Types of Cheviot plutonic rock

The classification of Cheviot igneous rocks

The variety of different ‘granites’ in the Cheviot pluton has been noted in previous studies since at least the early 20th century. Some classification is necessary for mapping. This page shows our own attempts at classification but it should be regarded as a work in progress as our field observations have already altered, and may quite likely alter again how we divide up the varieties of rock as our walking mileage increases.

In 1942, Jhingran distinguished three varieties of granite in the Cheviot Hills (Marginal, Standrop and Granophyric). In 1982, Al-Hafdh further subdivided these into six varieties (Marginal, Dunmoor, Standrop, Linhope, Hedgehope and Woolhope). Marginal corresponded to Jhingran’s Marginal; Dunmoor, Hedgehope and Woolhope corresponded to Jhingran’s Granophyric; and Standrop and Linhope corresponded to Jhingran’s Standrop.

Al-Hafdh did extensive sampling and analysis by both thin section and chemistry. His Stanhope and Linhope varieties are very similar in both appearance and content, as are his Hedgehope and Dunmoor varieties to each other. He distinguished them as separate intrusion cycles. His analysis did establish clearly that only the ‘Woolhope’ is an unequivocally true granite in its chemical and mineralogical content. He classified all the other varieties as granodiorites. He also noted how many of the rocks were extensively altered, sometimes beyond recognition, by hydrothermal alteration.

QPA diagram for the classification of granitic rocks

Quartz-Plagioclase-Alkali Feldspar (QPA) tertiary diagram for the classification of plutonic rocks

Our own attempt at classification started from the basis of Al-Hafdh’s mapping. However, we found his distinction between Linhope and Standrop varieties impossible, and between the Dunmoor and Hedgehope varieties difficult to determine. So we initially settled for 5 main varieties: Marginal (which corresponded with Al-Hafdh and Jhingran’s Marginal); Evolved Granular (which corresponded with Al-Hafdh’s Woolhope); Coarse-grained Porphyritic; Medium-grained Porphyritic; and Fine-grained Porphyritic. Many of the rocks show a tendency towards micrographic or ‘granophyric’ texture when viewed in thin section.

The more we walked the Cheviot Hills during 2015-16, the more dissatisfied we became with this classification. It seemed inappropriate to use the term ‘porphyritic’ as a distinguishing factor when the other varieties also showed porphyritic characteristics. We also seemed to be mixing locational classification (‘marginal’) with appearance classification (‘porphyritic’) in the other varieties. Then there was the problem of the variation and the many exceptions within each. Finally, there was the difficulty that there is so little reliable exposure; boulders may have been transported by water or ice; and over 90% of the landscape is covered in glacial drift or blanket bog. This last problem explains why we have settled for a relief map with individual location marker pins without the usual generalised colour-coded areas. When there is so little exposure, anything more than placing marker pins on a map, amounts to, at this stage, unscientific speculation on our part.

As a result of our more recent fieldwork, we have settled for three main rock groups based on geographical location, Marginal, Central Belt, and High Cheviot. The first corresponds to Jhingran and Al-Hafdh’s ‘Marginal’ variety; the second to Al-Hafdh’s ‘Dunmoor’, ‘Standrop’, ‘Linhope’ and ‘Hedgehope’ varieties or our own ‘porphyritic’ ones; and the third to Al-Hafdh’s ‘Woolhope’ or our ‘evolved granular’. Even so, we have found it necessary to subdivide these groups into various types that recognise differences in mineral content, grain-size and texture.

Marginal

A sample from Cunyon Crag

This is the one group on which all writers until now have agreed. It has been seen as a discontinuous outer ring of the pluton emplaced before the inner rocks. However, our thin section analysis has revealed that the Marginal rock along the southern edge of the pluton extending from Dunmoor Hill to beyond High Bleakhope, is a quite different rock from the Marginal at the pluton’s northern boundary at Bellyside Crag and Hill. Both types seem to fall within the quartz-monzonite classification. Both types have only 10-20% quartz, and in excess of 15% mafic mineral. However, the southern type generally shows a higher proportion of plagioclase to alkali feldspar than the northern. Some of the samples from Cunyon Crag have so much plagioclase that they approach the quartz-monzodiorite classification. The biggest difference between the two lies in the mafic content. The southerntype has a roughly equal balance of biotite and pyroxene while the northern type’s mafic content is dominated by pyroxene with very little biotite. This suggests that the two types may have had their origins in different magma or that that their shared original magma consolidated under very different conditions. This makes Al-Hafdh’s ring intrusion hypothesis more difficult to accept. Grain size of the ground mass is typically 0.125-0.25mm with phenocrysts varying between 1 and 2mm. In the area below Cunyon Crags at the east end of Dunmoor Hill, the rock has been subject to extensive alteration.

