Minor intrusions, Shetland
|Mykura, W. 1976. British regional geology: Orkney and Shetland. Edinburgh, Her Majesty's Stationery Office.|
Swarms of sub-parallel roughly north—south trending acid, intermediate and basic dykes cut the plutonic complexes and adjoining metamorphic and sedimentary rocks of North Roe, Northmaven, Muckle Roe and the northern part of the Walls Peninsula (P915589). The acid dykes comprise feldspar-phyric porphyrites, quartz-feldspar-porphyries and a great variety of felsites which are generally either ﬂow-banded or spherulitic. In North Roe there is a suite of felsite dykes which contain the sodic minerals riebeckite and aegirine. These rocks have a striking blue or bluish green colour and are, in many instances, spherulitic. The thickness of individual acid dykes ranges from a few centimetres to about 18 m. A few have lenticular outcrops and some of these are locally up to 55 m wide. The basic dykes are principally basalts and dolerites, but many of them contain hornblende as the dominant dark mineral instead of pyroxene. A great deal of the hornblende is secondary and may have formed during a late hydrothermal period of uralitisation. Some of the basic dykes contain appreciable quantities of quartz and these have been classed as quartz-dolerites. There are also a small number of thick dykes of coarse pyroxene-porphyrite. Some of these have a highly irregular outcrop. Dykes of intermediate composition are relatively rare; they include keratophyres, spessartites and microdiorites.
Both the basic and acid dykes generally form parallel swarms with trends ranging from north-north-west to north-north-east. A few of the dykes have curving courses within that sector. Exceptions to this general north-south trend are found on Vementry Island, where acid dykes tend to radiate from the Vementry Granite and in the northern part of the Walls Peninsula where some acid and basic intrusive bodies trend parallel to the strike of the Old Red Sandstone sediments. In the ground extending from North Roe to Muckle Roe the acid dykes are restricted to a north—south belt which coincides in width with that of the outcrops of the granites and granophyres. The distribution of the riebeckite-bearing felsites is even more restricted to a median north—south trending zone in and adjoining the Ronas Hill Granite. The basic dykes, on the other hand, are fairly evenly distributed over a rather wider area. On Muckle Roe acid and basic dykes form alternate swarms. Composite acid-basic dykes are relatively rare, but a number of acid dykes have keratophyric margins.
Most acid and basic dykes are younger than the plutonic masses which they cut, but not all dykes are younger than the latest granitic members of the complex. There is evidence for the existence of some basic dykes which are earlier than the adjoining granite. In North Roe the evidence of chilling of adjoining dykes and the intersection of dykes has shown that in most cases the basic dykes, spessartites and microdiorites are older than the quartz-feldspar-porphyries and these are older than the felsites. The blue riebeckite-felsites are the youngest of all. Further south the evidence for the age relationships of the dykes is less clear, and in the Walls Peninsula the small number of dyke intersections show basic dykes cutting acid ones. It is believed that in Northmaven and Muckle Roe the emplacement of acid and basic dyke-swarms proceeded in a series of alternating pulses. The close compositional and areal relationships between the late Caledonian plutonic complexes and the dyke swarms suggests that the dykes of Western Shetland are also of late-Caledonian and possibly Upper Devonian age.
In contrast to the northern plutonic complexes, the Sandsting Complex (P915574) has few minor intrusions either cutting it or associated with it. There are a number of small dykes of melamicrodiorite, some of which have the aspect of uralitised dolerites. Felsite dykes are rare within the complex and in the sandstones adjoining the granite, but on the shores of Bixter Voe (HU 33 51) and Gruting Voe (HU 27 48) there are dyke-swarms of granophyre and graphic microgranite, which appear to have been derived directly from the granitic magma.
Two suites of minor intrusions are present in the metamorphic rocks of east Mainland but, with the exception of one trachytic dyke, there are no igneous intrusions in the eastern Old Red Sandstone. The earlier suite consists of dykes which are metamorphosed and are mainly microdiorites, lamprophyres, and more rarely felsic porphyrites. These dykes possess a schistosity which is parallel to their margins, but does not continue into the country rock. It is thought that the dykes were deformed immediately after their intrusion and that they acted as movement zones in which differential slip gave rise to the schistosity and probably accelerated the process of crystallisation.
The second suite of minor intrusions comprises unmetamorphosed dykes and sills. They have been loosely grouped as lamprophyres, mainly spessartites and while most are not more than a few metres thick, some can be followed along their length for hundreds of metres. Though these dykes are generally unaffected by metamorphism they are sheared and even schistose in areas where they are involved in shear zones, such as those separating the individual tectonic blocks of Unst and Fetlar (see Metamorphic rocks of Shetland: Yell, Unst and Fetlar). ‘Lamprophyre’ dykes are particularly common in the Graven Complex of northern Delting (P915573) where they appear to have been intruded before the emplacement of the ‘Inclusion Granite’.
Sheets of quartz-porphyry cut the metamorphic country rock in the Channerwick district of south Mainland (P915573), where outcrops occur both north and south of the Channerwick Granite and the sheets are clearly associated with the plutonic mass.
The metamorphic rocks exposed along the east coast of Foula are cut by a network of sills and dykes of pink porphyritic microgranite. The sills are thickest and most abundant near the northern end of the outcrop where one thick sill forms the 650 m-long stretch of foreshore at Ruscar Head (P915577). The groundmass of the microgranite is slightly sheared and granulitised. None of the granite intrusions cut the Old Red Sandstone of Foula, suggesting that the microgranite was emplaced prior to its deposition.
The west and south coasts of Fair Isle are cut by a number of dykes of partly uralitised dolerite and basalt. Both porphyritic and ophitic dolerites are present. At three localities the basic dykes are closely associated with dykes of microgranite and felsite. Most dykes are emplaced along west-north-west trending crush belts (P915580). Sodic scapolite, together with calcite, and locally analcime, apatite and sphene form veins within and close to the basic dykes and the crush belts. Copper ores and pyrite are associated with some of these veins.
The Fair Isle dykes are similar to the late-Caledonian dykes of western Mainland, and the scapolite mineralisation appears to be connected with a plutonic complex which may lie a short distance offshore and which may be related to the plutonic complexes of west Shetland.