course-grained porphyritic Archives - Geology North

Great Standrop again

Stephen — Tue, 11 Aug 2015 15:43:54 +0000

We return to the collar of boulders and outcrops below Great Standrop to confirm whether the fine-grained rock is indeed a chilled margin. It turns out to be yet another aplite dyke.This trip confirms that the coarse-grained porphyritic ‘Standrop’ variety of granite. changes to the pink, medium-grained porphyritic variety (Al-Hafdh’s ‘Hedgehope granodiorite’) at the base … Continue reading Great Standrop again →

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Great Standrop

Stephen — Thu, 30 Jul 2015 15:38:30 +0000

I visit Little and Great Standrop to confirm that these fine tors consist of coarse-grained porphyritic ‘granite’.
The rock here is the classic grey type with large white phenocrysts of andesine feldspar. At Great Standrop aplite dykes are apparent cutting through the coarser rock which extends down to the rocky collar ringing the slopes above the Linhope Burn at around NT 944 177. At the foot of the collar, the rock appears to change to the medium-grained pink ‘granite’.
At NT 94590 17775, I find a fine-grained pink rock which I take to be a chilled margin of the pink medium-grained rock against the coarser material of the main Standrop ridge.
The characteristic coarse-grained ‘Standrop’ rock proves difficult to classify. Quartz content is roughly 20%, and in places plagioclase exceeds K-feldspar. The rock lies on the granite/syenite/monzonite/granodiorite boundary.

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Rigg Cairn, High Cantle to High Bleakhope

Stephen — Mon, 03 Nov 2014 16:26:03 +0000

We find a coarse-grained granitic rock at NT 94016 16737. This is pinker with fewer large white plagioclase phenocrysts than the rocks of Hedgehope Hill and Great Standrop but it has a similar grain size. Most of the terrain is trackless peat bog lacking any exposure of bedrock.
Descending towards High Bleakhope, the pink medium-grained rock outcrops at NT 92469 16365. The outcrops around NT 92643 16262 are all of high colour index and correspond with the dioritic ‘marginal’ variety found on Dunmoor Hill and at Linhope Spout.
In places there are xenoliths of what appear to be hornfelsed andesite.

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Dunmoor Hill

Stephen — Sun, 05 Oct 2014 15:24:52 +0000

We make a return visit to Dunmoor Hill. The summit tors of the hill present a medium-grained pink granitic rock that look very similar to that of the summit of Hedgehope Hill.
Long Crag (not the same as Long Crags, near Housey Crag in the Harthope valley) reveals a very similar rock. Al-Hafdh classifies the former as ‘Hedgehope granodiorite’ and the latter as ‘Dunmoor granodiorite’. Actually, both in hand specimen and thin section, they are very similar in appearance and mineral content – an observation born out by Al-Hafdh’s own chemical analysis of the types.
We remain unconvinced that they can be separated into two types belonging to separate intrusions.
Al-Hafdh argues that the two are separated by a belt of coarse porphyritic granodiorite (‘Standrop’ variety). A tor containing the coarse rock certainly appears at NT 96890 17969 about half way between the summit and Long Crag, but the extent of peat bog over the hill precludes any way of establishing whether this is part of a wider belt separating the summit of Dunmoor Hill from Long Crag.
A further descent down the south side of the hill reveals again the junction between the pink medium grained porphyritic rock ‘(Dunmoor’/’Hedgehope’) and the dioritic ‘Marginal’ variety.
Cat Crag is definitely composed of the ‘Marginal’ variety.
We look for the hornfelsed andesite that geological map locate close to Long Crag, but we don’t find it.

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Hedgehope Hill

Stephen — Tue, 22 Jul 2014 15:05:29 +0000

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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Knock Hill and Upper Linhope Burn

Stephen — Thu, 03 Apr 2014 21:39:34 +0000

Knock Hill (NT 99584 16499) with its attendant gorge is an impressive feature on the road towards Linhope. It is composed of ignimbrite, and gives evidence that the Cheviot volcanic system was explosive, and produced abundant pyroclastic surges and ash fall, as well as andesite lava flows.
The 1” OS geological map notes a ‘hypersthene-porphyrite’ dyke which we locate at NT 99584 16499 on the south side of the Breamish burn, just beyond the road bridge.
From here, a walk to the upper reaches of the Linhope Burn (around NT 94094 17459) and on to the Standrop Burn. (NT 93794 17839) There is a dacite dyke showing in the streambed just above the two burns’ confluence.
The streambed reveals a variety of rock types. The coarse-grained (Standrop) and finer-grained pink (Dunmoor) varieties in evidence. There is also a much finer grained pale rock similar to the ‘evolved’ granophyre from the Cheviot.
According to Al-Hafdh, this is ‘Woolhope’ granite, part of a ring dyke that circles from the upper Standrop Burn area through to the upper Dunmoor Burn area.

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Al-Hafdh, Linhope Spout and Linhope Burn

Stephen — Mon, 24 Mar 2014 22:19:31 +0000

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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