Minor intrusions, Orkney
|Mykura, W. 1976. British regional geology: Orkney and Shetland. Edinburgh, Her Majesty's Stationery Office.|
Over two hundred dykes and a small number of sills have been recorded in the Orkney Islands. Most of the dykes are of dark lamprophyric rocks which are classed as camptonites and monchiquites, but there are also a small number of highly feldspathic dykes and sills which have been termed bostonites. Dykes of these three types do not occur in Shetland and only a few have been recorded in northern Caithness. Even in the Orkney Islands they have a restricted distribution and are mainly conﬁned to the south and west of the archipelago (P915590). The camptonites are most common in the West Mainland and in Rousay; the monchiquites in the south islands. The trend of the camptonites is predominantly east-north-easterly, that of the monchiquites is north or north-north-east. It seems most likely that the dykes came from a focus that lies somewhere to the south or south-west of the island group.
The ﬁeld relationships of the dykes indicate that they are younger than the faults which cut the Old Red Sandstone. Recent radiometric potassium-argon dates by N. J. Snelling (personal communication) have suggested that their most likely age is around 283:9 million years, which is late-Carboniferous. This accords with the ages of similar dyke suites in the West Highlands and the Midland Valley of Scotland. There is no evidence that any of the dykes ever gave rise to surface lava ﬂows. On the other hand, some of the small volcanic vents or cryptovents in the south-east of the island group are either intimately connected with monchiquite dykes or contain clasts or lapilli of basic igneous, possibly monchiquitic material. This suggests that the formation of these vents was connected with the intrusion of monchiquite magma.
Two intrusions of alkaline olivine-dolerite in Deerness (East Mainland) and on the Black Holm of Copinsay, which are intimately connected with the volcanic rocks in the Eday Flags, are described on Eday volcanic rocks.
The camptonites are the most numerous of all the Orkney dykes and they consist of phenocrysts of olivine, augite and hornblende set in a groundmass composed of augite, hornblende, feldspar and iron ores. Porphyritic olivine is most common in the ﬁner-grained dykes, but it is hardly ever found in an unaltered state. The augite phenocrysts generally form near-euhedral crystals which have a deep green core and a pale mauve or lilac outer zone when seen in thin section. The hornblende, when present, is generally of a deep brown colour in thin section, and the larger hornblende crystals may have rounded edges.
In the groundmass near-euhedral augites and dark brown hornblendes are abundant. The feldspars, which are generally zoned, with labradorite in the centre and oligoclase or alkali-feldspar on the margins, ﬁll the spaces between these earlier minerals. A characteristic feature of the groundmass of the camptonites is the presence of numerous light-coloured rounded ocelli which may represent steam cavities partly ﬁlled by alkali-feldspar during a later crystallisation. Many of these ocelli have a central residual cavity which is ﬁlled with carbonates or analcime. Most of the camptonite dykes are in a highly decomposed condition and the freshest material can be obtained from pebbles in boulder clay or shore gravel.
The most remarkable camptonite dykes of Orkney are those exposed at Hoxa in South Ronaldsay. These are crowded with phenocrysts of augite and hornblende which are up to 12.5 mm in diameter.
The monchiquites are rocks which consist mainly of olivine and augite. The former forms microphenocrysts which are up to half a millimetre in size and which are in many cases altered to serpentine, chlorite, and carbonates. The augite makes up about two-thirds of the rock and forms small near-euhedral crystals. Brown hornblende is a minor constituent of some monchiquites, and small plates of biotite are present in most dykes. In the freshest rocks the groundmass is a brown glass, perhaps analcimic in composition, with curving microlites and small grains of iron ore. The original groundmass is, however, rarely preserved and in most dykes it is replaced by an aggregate of carbonates with serpentine, chlorite, zeolites and a good deal of analcime. Ocelli are not as common as in the camptonites. When present they contain some crystals of feldspar together with hornblende, biotite and nepheline.
Most monchiquites are less coarsely porphyritic than the camptonites, but there are some monchiquite dykes with conspicuous phenocrysts. A dyke on Corn Holm, Copinsay, for instance, has crystals of augite, hornblende and olivine which are more than 2.5 cm in diameter.
In about twenty of the monchiquite dykes fresh nepheline has been found to occur as a late crystallisation product in the groundmass or in the outer parts of certain ocelli. These nepheline-monchiquite dykes have no separate area of distribution but occur among and along with the other monchiquites. Though some biotite is present in most monchiquite dykes, there are a few dykes with ophitic ﬂakes of golden-yellow biotite which are up to 1 mm across. These biotite ﬂakes are commonly associated with olivine. Other minerals present in the biotite-monchiquites are granular carbonates, augite, perovskite, apatite, iron ores and spinel. The biotite-monchiquites have a close aﬂinity with the alnöites.
Bostonite intrusions are much rarer than those of the two rock types described above. The two ﬁnest examples are a broad dyke at the Haven, Swona and a sill which cuts the Stromness Flags just beneath the Hoy Volcanic Rocks at the Bay of the Stairs in north-west Hoy (P915587, P000596). The Swona bostonite consists of almost pure feldspar, principally orthoclase, and is one of the most potash-rich igneous rocks in Britain. The sill on Hoy is similar in composition but contains, in addition to feldspar, some chlorite and carbonate.
Some dykes in Stenness and at the Loch of Skaill, West Mainland, are transitional in composition between bostonites and camptonites.
A small number of dykes of olivine-basalt have been recorded in Orkney. They crop out at Firth, near Finstown, and at Loch of Harray and differ from the camptonites in that they contain porphyritic plagioclase feldspars and no brown hornblende. They do not bear a close resemblance to the Tertiary basalt dykes of Western Scotland and are probably of the same age as the Orkney camptonites and monchiquites.