This excursion provides opportunities to investigate the rock at the granite/lava border. An exposure where the junction is visible would be a great find but, given the degree to which the Cheviot bedrock is cloaked in peat, this is unlikely.
Another point of interest is an inlier of andesite that has been mapped between Long Crag and Cat Crag on Dunmoor Hill but that we haven’t, as yet, succeeded in finding. it would be interesting to mark the degree to which the lava and the granite were affected by their direct contact.

Map showing the excursion route, locations and igneous rock types

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We park out of the way of vehicles and gates on the grass at the roadside a short distance up the hill past Greenside Farm and walk back down to the start of the footpath that leads north-west then north-east towards the site of the medieval village at the foot of Dunmoor Hill. It is easy walking and as we approach the south-east side of the hill, we see a number of rock exposures ranging up the slope that is topped by Cunyan Crags, a sizeable outcrop of the Cheviot pluton. Somewhere between them, the lava/pluton junction runs south-west towards Linhope.

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Altered andesite with pyroxene-rich veins

We begin by investigating some of the exposures that we saw from the track. The Geological Survey mark these as andesite.
The first one we examine at NT9843178 immediately takes our interest because it is a very hard and variegated rock that looks distinctly granitic in parts. We wonder if this is a mixture of ‘Marginal’ plutonic rock and hornfelsed andesitic lava.
Thin sections will reveal that the main body of the rock is a reddish (haematite-rich), hornfelsed andesite with largely albatised plagioclase phenocrysts and altered pyroxenes together with occasional quartz crystals all set in a fine-grained, opaque-rich ground. The andesite has been intruded by a dark-grey, very fine-grained rock as well as by pyroxene-rich veins.
In these veins, the pyroxene appears alongside feldspars together with a little quartz.
Both the intrusive rock and the veins may be varieties of the dark, pyroxene-rich ‘Marginal’ rock that we have seen near Low Bleakhope in the Breamish Valley.

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The rock at the second exposure looks more obviously andesitic – it is darker and we see phenocrysts. However, thin sections present us with a surprise. Most of the ‘phenocrysts’ are quartz with few of the large plagioclase phenocrysts that we would have expected to see.
The rock is obviously altered; amphibole minerals have replaced primary mafic minerals, much of the groundmass has been altered to tiny flakes that appear orange in thin section and have parallel extinction, and the presence of so much quartz suggests that it is granitic.

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The rock at the third exposure we sample is hornfelsed andesite. Here, we do see comparatively large plagioclase phenocrysts in a  groundmass that is even more altered. Here are the same, although no quite so extensive, patches of quartz that we saw in the rock at the previous location. The surprise comes in the presence of tourmaline. It is unlike the tourmaline that we have seen elsewhere in the Cheviots in that it is grey-brown rather than the usual dark blue-green and it is less strongly pleochroic. The presence of tourmaline with quartz points to hydrothermal alteration in addition to the metamorphic effects associated with heat from the nearby pluton.

Specimen 4

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At this location, andesitic rock gives way abruptly to granitic rock offering some samples that are intensely dark and fine-grained while others appear to be porphyritic (although the phenocrysts may be xenocrysts ripped off coarser-grained host rocks) and still others approach the look of the sub-equigranular rocks up on Cunyan Crags.
As we move up the hillside, we see a transition from the fine-grained ‘porphyritic’ variety outcropping close to the pluton-andesite boundary to the medium-grained sub-equigranular variety that then continues up the crags.

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Specimen 1. Chilled porphyritic ‘Marginal’ plutonic rock

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Specimen 2

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Specimen 3. Medium-grained, subequigranular ‘Marginal’ rock with very occasional clusters of fine-grained material

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Specimen 4. Junction of sub-equigranular ‘Marginal’ host rock and a fine-grained mafic intrusion

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Cunyan Crags in the area of NT977181 presents substantial exposures of the Cheviot’s typical ‘Marginal’ rock that is dark grey/brown. sub-equigranular and allotriomorphic.

