Bingfield Dyke


Location of the Bingfield dyke

Bingfield dyke is exposed in the Redhouse Burn limestone on the south bank of the Redhouse Burn south-east of Bingfield.

The Redhouse Burn at NY971719 where tholeiite basalt stones and boulders lie in the stream bed

Map showing the location of the dyke and the surrounding rock types

Petrography and Petrology

Teall gave an excellent account of the dyke but mistakenly believed he was describing what Lebour had previously called the Brunton Dyke, a dyke now known as the St. Oswald’s Chapel Dyke, one of the Whin Sill-related Carboniferous dykes with typical quartz-dolerite features.
He described the ground-mass of the Bingfield dyke rock as being dark or greenish-grey and finely crystalline ‘portions of which bear a very close resemblance to the ground-mass of the Tynemouth dyke’. He estimated the width of the dyke to be 20 t0 25 feet with a 12 to 14 feet margin at the north-west that was highly altered with poorly crystallised feldspars, zones of iron oxide at bounding surfaces and joint planes, and abundant amygdaloids of calcite and chalcedony.
Feldspar crystals in the unaltered portions were seen to belong and narrow while the pyroxenes crystals wereirregular and almost colourless.
He noted that the iron-titanium oxides had remained part of the interstitial material.

Thin section of tholeiite basalt at Bingfield Dyke, NY972719
Thin section viewed in plane polarised light
The same thin section viewed with crossed polarising filters

Holmes and Harwood made the Bingfield dyke as the type-example of the ‘Brunton’ type of tholeiite basalt.
The comparatively small plagioclase phenocrysts that make up the few glomerocrystys in the rock show external zoning and an interior composition of approximately An<sub>88</sub>.
The many randomly distributed felspar laths in the rock matrix have an approx. An<sub>60</sub> composition at their centres of crystals ranging to approx. An<sub>40</sub> at their borders.
Holmes and Harwood wrote that the pyroxenes in the basalt were colourless to pale grey-green plates and interdigitated granules with augite prevalent and enstatite or hypersthene present in smaller quantities.
They agreed with Teall that the skeletal crystals and tapering rods of the iron-titanium oxides, along with some pyrites, appear only in the dark brown mesostasis that constitutes about a third of the rock.
They noted there were areas of the rock in which the mesostasis cleared to patches of calcite including or bordered by pyrite and also that calcite commonly occurred in the amygdaloids along with chalcedony and pyrite.

Their modal analysis of the Bingfield dyke established mineral proportions of:
34.7% Mesostasis
32.3% Pyroxene
24.6% Feldspar
– and a chemical composition that was very similar to the Kielderhead dyke.

Zoned plagioclase glomerocryst in Bingfield dyke basalt
Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)
The same area viewed with crossed polarising filters

The plagioclase in our samples is mostly in the form of broken, twinned laths with a very few plate-like crystalsand even fewer crystals that could be said to be glomerocrysts.
In common with the other glomerocryst-bearing tholeiite basalts of Northumberland, these are more calcium-rich than the laths and are clearly zoned.

Zoned plagioclase glomerocryst in Bingfield dyke basalt
Section viewed with crossed polars (FoV 1.2 x 0.8 mm)

The percentage of clinopyroxene is much greater than that of orthopyroxene in our samples. The former tends to be in granular masses that often show signs of alteration, while crystals of the latter are stumpyandrelatively unaltered.

Orthopyroxene in Bingfield dyke basalt
Section viewed in plane polarised light (FoV 1.2 x 0.8 mm)
The same area viewed with crossed polarising filters
Augite in Bingfield dyke basalt
Section viewed in plane polarised light FoV 1.2 x 0.8 mm)
The same area viewed with crossed polarising filters

The iron-titanium minerals tend to form clearly defined crystals in association with the brown mesostasis.
Amygdaloids are frequent and are composed of a mixture of calcite, a quartz mineral, possibly chalcedony, and an almost opaque mixture that is full of opaque particles, rods and micro-crystals.

Iron-titanium oxides in association with brown mesostasis in Bingfield dyke
Saection viewed in p[lane polarised light (FoV 2.3 x 1.5 mm)

Amygdaloid in Bingfield dyke basalt
Section viewed in plane polarised light (FoV 2.3 x 1.5 mm)

The same area viewed with crossed polarising filters

Some of the minerals in the rock show signs of alteration with some crystals appearing as pseudomorphs, possiblyafter pyroxene.
The Bingfield dyke is intruded into limestone and number of calcite veinlets traversed our samples with some quartz, perhaps chalcedony, appearing towards the edges.

Alteration in Bingfield dyke basalt
Section viewed in plane polarised light (FoV 1.2 x 0.8 mm)
The same area viewed with crossed polarising filters
Calcite vein, with accompanying quartz, in Bingfield dyke basalt
Section viewed in with crossed polarising filters (FoV 2.3 x 1.5 mm)

Fossils in the Redhouse Burn Limestone in the banks of the Rehouse Burn at NY971719 Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)
Limestone with fossils and quartz grains in the banks of the Redhouse Burn at NY971720 Section viewed in plane polarised light (FoV 4.6 x 3.0 mm)

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.

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.

British Geological Society, Sheet 13 Bellingham, http://www.largeimages.bgs.ac.uk/iip/mapsportal.html?id=1001481


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