarity has been taken as evidence that Avalonian mantle was underthrust beneath Laurentian Scotland during the final destruction of the Iapetus Ocean.
The microdiorite, microgranodiorite and microgranite dykes are locally common and are characterised by high plagioclase content and a low proportion of mafic minerals. They are generally highly altered, range from andesitic to rhyolitic in composition and comprise varying proportions of feldspar, amphibole, biotite and quartz phenocrysts set in a fine-grained quartzofeldspathic groundmass. Corroded clinopyroxene phenocrysts may also be present within the microdiorites. Coarser grained dioritic rocks form major dykes (up to 100 m wide) and small irregular shaped intrusions. This dyke suite, although calc-alkaline in character, is petrogenetically unrelated to the lamprophyre suite and was probably produced by melting within the lower crust or upper mantle. Only a few of the pervasively altered rhyolitic dykes may represent felsic differentiates of the lamprophyric magmas. In general, the felsic, late Caledonian minor intrusives are spatially associated with, and have chapter four: caledonian structure and magmatism similar chemical compositions to, the larger granitic plutons, and were probably derived from the same magma chambers that ultimately produced the plutonic rocks.
Associated with the dykes are a number of volcanic vents and breccia pipes. Most are poorly exposed bodies mainly composed of highly altered sandstone-siltstone agglomerate with diffuse patches of tuffisite. One of the largest examples crops out on the west side of Kirkcudbright Bay at Shoulder O’Craig (NX 663 492) where a vent agglomerate, mostly of sandstone debris, is associated with a number of basaltic minor intrusions and kersantite sheets. Elsewhere, a particular concentration of vents, and of minor intrusions in general, forms the Black stockarton Moor subvolcanic complex to the north-east of Kirkcudbright (P912339). The complex comprises a mass of intersecting lamprophyre and microdiorite dykes, granodiorite sheets, small granodiorite stocks and breccia pipes intruded into the Carghidown Formation. The development of the complex involved several phases of intrusion (See table
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) and overlapped with the emplacement of the nearby Bengairn Complex and Criffel–Dalbeattie Pluton (see below). A sygmoidal intrusion pattern and locally developed foliations suggest that intrusion of some dykes was accompanied by movement on the Caledonian fault network, though in this case reactivation of the faults may have been caused by the intrusion process. Compositionally, the subvolcanic complex and the Criffel–Dalbeattie Pluton show similarities that suggest they both arose from the same evolving magma(s).
- 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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