The Morpeth Dyke

The Morpeth dyke has been, and continues to be, something of a mystery.
The 19th and 20th century geologists who investigated the minor igneous intrusions of Northern England found it’s especially dark colour, high density, variable texture and it’s olivine content, very puzzling.
Presently, we are finding its location a bit of conundrum.

Location of the Morpeth dyke

In ‘Outlines of the Geology of Northumberland’ published in 1878, G.A. Lebour located the dyke close to Morpeth railway viaduct, writing, ‘The dyke … crosses the Wansbeck a few yards below the viaduct of the North Eastern Railway.’ The 1871 geological map of the area that Lebour helped to create provides us with a more definite location for the dyke’s intersection with the Wansbeck as well as showing its length and trend. Subsequent maps, up to the one published in 2001, present more or less the same information.

A map showing the location of the Morpeth dyke and bedrock of the area

Map based on pre-2001 BGS maps showing the location of the Morpeth dyke and the bedrock of the area

Key to bedrock types

Key to the bedrock types on the map showing the location and trend of the Morpeth dyke

In 1948, as part of a larger study, Bruckshaw and Roberson investigated the residual magnetism of the Morpeth dyke – the magnetism that remains after its induction in the rock’s magnetite by the earth’s magnetic field at the time of its crystallisation. [The study confirmed that in Paleaogene times, the earth’s poles were the reverse of their current positions]
Referring to the echelon of dykes that runs through Morpeth, Kielderhead and Beith in Ayrshire as ‘the Morpeth dyke’, they write, ‘very few exposures of the Morpeth dyke exist but as this one is the only olivine-bearing member of the (palaeogene tholeiite) series it is readily identified.’ They also note that the restricted exposure meant they were unable to fully investigate the dyke and that, ‘the magnetic anomaly at Beith and Morpeth on the Morpeth dyke was found some distance (up to 350 ft.) from the position marked on the geological map,’ suggesting that the dyke was present but not visible.
The position and trend of the dyke is shown on the map that accompanies their study but it is too imprecise to help with its accurate location in the field.

Lawrence and Jackson make a brief reference to the dyke at Morpeth in their 1990 BGS technical report for the Department of Environment. They write that it is likely to be a continuation of the West Sleekburn dyke, it had been over 4 metres in width and that it is, ‘said to have been quarried west of Climbing Tree at grid ref. 2614 8127.’
Presumably, this reference should have read 2164 8627 so as to correlate with their location of the south-east portion of the dyke on their map of tholeiite dykes.

The dyke does not appear on the 2001 map that followed the 1968-1996 re-survey and it does not appear on the current British Geological Survey online map.

Looking up the side of the gorge to the probable location of the old  Morpeth dyke quarry left of the central mound.

Possible location of the SE end of the Morpeth dyke: looking up the east side of the gorge from the public footpath at NZ215862.
We found dyke material in the channel to the left of the central mound.

We have looked for the south-east end of Morpeth dyke from ‘a few yards below the viaduct’ to the location at Climbing Tree and beyond, but we haven’t found an outcrop..
West of Climbing Tree, a few hundred yards downstream from the viaduct, a channel covered in surface deposits and vegetation runs down the east side of the gorge to the River Wansbeck; perhaps this is the outcome of quarrying for the dyke rock and so marks the course of this portion of the dyke. This channel appears to continue for a short distance on the opposite bank of the river.

Although we haven’t found an outcrop of the dyke at the south-east end, we have found what appear to be pieces of it. Our first find was in the river bank directly in line with the channel, and later, in early March 2016 when vegetation was low, we found a second piece half-buried in the ground half-way up the channel. Both samples of rock conform to the descriptions of the Morpeth dyke petrography that follow – some evidence that supports the report of the dyke having been quarried here.

Finding evidence for the location of the north-west end of the dyke presents even more difficulty.
In a particularly dry spell in the summer of 2015 when the river was very low, we were able to get out some way from the bank of the river at NZ 208 865. There we found a number of very dark-looking basalt rocks measuring up to 20 cm embedded into the river bed. These basalt pieces didn’t appear elsewhere either side of the location. We also found smaller pieces of black glassy material.

The river bed at the location- of the north-east end of the Morpeth dyke NZ208865

The river bed at the location of the north-west end of the Morpeth dyke, NZ208865

Macroscopically, the texture of this basalt differs from that found at the south-east end. The iron-titanium crystals are larger and more isolated and dark interstitial glass is all but absent. However, It does have sparse, individual plagioclase phenocrysts of a labradorite composition. Microscopically, it shows the same pleochroic alteration material as appears in the rock found at the south-east end although there is less of it. So, we are uncertain whether this rock originated in the Morpeth dyke – if it did, then the character of the dyke rock here at the NW end is different to that at the SE end.

