Types of Cheviot plutonic rock

The classification of Cheviot igneous rocks

Differences in Cheviot ‘granites’ have been noted in all previous studies. These differences give clues to the different physical and chemical processes that formed and altered the rocks leading to hypotheses about the origins of the Cheviot complex. Geologists have tried to classify the different rock types and map their occurrences. This page communicates our own attempts at a classification that we regard as a work in progress.

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

Al-Hafdh did extensive sampling and analysis by both thin section and chemistry. His ‘Stanhope’ and ‘Linhope’ types are very similar in both appearance and content, as are his ‘Hedgehope’ and ‘Dunmoor’ types 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 types 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 attempts at classification started from the basis of Al-Hafdh’s but we found the distinction between ‘Linhope’ and ‘Standrop’ types impossible to recognise and that between the ‘Dunmoor’ and ‘Hedgehope’ types difficult to determine. So we initially settled for 5 main types:
‘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’.
We avoided the term ‘granophyric’ because rocks of all types show coarse or fine granophyric/micrographic textures when viewed in thin section.

The more we walked the Cheviot Hills during 2015-16, the more dissatisfied we became with our classification. It seemed inappropriate to use the term ‘porphyritic’ as a distinguishing factor as most of the rocks that we were investigating appeared to be more or less porphyritic over the area of their occurrence. Our system also had the problem of mixing classification based on location (‘Marginal’) with classification based on appearance (‘Porphyritic’).
As a result of our more recent fieldwork, we have settled for the time being on three main rock groups based on geographical location, ‘Marginal’, ‘Central Belt’, and ‘High Cheviot’ groups.
In our ‘Marginal’ group are the rocks at the perimeter of the pluton that tend to be quartz monzonite,
in our ‘High Cheviot’ group are predominantly the quartz-rich rocks that occurs high on Cheviot,
and in our ‘Central Belt’ group are the predominantly intermediate rocks that we find between the ‘Marginal’ and the ‘High Cheviot’.
Our system isn’t ideal. We find quartz-rich granite in marginal and central locations, and mafic ‘Marginal’ type rock centrally, and there is variation in mineral content, grain-size and texture in all our types – but even so, our three-group system helps conversation about our findings.

A word on our mapping of these three groups. We have settled on representing our findings on the relief map with individual location marker pins for each sample. We have decided against presenting the usual generalised colour-coded areas for each group and type at this stage since, given the relative scarcity of outcrops and the great variety of rock types that can occur within quite small areas in the Cheviots, any generalised map we could offer at this stage would be extremely speculative. As our knowledge and understanding increase, we might find ourselves, at a later date, able to construct a useful generalised map of Cheviot rock types.

‘Marginal’ Group

A sample of ‘Marginal’ rock from Cunyan Crag

This is the one sort of rock on which all previous writers have agreed. It has been seen as a discontinuous outer ring of plutonic rock emplaced before the inner rocks. However, our thin section analysis has revealed that whilst the ‘Marginal ‘ rock that extends along the southern edge of the pluton from Dunmoor Hill to beyond High Bleakhope and the ‘Marginal ‘ rock at the pluton’s northern boundary in the vicinity of Bellyside Crag and Bellyside Hill have some important features in common, they also have quite marked differences too. The southern type appears to fall within the quartz monzonite classification whilst the northern type tends more to the composition of a quartz syenite. Both typeshave around 10-20% quartz, and 10-15% mafic mineral but the southern type has a much higher proportion of plagioclase to alkali feldspar than the northern. Some of the samples from Cunyan Crag have so much plagioclase that they approach the quartz monzodiorite classification. Both types have plentiful pyroxene and both are low in primary biotite but the southern ‘Marginal’ has more secondary biotite than its northern counterpart. This suggests either different magma sources, or a shared original magma that consolidated under different conditions, or that the two types have experienced different alteration processes, or a combination of some of these. The first two possibilities cast doubt on Al-Hafdh’s ring intrusion hypothesis.
In both types, there is further variation in grain size, the degree of granularity and the degree to which it is granophyric.

‘High Cheviot’ Group

A sample of ‘High Cheviot’ rock 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 the other Cheviot groups. 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 rock, some of which resemble rock from the Marginal group, and they may have had their origins close to the roof of the pluton although some may be dyke material. 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.

‘Central Belt’ Group

Typically, the rocks of this group show a coarser structure than the High 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 inrocks within this group. Quartz content is usually over 20% but not always so and plagioclase and alkali feldspar which means that the more mafic samples range from quartz syenite to quartz monzonite while the more felsic samples range betweensyenogranite and monzogranite. 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. Again, we need to 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 of ‘Central Belt’ rock 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 varieties of this type of rock. The material from Long Crag and nearby outcrops on Dunmoor Hill often has distinct small white plagioclase and shiny black biotite phenocrysts. However, rock from the summit tors of Dunmoor Hill is more ‘Marginal’ in appearance and has a quartz monzonite character.
Similar rocks to the Long Crag sample appear on the summit of Hedgehope Hill and in other places through the central belt of the Cheviots. These rocks usually contain more than 20% quartz but are low in plagioclase.

A sample of ‘Central Belt’ rock 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. 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 of variety ‘Marginal’ quartz monzonite, it has a higher proportion of plagioclase than other types within the Cheviot pluton. Depending on the sample, this type may be either monzogranite or quartz monzonite.

A sample of ‘Central Belt’ rock from 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 ‘Central Belt’ 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 the ‘Central Belt’ types. The ‘Central Belt’ rock of Dunmoor Hill and the darker ‘Marginal’ rock are in close association right across much of the south side of Dunmoor Hill. On Shiel Cleugh Edge, a sharp boundary between a generally coarser-grained ‘Central Belt’ rock and a finer-grained variety of the same type can be detected with the finer-grained rock showing a chilled margin against the coarser.

No vestige of a beginning, – no prospect of an end

error: