Regional setting of the Skye Central Complex

From: Bell, B.R. and Harris, J.W. An excursion guide to the geology of the Isle of Skye : Geological Society of Glasgow, 1986. © 1986 B.R. Bell & J.W. Harris. All rights reserved. October 2022 note: A new edition of the guide is now available.

Chapter 13 The regional setting of the Skye Centre

Figure 13 Geological sketch-map showing the position of intrusive complexes and dyke swarms within the British Tertiary Volcanic Province

The regional setting of the Skye Centre

The volcanic centre on Skye is one of a group of Lower Tertiary intrusive complexes, which includes: Rhum, Ardnamurchan, Mull, Arran, St. Kilda, Slieve Gullion, Mourne, and Carlingford (Figure 13). They are all over 500km from the oceanic crust of the North Atlantic. The lava field associated with the Skye Centre covers an area of approximately 1500km² and lies within a shallow, faulted sedimentary basin, elongated NW-SE. This basin was probably initiated during Triassic times and suffered uplift and erosion during the Palaeocene, prior to the volcanic activity. The feeders to the lava pile are believed to have been dykes and sills, without the formation of true subvolcanic magma chambers (Thompson et al. 1972).

According to Thompson et al. (1980a), the parental mantle rock which gave rise to the Lower Tertiary volcanic and intrusive rocks preserved on Skye was a spinel lherzolite, depleted in various stages during magma extraction (see Section (12B) of Chapter 12). Prior to these Lower Tertiary melting events, a small melt fraction was removed from the same mantle volume, depleting it of highly incompatible elements. Morrison et al. (1980) suggest that this event was marked by the intrusion of highly alkaline lamprophyre dykes throughout NW Scotland in Permian times. This link between the Lower Tertiary volcanic activity and Permian lamprophyre dykes infers that NW Scotland was subjected to a tensional tectonic regime since at least Upper Palaeozoic times, causing intermittent magmatism. Significantly, these dykes are not found in the immediate vicinity of the individual centres of the province, implying that some form of discontinuity has moved the crust down to the west.

The Lower Tertiary central intrusive activity commenced with the development of the Cuillin Complex. Geophysical evidence (Bott and Tuson 1973) indicates that the deep structure of the Skye Centre consists of a large, vertical cylinder of basic or ultrabasic material extending downwards to a depth of at least 14km. The associated positive gravity anomaly is of the order of 50 milligals. Below depths of approximately 14km the density contrasts between basic and ultrabasic igneous rocks and basement Lewisian Gneiss are not easily determined and it is, therefore, possible that this cylinder continues to much greater depths.

Walker (1975) suggests that the surface expression of such a cylinder is the layered basic and ultrabasic units of the Cuillin Complex, itself.

The formation of the Cuillin Complex was followed by three subvolcanic centres which, at the present level of erosion, are dominated by acid rocks and pyroclastic material. These are: the Srath na Creitheach, Western Red Hills, and Eastern Red Hills Centres. Aeromagnetic data (Brown and Mussett 1976) indicate that the material underlying these centres has a net positive remnant magnetisation, in contrast to the net negative remnant magnetisation of the Cuillin Complex. Thus, the basic/ultrabasic material below the Skye Centre must have developed over at least two periods of magma uprise, between which a change in the Earth's magnetic polarity took place. Brown and Mussett (1976) and Dickin (1981) suggest that the central intrusive activity on Skye occurred over a protracted period, possibly 5 m.y., or more.

Most of the products of explosive volcanism associated with the Skye Centre, such as the acid tuffs and ignimbrites of the Kilchrist area (see Section (8D) of Chapter 8), are preserved only locally, within down-faulted blocks. However, other significant deposits of acid tuffs have been recognised in Palaeocene sediments within the North Sea Basin and the Faeroe Trough (Knox 1979; Thiede et al. 1980; Sigurdsson and Loebner 1981), suggesting that this phase of volcanic activity was a widespread event.

The average age of the intrusive units on Skye (59–60Ma) and some of the oldest ocean floor basalts from the North Atlantic (the so-called Anomaly 24 basalts) is approximately the same. According to Brooks (1980), during the period 65 to 60Ma, major rifting, uplift and volcanism took place between Greenland and NW Europe (the Rockall Plateau). These events gave rise to the lava field of the Kangerdlugssuaq region, East Greenland and large linear dyke swarms, as exemplified by the major coastal swarm, also of East Greenland (Larsen 1978; Nielsen 1978). Brooks (1980) concludes that the volcanism which took place marginal to this ocean floor spreading was episodic, with major events at 60, 55, 50, 35 and 28Ma. The volcanism on Skye correlates with the 60 and 55Ma events.

Information about the regional setting of the British Tertiary Volcanic Province (BTVP) may also be obtained from geometrical analyses of the NW-SE -trending dyke swarms associated with the Skye and other related Lower Tertiary intrusive centres (see above). The Skye dyke swarm developed during the formation of the lava pile and the intrusive centres (see Section (9B) of Chapter 9). Furthermore, the subvolcanic intrusions are located at the point of maximum crustal dilation within the swarm. The orientation of the dyke swarms suggests that the regional stress field during the volcanic activity was such that the maximum compressive strecs was horizontal and parallel to the swarm and the minimum stress acted perpendicular to the trend of the swarm (i.e. NE-SW).

Another well-defined feature of the BTVP is the N-S alignment of the individual volcanic centres, from Lundy in the south, to St. Kilda in the north, a distance of over 1000km. The width of this Lower Tertiary zone of volcanic activity is of the order of 100km. This suggests that at 60Ma the lithosphere in the vicinity of the BTVP, together with the subjacent mantle, were subjected to anomalous stresses and a higher-than-normal heat flux. These conditions were most likely accompanied by major ocean floor spreading in the North Atlantic. During this time, mantle melts took advantage of lines of weakness in the lithosphere to form large, vertical cylinders of basic/ultrabasic material, above which developed individual volcanic centres.

J.D. Bell (1976) suggests that the position of the Skye Centre was controlled by the intersection of the Camasunary Fault (see Section (10D) of Chapter 10) with the line of weakness utilised by the Skye regional dyke swarm. Other similar correlations have been advanced by Gass and Thorpe (1976), including: the Rhum Centre also on the Camasunary Fault; the Ardnamurchan Centre on the Strathconon Fault; the Mull Centre on the Great Glen Fault; and, the Arran Centre on the Highland Boundary Fault (Figure 13). Where these pre-Tertiary faults intersected the approximately equidistant regional dyke swarms, major upwelling of magma occurred.

The volcanic activity which ensued was an attempt, albeit a failed attempt, at rifting and, presumably, ocean floor spreading. Separation of the continental crust finally took place some 500–1000km further to the west of the Scottish mainland. The volcanic centres of the BTVP, therefore, moved laterally away from the area of active mantle upwelling and volcanism ceased.

Appendix 3: Glossary of place names and grid references

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