High Cheviot

A sample fromWoolhope Crag

This is generally the most fine-grained and acid of the rock groups. The grains in the groundmass vary from 0.1 to 0.25mm with small phenocrysts reaching a maximum of 2.3mm, although the distinction between ‘groundmass’ and phenocrysts is less obvious than in most other Cheviot types. The rock has a higher quartz and orthoclase content than any of the others, and is usually lacking in pyroxene. Al-Hafdh’s chemical analysis which is confirmed by our own thin section assessment, suggests that plagioclase content may be so low (under 10%) that the rock should be classified as an alkali-feldspar granite. However, its relatively fine-grained texture means that some specimens fall almost into the IUGS fine-grained classification and therefore really should be called felsite. Al-Hafdh describes its texture as ‘saccharoidal’ which is apt in places. Its mafic content is low, and generally consists of biotite with chlorite occurring at Woolhope crag. This group is found widely on the north slopes of the Cheviot on the spur from Woolhope Crag upwards, on the east slopes approaching the summit and onwards to the south-west side of Cairn Hill. It is interrupted from time to time by much darker rocks, some of which resemble the Marginal type, that may have had their origins close to the roof of the pluton, and some appear to be dykes. The lower end of Woolhope Crag shows a darker hue than the rock of the upper end, but thin section analysis suggests little mineralogical difference, the darker appearance apparently being due to finer grain size.

Central Belt

Typically, the rocks of this group show a coarser structure than the Upper Cheviot group, and are generally pink-purple in colour with, in some areas, white phenocrysts of andesine feldspar. Some minerals are easily visible without a microscope but there is usually a finer groundmass containing quartz and alkali feldspar. There is a considerable variation inrock types within this group. Quartz content is usually over 20% but not always so which means that some samples may be quartz-monzonites and some granites. Plagioclase sometimes exceeds alkali feldspar, and sometimes vice versa. This makes differentiation even more difficult as some of the specimens can be classified as syeno-granite and others as monzo-granite. Al-Hafdh classified them all as ‘granodiorites’ but that was before the IUGS determined that a granodiorite should have a ratio of plagioclase to alkali feldspar which exceeded 65%:35%. As far as we can tell no rocks in this group are that rich in plagioclase. We should stress that we do not have access to a laboratory equipped for professional chemical analysis or mass spectrometry so precise determination of feldspar proportions is not possible for us.

A sample from Long Crag on Dunmoor Hill

Grain size for the pink-purple type of rock within the Central Belt group is mainly 0.5mm for the groundmass with phenocrysts varying from 1 to 4mm. The groundmass is approximately equal in area to the phenocrysts. There are fairly distinct localised types of this rock. The material from Long Crag and surrounding tors on Dunmoor Hill, often has distinct small white plagioclase and shiny black biotite phenocrysts. The rock from the summit tors of Dunmoor Hill has a similar appearance but lacks the white and black phenocrysts. Similar rocks to the latter appear on the summit of Hedgehope Hill and in other places very widely through the Central Belt. These rocks usually contain more than 20% quartz but are low in plagioclase. They may fall into the syeno-granite classification.

A sample from the summit tors of Dunmoor Hill

The other distinctive type within the Central Belt group is that which Al-Hafdh called ‘Standrop’. It is found at Great and Little Standrop and on the distinctive boulder collar below the summit of Hedgehope Hill. Good examples of this type also occur in the upper Linhope Burn in grid square NT 9417. The type has very distinctive large white phenocrysts of plagioclase (andesine) feldspar which detract from the purple colour to give an impression of greyness. It is the coarsest type found in the Cheviot pluton. Grain size varies from 1 to 4mm, and there is no groundmass. Along with the southern Marginal variety, it has a higher proportion of plagioclase than other types within the Cheviot pluton. Depending on the sample, this may be either monzo-granite or quartz-monzonite.

A sample from Great Standrop

All rock types of the Central Belt group have been subjected in places to powerful hydrothermal activity. Some of the rocks from the summit plateau of Hedgehope Hill have very little clear structure remaining. Sericitisation and tourmalinisation are common forms of alteration. Some specimens show extensive breakdown of plagioclase feldspar to epidote and sericite, while mafic minerals have been altered to secondary amphibole. The extent of hydrothermal alteration is very striking in the area around Harthope Linn (NT 92 20) in the upper Harthope Valley.

Hydrothermally altered rock from the summit of Hedgehope Hill

Strongly brecciated rocks typically occur in areas of fracture and faulting e.g. along the Hawsen Burn and, much more extensively, in the upper Harthope Valley. The breccias consist of fragments of granite and possible andesite which have been cemented together by silica and haematite with some tourmaline veining occurring in places.
The presence of silica veining along with haematite in the fractures and faults in the Cheviot complex has been associated with tectonic processes in the Hercynian, occurring between 325and 300 Ma. These physical and chemical processes, constituting the last phase of igneous activity, are thought to have enabled the fluid transport and deposition of silica through lava, granite and dyke rocks causing widespread mineralogical change including the low-temperature oxidation of iron-titanium oxides to haematite together with their re-magnetization.

Breccia cemented by quartz and haematite from the upper Hawsen Burn

In the places where bedrock reaches the surface, there is often a bewildering variation amongst theCentral Belt types. A clear boundary between the Central belt rock of Dunmoor Hill and the darker Marginal rock can be traced to within a few metres right across much of the south side of Dunmoor Hill. On Shiel Cleugh Edge, a sharp boundary between a generally coarser-grained Central Belt type and a finer-grained variety of the same type can be detected with the finer-grained rock showing a chilled margin against the coarser.

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