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The Little Whin Sill

The Little Whin Sill is a unique component in the Whin Sill complex on account of the phenocrysts of olivine and its pseudomorphs, albeit tiny and restricted to only some parts of the sill, that are reported to be part of its composition. This most basic and the least evolved of the whinstones outcrops only in the vicinity of Stanhope in Weardale, always at a higher stratigraphic level than the Great When Sill. It is visible in the water-filled, disused Greensfoot quarry at Stanhope (out of bounds to the public), and also along the east side of the Rookhope Burn above Eastgate. At Turn Wheel Linn on the Rookhope Burn the Little Whin Sill is exposed and accessible, projecting out from between the top two layers of the Three Yard Limestone.
The sill has also been encountered in three Geological Survey boreholes in the vicinity, the first at Rookhope (NY 9386 4273) in 1961 under the direction of Kingsley Dunham, a second overseen by R.K. Harrison at Woodland (NZ 09096 27694) in 1962 and a third at Crawleyside (NY 99625 40100) overseen by N.G. Berridge in 1982.

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Map showing the excursion route, locations and igneous rock types and map key

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Location 1. The Little Whin Sill at Turn Wheel Linn

We arrive at Turn Wheel Linn by taking the minor road north towards Rookhope and parking on the verge at NY 94486 40555 close to field gate where the footpath meets the road. We follow the narrow path down the hillside to the west bank of the Rookhope Burn, through woods and then long the edge of an open field to the little waterfall on the edge of more woodland. Wellington boots come in handy because the Little Whin Sill outcrops on the east side of the burn.

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Here, the sill is about two metres thick, fine grained and with marginal chilling. The host limestone shows some hardening through metamorphism while the whinstone itself looks mostly fresh with signs of carbonisation to white whin where it is in contact with the limestone with some general carbonate veining.

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Petrology and petrography of the Little Whin Sill at Turn Wheel Linn

There have been two two relatively recent petrological studies that focussed only on the very fine-grained chilled margin of the Little Whin Sill at Turn Wheel Linn. The first by Dunham and Kaye in 1965 recorded, amongst other things, the amount of groundmass (94.7%) relative to that of microphenocrysts; olivine (2.7%), orthopyroxene (1.5%), plagioclase (0.7%) and clinopyroxene (0.4%). The second, by Dunham and Wilkinson in 1995 used electron probe analysis to determine, amongst other things, the composition of the fresh olivine in the chilled margin. They found it to be 69.6% forsterite, the magnesium-rich end member of the olivine solid solution and 30.4% Fayalite, the iron-rich end member. The maximum size of the olivine microphenocrysts was 0.2mm.Two separate series of experiments in the 1970s concluded that the olivine began to crystallise between 1120°C and 1156°C.
All of the olivine studied had been subject to some alteration to talc and iron-magnesium carbonate. Some crystals bore only alteration rims but most of them showed additional alteration along their internal fractures.

In contrast, R.K. Harrison found no fresh olivine in sections from the Little Whin Sill at Woodland, reporting that only rare “pseudomorphs of chlorite (E 34134) and pyrite appear to be after olivine…”.
E 34134 refers to the thin section that was made from material from the borehole and is now available for viewing on the BGS website. On close inspection we find two pseudomorphs in the lower right hand corner that might be those that Harrison was referring to.

Dunham and Wilkinson suggested that olivine might well be the source of some of the pseudomorphs that appear in whinstones throughout the Whin Sill complex – the others being known to be alteration products of pyroxene. Much of the clinopyroxene in the chilled margin they studied occurred at the margins of altered orthopyroxene with groundmass material appearing between the two pyroxene types. Skeletal augite and pigeonite phenocrysts of variable composition were also found.
The plagioclase in the chilled margin was identified as labradorite with some crystals altered, probably by hydrothermal activity, to kaolinite and tiny grains of potassium feldspar.
The altered groundmass of the margin contained chlorite, kaolinite, carbonates and pyrite.
Dunham and Wilkinson related their findings to the magma that gave rise to this rock, arguing that the euhedral shape and uniform composition of the olivine and olivine-pseudomorph crystals with the skeletal nature and variable composition of the pyroxenes suggested that the magma was intruded bearing olivines, but that plagioclase and pyroxenes formed after intrusion and also that the original magma was relatively siliceous and that this was an important factor in the alteration of olivine to talc plus carbonates soon after intrusion.

