chilled margin Archives - Geology North

Shielcleugh Edge

Stephen — Wed, 06 Jul 2016 09:13:01 +0000

SHIEL CLEUGH EDGE

Ian made an expedition to Shiel Cleugh in 2015 and noted the rather complex variety of granitic rocks here. We return to check this out, and also to see if we can find where Al-Hafdh (1985) reckoned he had found the ‘Standrop’ type chilled against the ‘Dunmoor’ type (both names Al-Hafdh’s classification). If correct, this would indicate that the coarser ‘Standrop’ was intruded later than the finer ‘Dunmoor’, both of which Al-Hafdh interpreted as part of a series of ring intrusions.
In the course of the day we climb to the top of Shiel Cleugh Edge, and notice wistfully in the distance the prominent tor of Coldlaw Cairn. This is within the plutonic area and needs a visit but it is very remote and only reached over trackless blanket bog.
We find a rather bewildering confusion of rock types. There is no clear cut boundary between finer (Dunmoor) and coarser (Standrop) types. Both are found widely over the Southern side of Shiel Cleugh. The position is complicated by the limited exposures, many of which cannot be accepted with certainty as bedrock. However, a pattern does emerge as the day progresses. The coarser ‘granite’ predominates at a higher level, while the finer ‘granite’ predominates lower down. At the very bottom close to the parent Breamish burn. The rock becomes much more mafic and dioritic. This appears to be a continuation of the Marginal type which is found extensively on the Southern slopes of High Cantle.

From left to right: the succession of rock=types down the Shiel Cleugh Burn
We see increasing mafic content as the burn is descended to the River Breamish.

However, there is little sign of clear boundaries. The rocks which are fairly well exposed in the Shiel Cleugh burn which flows off the southern slope of the Edge into the Breamish, show a gradual change from coarse to finer pink ‘granite’ and then a gradual increase in colour index towards Marginal rock.
There however some rather exciting dicoveries. At NT 91922 16767 we do find a distinct boundary between coarser and finer rock.

Contact between the coarser and finer-grained rock

At NT 91847 16893 and NT 91825 16916 we find further junctions between the two types. At these locations, there is evidence for compression of one against the other together with some flow structure. Significantly there appears to be a chilled margin, and later thin section analysis confirms this. However it is the finer (‘Dunmoor’) which is chilled against the coarser (‘Standrop’). This suggests that the coarser rock cannot have been intruded into the finer.

Thin section with crossed polars showing the chilling of the finer-grained rock against the coarser-grained

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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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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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Dunmoor Hill

Stephen — Sat, 03 May 2014 22:15:28 +0000

We go to Dunmoor Hill in search of Al-Hafdh’s hitherto elusive chilled margins. Dunmoor Hill is a particularly good site as exposure, unlike in many areas of the Cheviot hills, is plentiful. We are finding Al-Hafdh’s thesis on the Cheviot pluton immensely stimulating. It has provided clear direction to our own research, although we are beginning to doubt his proposal that the Cheviot pluton consisted of a series of ring dykes.
The area below Cunyon Crags has plenty of small outcrops where we find a bewildering mixture of felsic and mafic fine-grained material. At first, we think that the mafic rock is altered andesite. However, thin sections show very well-developed granophyric texture in the felsic rock which imply late intrusion into already established plutonic bodies. The mafic rock has a very high biotite/opaque iron oxide content, and appears to be restite.
Subsequently, we are able to trace the junction between the pink rock (Al-Hafdh’s Dunmoor type) and the more mafic Marginal rocks right across the south slope of Dunmoor Hill. The junction doesn’t show a clear cut boundary but it does present frequent inter-penetration of the two types. The implication is that neither type was fully consolidated when the intrusion took place.
We are still unable to find any chilled margins to confirm an intrusion sequence. Finer rocks always turn out to be aplite veins or small dykes.

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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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