Specimen 1

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Specimen 2. A sample of the same variety of the ‘Marginal’ type from a different part of the crag

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Next, there’s a long boggy stretch that crosses, according to Robson’s map, the boundary between the outer ‘Marginal’ quartz monzonite and the ‘Central Belt’ medium-grained,  porphyritic granites that lie closer to the centre of the area occupied by the pluton.
We are heading for an outcrop at NT970178 where Al-Hafdh records his porphyritic ‘Hedgehope’ granodiorite chilled against the ‘Marginal’ quartz monzonite that evidences  his sequence of multiple intrusions. We search but we do not find a chilled porphyritic rock against a more mafic granular one, but we do find a great variety of rocks in this vicinity. Over a few square metres below NT970178, we find;
granular, medium-grained ‘Central Belt’ rock that is less mafic than the ‘Marginal’ type, sub-equigranular ‘Marginal’ rock, mixed fine-grained and coarser-grained ‘Marginal rock, and ‘Marginal’ rock that has been invaded by a fine-grained, felsic material.
Some of these rocks are strongly granophyric and some are not.
It’s tempting to see the redness of some of these rocks together with  the presence of  large crystals and crystal masses and take them to be of the ‘Central Belt’ type – since these are always reddish and often porphyritic. However, under the microscope, the ‘phenocrysts’ in most of the rocks look more like inclusions in the fine-grained component rather than phenocrysts and glomerocrysts that have crystallised out of the same magma.

Specimen 1. ‘Central Belt’ rock

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Specimen 2

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Specimen 3. Mixed fine-grained and coarser-grained granophyric ‘Central Belt’ or ‘Marginal’ rock

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Specimen 4

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Specimen 5. Sub-equigranular ‘Marginal’ rock

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Specimen 6

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From location 4, we head over rough ground towards the tors that mark the north end of Long Crag and the slightly higher ones to our right that lie close to the fence and footpath that runs down to Cunyon Crags. We pass by both exposures, for now, and take the rough track leading from Long Crag to the summit tors of Dunmoor Hill.

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The sample we take from the summit tors is a very interesting one. It reveals a junction between a coarser-grained, fairly mafic, ‘Central Belt’ rock and a finer-grained variety that, had it been closer to the plutons periphery, we might easily have classified as ‘Marginal’ on account of its even lower quartz and  high mafic mineral content.
In seeking to explain the presence of such a mafic rock here, perhaps we need to remember that the higher ground in the Cheviots represents plutonic rock that was close to the andesitic roof of the pluton, Housey Crag andesite exposures evidence this.  So, this rock may be ‘Marginal’ in respect of it’s proximity to the upper margins of the pluton.
In the hand sample, the finer-grained material is the darker of the two with patches of lighter coloured, larger crystals within it.
In thin section it is clear that the two varieties contain the same minerals; much plagioclase, some K-spar and quartz, with biotite, pyroxene and opaques. In both materials, the opaque oxides accumulate within and at the edges of altered pyroxene and biotite and as the finer-grained one has the higher percentage of these, it also has the higher percentage of opaques.

Al-Hafdt’s ‘Distribution Map of Cheviot Granites’ draws boundary lines between two different rock types fairly close to this location – that between his fine-grained, porphyritic ‘Hedgehope’ type and his coarse-grained, granular ‘Standrop’ type (although he doesn’t record having sampled this exposure). The coarser-grained rock in our sample does resemble his ‘Standrop’ type, but the finer-grained material is more granular than porphyritic and therefore doesn’t align with the porphyritic character of his ‘Hedgehope’ type.
Robson’s distribution map, drawn earlier than Al-Hafdh’s, shows a junction very close to this location between the non-granophyric ‘Standrop’ type and the ‘Granophyric’ type.
The quartz in both varieties in our sample is coarsely granophyric.

Comparing Al-Hafdh’s and Robson’s maps that show the distribution of Cheviot granites in this area

Possibly, this junction is only a small scale feature, but on the other hand it may be one of those rare occurrences in the Cheviots – a visible junction of two different masses of magma. We aim to return to determine the extent and orientation of the junction.

Junction of coarse-grained and medium-grained rocks

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Click here to read an account of the second part of this excursion

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References

Al-Hafdh N.M. 1985. The Alteration Petrology of the Cheviot Granite. Thesis submitted for PhD. at Newcastle University.

Robson D.A. 1976 A Guide to the Geology of the Cheviot Hills Transactions of the Natural History Society of Northumbria, Newcastle Upon Tyne Vo. 43, No. 1.

British Geological Survey  Online geology map http://mapapps.bgs.ac.uk/geologyofbritain/home.html

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