Tynemouth Dyke

Location of the Tynemouth dyke

The Tynemouth dyke is exposed on the intertidal zone of the shore just to the north of the west end of the Tynemouth North Pier at NZ 3750 6929 where it is almost 4 metres wide and also by the side of the road leading to the pier at NZ 3738 6931 where it is almost 3 metres wide. The dyke is also reported to outcrop in the cutting to the north of Tynemouth station after which it continues underground westwards almost as far as Killingworth.

A map of the Tynemouth area showing the roads, the bedrock, the sands and Tynemouth dyke.

Location of theTynemouth dyke

Key

Key to the Tynemoth dyke map
The Tynemouth dyke at  NZ 3750 6929 viewed from the platform at the north-west end of the pier.

The Tynemouth dyke at NZ 3750 6929 viewed from the platform at the north-west end of the pier.
The dyke is to the right of the red-brown sandstone that it intrudes.

The black tholeiitic basalt of the Tynemouth dyke, largely covered by boulders and pebbles, projecting out into the sea roughly in line with the pier.

The black tholeiitic basalt of the Tynemouth dyke, largely covered by boulders and pebbles, projecting out into the sea roughly in line with the pier

Tynemouth dyke outcropping by the side of the road that leads to the pier at NZ 3738 6931

Tynemouth dyke outcropping by the side of the road that leads to the pier at NZ 3738 6931

Tynemouth dyke outcropping behind the monument on the pier road at NZ 3738 6931 .jpg

Tynemouth dyke outcropping behind the monument on the pier road at NZ 3738 6931

Petrography and Petrology

The dyke rock exhibits the Brunton texture in which small, typically 0.4 x 0.04 mm, plagioclase laths are partially embedded in granular clusters of augite. The laths are labradorite in composition and although their orientation is in the main random, they can occasionally be seen to extend radially from the augite. D.H. Land who surveyed the area between 1959 and 1962, made the observation that those parts of the plagioclase laths in contact with augite are less well developed than other parts, probably due to suppression of plagioclase crystal growth by developing augite crystals. Labradorite that developed later than the augite tends to form ‘anhedral plates without twinning and undulatory extinction’.
Plagioclase laths can also be seen to have been re-oriented tangentially around vesicles as these expanded in the magma under pressure of trapped gases.

Whole slide sample of Tynemouth dyke rock  from fore-shore viewed in reflected light.

Whole slide sample of Tynemouth dyke rock from fore-shore viewed in reflected light

Whole slide sample of Tynemouth dyke rock from fore-shore viewed in plane polarised light.

The same slide viewed in plane polarised light

Whole slide sample of Tynemouth dyke  rock  from fore-shore viewed with crossed polarising filters.

The same slide viewed with crossed polarising filters

The Tynemouth dyke tholeiitic basal  showing plagioclase laths in association with granular augite, calcium-rich plagioclase phenocrysts,mesostasis and amygadales.

The Tynemouth dyke tholeiitic basalt showing plagioclase laths in association with granular augite, calcium-rich plagioclase phenocrysts, mesostasis and amygadales.
Sample viewed with crossed polarising filters at x10

Amygdale in Tynemouth dyke viewed with crossed polarising filters. Plagioclase lathes adjacent to the amygdale have been re-oriented tangentially.

Amygdale in Tynemouth dyke viewed with crossed polarising filters
Plagioclase lathes adjacent to the amygdale have been re-oriented tangentially.

In addition to the optically related augite and plagioclase laths, phenocrysts of unaltered calcium-rich plagioclase occur. These are zoned with composition changing, according to Land, from approximately 90% anorthite at their cores to approximately 60% anorthite at their margins. In one sample, Land identified spinel inclusions in stumpy 0.6 x 0.3 mm anorthite crystals which led him to speculate that these minerals formed together, deep in the magma chamber ‘through the metamorphism of aluminous xenoliths by the tholeiite magma.’
Rarely, augite-rimmed orthopyroxene can be found in the groundmass and very rarely, chlorite occurring most likely as a replacement for olivine.
The interstitial grey-brown mesostasis contains most of the opaque iron-titanium minerals in the rock along with fine feldspar needles and pyroxene granules.
Spheroidal amygdale are common. They typically measure 1 mm in diameter and contain mesostasis, chlorite or carbonates. Teall observed that chalcedony and calcite tended to fill vesicles that had only partially been filled with mesostasic glass and that the number of amygdale is in inverse proportion to the number of anorthite-rich plagioclase phenocrysts.
Land wrote that the dyke rock in contact with the host sandstone on the foreshore has been altered so that ‘augite has been altered first to carbonate, owing to the reaction of volatiles (particularly carbon dioxide) with Mg and Ca to produce dolomite and calcite, releasing Fe to form goethite and SiO2, to chalcedony.’
More advanced alteration can result in the mesostasis and augite being altered to carbonates and clays.

Twinning in a cluster of calcium-rich plagioclase in Tynemouth dyke rock. Sample viewed with crossed polarising filters.

Twinning in a cluster of calcium-rich plagioclase in Tynemouth dyke rock.
Sample viewed with crossed polarising filters.

References

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.
Land D.H. et al 1974.Geology of the Tynemouth District (Explanation of One-Inch Geological Sheet, 15, New Series) HMSO.
British Geological Society, Sheet 15 Tynemouth, http://www.largeimages.bgs.ac.uk/iip/mapsportal.html?id=1001485

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