June 2017 Summary

After several years of field work in the Cheviot Hills, it is appropriate to offer some conclusions from our studies. We lack access to, and expertise in many of the contemporary chemical and isotope procedures, so we accept that our conclusions must remain tentative. We would welcome informed suggestions and corrections.

We have noticed the following striking and somewhat unusual aspects to the Cheviot pluton.

  • There was a huge outpouring of lavas, mainly of andesite. The extent of this is now believed to reach up to 600 sq km and, before erosion, at least 2000m deep. Observed layering of tephra deposits in relation to lava confirms the traditional understanding that the outpouring of lava was preceded by an explosive phase. The depth of ignimbrite in the Knock and Brough Hill areas, even after 400 million years of erosion, suggests that the Cheviot volcanic system may have achieved a pretty high VEI rating.
  • There is a very marked absence of volcanic necks or vent agglomerate. The brecciated rocks which we have found in the upper Harthope and Hawsen Burn valleys cannot be safely identified as vent material. The abundant veining of quartz, tourmaline and haematite could equally point to a hydrothermal origin.
  • There is an absence of evidence for compression except at exposures close to the North side of the summit plateau of the Cheviot, and at the junction of two types of granitic rock on Shiel Cleugh Edge above High Bleakhope. There is, however, widespread evidence of thermal and metasomatic action (but not pressure) on the lavas adjacent to the pluton.
  • There are some distinct boundaries within the pluton. These are especially noticeable on Dunmoor Hill. Elsewhere, exposures of bedrock are too scanty to trace boundaries.
  • We have only recorded one chilled margin within the pluton, at Shiel Cleugh edge. There is no obvious chilling at the junction of granitic rock types (Marginal and Central Belt types) on Dunmoor Hill. The Quartz-monzonite of Dunmoor Hill close to the junction with the lavas does show an increasing fineness of grain consistent with chilling of the pluton against the earlier lavas.
  • Recent research suggests that the pluton is at least 4km deep and 20km wide at that depth.
  • Most of the plutonic rock is medium-grained. The Upper Cheviot evolved granite borders on the finest and is therefore almost a felsite. Much of the granitic rock has larger phenocrysts mainly of andesine feldspar, and sometimes of augite and biotite, in a fine-grained groundmass of alkali feldspar and quartz.
  • Current dating by the BGS suggests a slightly earlier date of 395-400Ma which would make the Cheviot more or less contemporary with the Skiddaw, Shap and Criffel plutons.
  • Other contemporary plutons of the early Devonian period do not have an associated volcanic phase. The Cheviot is unusual in exhibiting both plutonic intrusion and volcanic extrusion.
  • There is widespread pyroxene in the ‘Marginal’ quartz-monzonite. This is especially prevalent in the Northern Marginal area.
  • Evidence for cauldron subsidence which is an obvious characteristic of the almost contemporary Ben Nevis volcano, is not apparent in Cheviot. Surviving outcrops of lava above the pluton show bedding parallel to the slope of the pluton. This does not support evidence for either stoping or cauldron subsidence.

In light of these observations, we would offer the following suggestions for interpretation of the Cheviot system.

The extent of the andesite lava flows is rather remarkable. Andesite is a relatively viscous lava which normally does not flow far from source; hence the steep-sidedness of andesite composite volcanoes. To achieve the extent of the Cheviot lava flows, multiple vents scattered over a wide area, must have been present. However, the lack of any evidence for vents is problematic. Even allowing for late Devonian erosion, it is surprising that any such evidence is missing. Looking across the Southern Uplands of Scotland from the upper slopes of Cheviot, the numerous volcanic necks of Southern Scotland’s Carboniferous volcanics are clearly visible. Yet these necks survive from only about 50 million years later. Perhaps the answer is that there were no necks in the Cheviot system, and that the massive volumes of andesite were expelled from a series of rifts valleys. Could it possibly be that the network of faults which mark the main river valleys in the Cheviot Hills, are the remnants of these rifts?

The impressive volume of the Cheviot pluton rules out the traditional view that it is a laccolith. There is a real possibility that it is the remnants of the magma chamber or one of the chambers which fuelled the Cheviot volcanic system. Presumably the less silica-rich fractions of the chamber being more fluid, were expelled first as andesite, leaving the more silica rich melt behind to crystallise out into varying forms of granitic rock. Whether this theory is correct or not will depend on evidence gathered from modern chemical and isotope analysis which is beyond our own resources.

It is also clear from previous research and from the BGS past surveys that the pluton is also an intrusive body. The sharp contact between the pluton and the lavas, and the obvious chilling of the quartz-monzonite at Cunyon Crags on Dunmoor Hill, suggest that the lavas had already been consolidated before the intrusion of the granitic rock. The exception is in the North near Goldscleugh where tongues of granite penetrate the andesite, suggesting a perhaps more fluid environment there. The lava shows evidence of thermal and chemical alteration but not pressure from the granites. This suggests that the intrusion took place in circumstances where the lavas were relatively easily pushed aside. The relatively fine-grained structure of the plutonic rocks suggests that consolidation took place close to the surface. There is, however, evidence that, close to the summit plateau of Cheviot, there was some compression of granite and andesite. This evidence taken with the composition of the Upper Cheviot rock as the only unequivocally true granite in terms of Kspar and SiO2 content, together with its relatively fine grain, suggests that it may have formed an incipient but failed rhyolitic lava dome which nevertheless failed to break through the andesite crust. It should be remembered that we can only record data from what is apparently the topmost layer of this large pluton. Different but unobserved conditions may well prevail at greater depth.

The overwhelming presence of glacial drift and blanket bog obscures most of the Cheviot pluton’s bedrock. However, in the few places where contacts can be seen between different types of plutonic rock, they appear clearly defined. Just above Linhope Spout there is a sharp contact between the darker Marginal rock (Quartz-monzonite), and the much pinker Central Belt rock (borderline monzonite/granite). The boundary between the two types can be traced across much of Dunmoor Hill although it is somewhat more fuzzy there with tongues of either type penetrating the other for several metres suggesting that neither were fully consolidated when contact took place. On Shiel Cleugh Edge, there is clear evidence that one phase of Central Belt rock has been chilled against another. All this evidence indicates that multiple intrusions of granitic magma into the top reaches of the pluton must have occurred.

Our final concern is with the abundant presence of pyroxene, both clino- and ortho-, in some parts of the pluton, especially in the Marginal rocks. We have not been able to find any satisfactory explanation how pyroxenes can form in a subductional melt without the injection of mantle material. Pyroxenes form at high temperatures, whereas subduction hydrous melts achieve lower temperatures and are likely to produce the lower temperature hydrous minerals such as amphibole and biotite. Biotite is a universal constituent of the Cheviot pluton with primary amphibole (rather than as an alteration product) more scarce. This is as would be expected in a calc-alkaline subduction hydrous melt. So where did the high temperature pyroxenes come from? The only reasonable explanation that we can find is that much hotter and more basic mantle melt became available in the Cheviot pluton possibly as a result of contemporary transtensional tectonic activity. This could also offer an explanation why the Cheviot pluton out of all its contemporary colleagues in Northern England and Southern Scotland, produced an extensive volcanic outpouring.