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Two reports of Little Whin Sill material from the Woodside and Crawleyside boreholes give us an analysis of the whinstone in its coarser-grained, unchilled state.
Harrison noted its petrological resemblance to Great Whin Sill material. Both have a generally subophitic texture although The Little Whin Sill is less dense with more alteration to its mineral content and it has a mesostasis that gives it a tholeitic character.

Plagioclase makes up 47% of the rock occurring as labradorite in the groundmass, as oscillatory and reverse zoned microphenocrysts usually with labradorite cores, and also as similarly composed, slightly larger, shadow-zoned phenocrysts with associated late-crystallising quartz.
Pleochroic orthopyroxene phenocrysts are present but rare and the clinopyroxene which makes up 23% of the rock occurs in granular and prismatic forms with some alteration to carbonate, chlorite and quartz. Quartz, orthoclase, chlorite and carbonates make up the the interstitial matrix.
Accessory minerals are hornblende, biotite and apatite.
As we said above, he found no fresh olivine and only rare pseudomorphs after olivine.

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The Crawleyside borehole identified the 20.6 metre thick Little Whin Sill material as occurring in the Three Yard Limestone and having two distinct layers – a relatively coarse-grained one measuring 6 metres lying above a fine-grained one that is chilled over it’s lower metre. Berridge wondered if, accepting that the coarse layer might, ‘relate to volatile accumulation towards the top of the sill’ these two facies, might suggest multiple intrusions of the Little Whin Sill in this vicinity.
He thought the sill material encountered here to have more in common with the Great Whin Sill material collected from the Throckley borehole to the east than that from the Little Whin Sill collected from the nearby Woodland borehole to the south. The coarse-grained component of the Little Whin Sill here was subophitic with, ‘three optically distinct clinopyroxene types, with some microphenocrysts showing zoning from augite to sub-calcic augite and others showing structurally continuous rounded cores of pigeonite’ – like those in the Great Whin Sill.
Berridge was keen to point out differences between the coarse- and fine- grained rock. Orthopyroxene is less common than the clino- variety in both but it tends to be prismatic and often uralised in the coarse-grained material, but smaller and fresher in the fine-grained material. Again, there is abundant iron-titanium oxides in both facies but most of it in the fine-grained material is relatively large sub-hedral to sub-skeletal grains – Berridge noting this as an indicator of chilling in whinstones generally.

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Interestingly, he found a larger mesostasis component in the coarse- grained facies than in the fine-grained one and identified that in the former, quartz is dominated by a micrographic inter-growth of quartz and potassium feldspar, while in the latter only quartz appears.
The two facies differ again in that accessory apatite is common in the coarse-grained rock while acicular actinolite is not, but in the fine-grained rock, the reverse is true.
Pleochroic biotite and hornblende is scarce but evenly distributed throughout the coarse-grained rock.
Berridge records no olivine or pseudomorphs after olivine in either facies.

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References

R.K. Harrison, 1962, Petrology of the Little and Great Whin Sills in The Woodland borehole, Co. Durham

N.G. Berridge , 1982, Petrography of the Little Whin Sill and Great Whin Sill, Crawleyside (H.1) borehole, Weardale, County Durham, Rep. Inst. Geol. Sci., No82/1, pp 55-57

G. A. L. Johnson and K. C. Dunham, 2001, Emplacement of the Great Whin Dolerite Complex and the Little Whin Sill in relation to the structure of northern England, Proceedings of the Yorkshire Geological Society, Vol. 53, Part 3, pp. 177-186.

Stone, P, Millward, D, Young, B, Merritt, J W, Clarke, S M, McCormac, M and Lawrence, D J D. 2010, British regional geology: Northern England. Fifth edition. Keyworth, Nottingham: British Geological Survey.

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