Petrography and Petrology

JJH Teall was the first to analyse the rock and grouped the Morpeth dyke with the Brunton, Tynemouth, Hebburn and Seaton and Hartley dykes because of their common; ‘porphyritic felspars and amygdaloids…… long lath-shaped feldspar sections, irregular grains and plates of pyroxene, and interstitial matter, with its various devitrification products.’ However, he thought the Morpeth dyke’s darkness, high density, ‘close’ crystallinity and, most importantly, its comparatively abundant fresh olivine’ together with the olivine’s ‘green and brown serpentinous pseudomorph,’ all contributed to making it especially interesting.

South-east end of the dyke, river bank

Polished section of rock found in the river bank at NZ 215 862. The portion shown is 32mm across and is viewed in reflected light.

Polished section of rock found in the river bank at NZ 215 862
The portion shown is 32mm across.

Whole-slide thin-section of rock found in the river bank at NZ 215 862 viewed in plane polarised light.

Thin section from the same sample viewed in plane polarised light.This section measures 42 mm across.

Morpeth dyke rock viewed with crossed polarising filters.

Portion of a thin section from the same sample viewed with crossed polarising filters

South-east end of the dyke, halfway up the bank

Tholeiite basalt from a small boulder found halfway up the channel at NZ 2161 8628.

Tholeiite basalt from a small boulder found halfway up the channel at NZ 2161 8628.
The specimen measures 90 x 50 mm.

Prepared hand specimen of tholeiite basalt found halfway up the bank at  NZ 2161 8628 viewed in reflected light.

Prepared hand specimen of tholeiite basalt found halfway up the channel at NZ 2161 8628 viewed in reflected light and measuring 45 mm across

Thin section of tholeiite basalt boulder found halfway up the bank at  NZ 2161 8628 viewed in plane polarised light.

Thin section from the same sample viewed in plane polarised light.

Thin section of tholeiite basalt boulder found halfway up the bank at  (NZ21618628)  viewed with crossed polarising filters

The same thin section viewed with crossed polarising filters

North-west end of the dyke, in the river bed

Prepared hand specimen of basalt from the location of NW end of the Morpeth dyke at NZ 208 865.

Prepared hand specimen of basalt from the location of NW end of the Morpeth dyke at NZ 208 865

Whole-slide thin section of a sample found at the NW location where the dyke is shown to cross the Wansbeck at NZ 208 865.</strong>

Whole-slide thin section from the same sample viewed in plane polarised light.

Basalt at Morpeth Dyke NW end with XP NZ 208 865 viewed with crossed polarising filters.

The same section viewed with crossed polarising filters

Heslop and Smythe’s analysis of the Morpeth dyke rock encouraged them to draw parallels between it, and the rocks of the Collywell, Crookdene and Tynemouth dykes.
They found almost identical levels of silica, titanium, magnesia, and alkalies with slightly more aluminium in the Morpeth dyke, and slightly less iron. Given these similarities they found it remarkable that the Morpeth dyke should be rich in olivine, while the others were free from it. By way of explanation, they suggested that ‘differences of physical conditions (pressure and temperature ?), rather than … difference of chemical composition’ determined the variation in mineral development.’

Feldspar, laths, augite and olivine,with green/brown glass in the Morpeth Dyke rock. Viewed in plane polarised light.

Feldspar, clinopyroxene and olivine with green/brown glass in a sample from the SE location (NZ215862)
The section is viewed in plane polarised light

The Morpeth dyke rock in thin section showing the main constituents viewed in XP (X25). These are: fresh olivine, feldspar laths and augite with serpentine pseudomorphs and green/brown and black black vitrification products

The same sample viewed with crossed polarising filters

Heslop and Smythe’s paper to the London Geological Society, referenced below, provides a remarkably full account of their samples. Here’s how they describe the olivine content:
‘Most of the olivines are decomposed, but many fresh specimens occur; they are colourless and are traversed by deep, dark green grooves roughly at right angles, and by occasional curved cracks. The interference-colours are brilliant.
When decomposed, the whole crystal becomes replaced by green pleochroic material, arranged apparently in bundles parallel to the cracks in the original crystal. It is not easy to fix the relative age of the olivine. The fresh pieces never have good outlines, and most of the decomposed pieces present the rounded appearance suggestive of magmatic corrosion.
Several sections have been observed included in augite, and therefore, if they belong to the ground-mass generation, they are certainly older than augite.’

