Tag Archives: hydrothermal alteration

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Upper Harthope Valley

On Tuesday 29th November we set out to make a more detailed study of the rocks in the upper Harthope Valley above Harthope Linn. We believe from preliminary samples that these may differ significantly from the granitic rocks of the Cheviot pluton. The day is bright and cold with a hard frost promising to make boggy ground easier to walk over. The furthest permitted parking is at the Hawsen Burn just below Langleeford. From there it is a long but easy farm track to Langleefordhope, and then about a quarter of a mile to the lower Harthope Linn close to the stell (circular sheepfold). After that, the going becomes much rougher with several potentially difficult burn crossings.

The rocks outcropping above the stell prove to be granitic Central Belt group although rather more mafic than usual. Similarly the dyke-like outcrop beside the path about 200 yards above the upper Harthope Linn, is also granitic. This rock seems to give rise to a fairly even rolling hillsides.

Looking down the Harthope Valley from near Harthope Linn.
The granitic rocks of granite/quartz-monzonite give smooth rounded slopes.

From this point onwards, the type of rock changes consistently. We checked this most of the way to the watershed. The rock appears to be a form of breccia. In many places it has obviously been severely shattered. In other places it seems more stratified and less disturbed. Because of the shattered nature of the rock, it erodes into gullies more readily giving a more irregular pattern to the slopes of the hills.

A view looking up the Harthope Valley from above Harthope Linn.
The erosion gullies formed from the breccia rocks can be clearly seen.

An outcrop of typical breccia.

An outcrop of stratified breccia.

The breccia has been cemented together by silica and red haematite. The silica occasionally has space to crystallise out into attractive quartz crystals, mostly rock crystal but a few specimens show a hint of mauve. Occasionally, veinlets of black tourmaline appear with the quartz.

The Harthope valley marks the line of a SW-NE fault almost bisecting the pluton. Lateral displacement can be detected from the plutonic/lava margins on either side of the fault, and can be measured to about a quarter of a mile. Vertical movement, if any, is unknown. The faulting begs the question whether the breccia is a product of a crush zone, or evidence for volcanic vent activity. We remain uncertain about this. However, certain conclusions can be drawn. The breccia must have been cemented together under high temperatures for crystalline silica and tourmaline to have been deposited. The flatter layers imply more stable conditions for at least some of the hydrothermal activity. The presence of haematite (ferric oxide) indicates oxidising conditions but whether these were the result of iron reacting with high temperature water vapour inside the magma chamber, or exposure to the air at a vent, is uncertain.

Severely shattered breccia outcrop

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The Hawsen Burn

We find plenty of fine-grained granophyric rock but we are not convinced that it really is of the same type as the ‘Evolved’ rock from the summit and north slopes of the Cheviot. Subsequent thin section analysis suggests that it and samples from the Standrop Burn are rather different from the ‘Evolved’ type. However, the Hawsen Burn samples do share the high quartz and low plagioclase content of the ‘Evolved’ rock of the Upper Cheviot area. At any rate, we now feel that there is insufficient evidence to argue that the fine-grained granophyric rocks are all part of a common ring dyke system. They are quite likely to be separate intrusions, some of them representing very late magmatic activity.
We locate a medium-grained but rather altered, pink porphyritic variety at NT 94299 23027 which seems to correspond with varieties found on Dunmoor and Hedgehope Hills.
There is plenty of evidence of altered andesite, hornfelsed by contact with the pluton as we would expect here as the Hawsen Burn runs along the boundary between the two in some places.

Altered yellowish andesite at NT 94504 23011 is stated in some guides to contain massive epidote but there was none in the samples that we thinned.

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Hedgehope Hill

A visit to Hedgehope Hill confirms that the upper Dunmoor Burn contains a very fine-grained granophyric rock. We have provisionally classified this as ‘Evolved’ granite but have since become increasingly doubtful whether it is really part of the same intrusion as the evolved granophyre on the upper slopes of the Cheviot, and whether it really does form part of a ring dyke as Al-Hafdh suggested. Exposure is too limited in the upper Dunmoor Burn to draw definite conclusions.
About 100m below the summit of Hedgehope Hill, the track crosses a large boulder field which is probably the result of periglacial activity. The great majority of these boulders belong to the coarse porphyritic type which Al-Hafdh named ‘Standrop granodiorite’. The summit of Hedgehope has a medium to fine-grained pink rock which is similar to that of Dunmoor Hill. Many samples from the summit show evidence of significant hydrothermal alteration.
Returning via the north side of Hedgehope Hill towards the Harthope valley, we find more of the coarse-grained ‘Standrop’ rock but, after much searching, fail to find the chilled margin between the finer and coarser varieties that Al-Hafdh says is visible there. The one example of really fine-grained rock chilled against the courser rock turns out to be another felsite or aplite dyke.

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Al-Hafdh, Linhope Spout and Linhope Burn

We have discovered Al-Hafdh’s 1985 PhD thesis on the alteration petrology of the Cheviot pluton and we’ve decided to chart and verify his rock types.
He claims to have found a series of chilled margins which establish the sequence of the various intrusions.
So, we make an excursion to the Linhope area where we find no convincing evidence for chilled margins but we do find the junction between Al-Hafdh’s Marginal and Dunmoor varieties just above Linhope Spout.
The ‘Felsite’ dyke marked on the 1” OS map at the top of Linhope Spout turns out to be a wider than normal version of the aplite veins which are frequent in the Cheviot pluton. This aplite dyke probably accounts for the hardness of the rock which has caused the waterfall feature.
About 100m up stream there is a slight cliff (about 5m high) at the junction between the pink granite and the marginal. This contains significant quartz veining, lending weight to the theory that hydrothermal activity took place along lines of weakness between the two types. Thin sections from this area reveal red crystals in the quartz which we first identified as rutile but now think more likely to be hematite.
Up stream to NT 94694 17249, we find a course-grained porphyritic rock in the streambed which Al-Hafdh classified as ‘Linhope’ granodiorite. We are not convinced by his differentiation of this from ‘Standrop’ granodiorite, and can not find evidence for a chilled margin between them.

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Woolhope Crag and the Cheviot

We go to see if the theory of the darker rocks making tougher landscape features, would hold at Woolhope Crag (NT 92264 22169).
The answer was ambiguous. There is certainly plenty of dioritic rock there but there is also a pale fine-grained granophyric rock. We subsequently made our way up the north slopes of the Cheviot, and found plenty of scattered exposures which revealed more of the granophyric rock. We have classified this type on the map as ‘Evolved’ granite.
Descending from Cheviot summit on the main path for Langlee in the Harthope valley, we found distinct quartz veining with tourmaline content (approx. NT 918 209). There is the possibility that this quartz veining is the result of hydrothermal penetration on lines of weakness between different types of plutonic rock.
We need to pay another visit to investigate this.

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A hypothesis and an excursion to Harthope Linn

 Harthope Linn (NT 92734 20229)

We are pursuing the idea that the rocky features in the mostly rounded Cheviot Hills were caused by a more mafic content making the rocks harder. This would account for waterfalls such as Harthope Linn and Linhope Spout. Field work has shown that the andesite at the Carey Burn Linn and at Davidson’s Linn are of the dark harder variety.

There seems to be considerable variety in the rocks of the Cheviot pluton. The 1” geological survey shows the whole area as ‘granite’ but it is also apparent that the visible mineral content make this classification doubtful. This has led me to begin working with Ian to investigate and map the area in an effort to find out what is there and to better understand how it came to be so .

Today’s work at Harthope Linn did reveal rocks with a high colour index and mafic content but there were also other paler varieties and some showing substantial alteration.