Alteration of olivine resulting in a serpentinite pseudomorph

Alteration minerals forming pseudomorphs in a sample from the SE location (NZ215862)
The section is viewed in plane polarised light

Alteration of olivine in Morpeth dyke rock resulting in serpentinite pseudomorphs: viewed with crossed polars.

The same sample viewed with crossed polarising filters

Pleochroic alteration mineral in a sample from the location at the NW end of the dyke (NZ208865) viewed in plane polarised light

Pleochroic alteration mineral in a sample from the location at the NW location (NZ208865)
Section viewed in plane polarised light

When the slide is rotated the centre of each cell can be seen to darken. Sample viewed in plane polarised light

With the slide rotated, the centre of each bundle darkens
Sample viewed in plane polarised light

Of the feldspar content, Heslop and Smythe write, ‘The porphyritic felspars are much smaller than those of the Tynemouth Dyke, and they occur in groups of two or three inter-grown or in single crystals.
They are always surrounded by a zone which, as a rule, furnishes the crystal faces.’
The porphyritic feldspar crystals in the sample from the south-east end do gather to form glomerocrysts while those in the samples that we collected from the north-west end are solitary.

Plagioclase Glomerocrysts in Morpeth dyke rock found in the river bank PPL

Plagioclase glomerocryst in the rock found in the river bank at NZ215862 viewed in plane polarised light

The same plagioclase glomerocryst viewed with crossed polars

The same plagioclase glomerocryst viewed with crossed polars

Plagioclase phenocryst with augite inclusions in a sample from the location at the NW end of the dyke (NZ208865)

Plagioclase phenocryst with augite inclusions in a sample from the north-west location (NZ208865)
The section is viewed with crossed polarising filters

‘The amygdaloids are usually darker at the circumference than at the centre, and between crossed nicols the latter remains isotropic, or is only faintly illuminated, while the circumference shows bright interference-colours and a spherulitic arrangement of fibres.’

An amygdaloid in the Morpeth dyke rock probably containing natrolite

An amygdaloid in the rock collected at the south-east location (NZ215862) viewed in plane polarised light

The same sample viewed with crossed polars (X25)

The same sample viewed with crossed polars
The spherulitic mineral towards the circumference of the amygdaloid has low birefringence and negative relief and is probably natrolite.

By 1929, Holmes and Harwood could relate the Morpeth-Blyth zone of dikes to those exposed at the headwaters of the North Tyne at Deadwater and Kielderhead and further north at Middleton (Lugton) on the Ayrshire coast, these forming a distinct belt of olivine-bearing dykes distinct from the Hebburn ( Brunton) dyke.
They classified the Morpeth dyke as a Salen type which is typically, ‘even grained finely crystalline dark-grey rocks, usually without any trace of porphyritic constituents. Microscopically, they are seen to be composed mainly of augite and labradorite felspar, with subordinate olivine, iron-ore, and a variable quantity of residual glassy matter of relatively late consolidation. The augite is, usually, at least as prevalent as the felspar.’
However, as we have seen, the Morpeth dyke has both glomerocrysts of plagioclase phenocrysts and olivine, so Holmes and Harwood suggested that. ‘the Morpeth dike thus represents an approach to the Brunton type, of which the Tynemouth dike is an anorthite-bearing example.’

References

Lebour, G A, 1878. Outlines Of The Geology Of Northumberland. M & M N W Lambert, Newcastle upon Tyne.
Teall, J J H. 1884. Peteological Notes On Some North-Of-England Dykes. The Quarterly Journal Of The Geological Society Of London, Vol. 40. Pp. 209-247.
Heslop, M K. and Smythe, J A. 1910. On The Dyke At Crookdene (Northumberland) And Its Relations To The Collywell, Tynemouth:, And Morpeth Dykes. The Quarterly Journal Of The Geological Society Of London, Vol. 66. Pp. 1-18.
Holmes, A and Harwood, H F. 1929. The Tholeiite Dikes Of The North Of England. The Mineralogical Magazine and Journal Of The Mineralogical Society, No. 124. Vol 22.
Bruckshaw, J McG., and Robertson, E I. 1948. The Magnetic Properties Of The Tholeiite Dykes Of North England. Geophysical Supplements to the Monthly Notices of the Royal Astronomical Society.
Lawrence, D J D and Jackson, I. 1990. Morpeth-Bedlington-Ashington Technical Report Wa/90/19, Geology And Land-Use Planning: Morpeth-Bedlington-Ashington Part 2. Geology. Keyworth, Nottingham: British Geological Survey.
British Geological Survey. Sheet 14, Morpeth. http://www.bgs.ac.uk/data/maps/maps

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