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Earth Science Conservation Review

Summary Full report
Mournes - Green Harbour Dyke No. 55Down
Site number:1112
ESCC:EF
Locality Type:Coastal section Status: ASSI
Grid Reference: J388248 Centroid
Google maps: 54.15393,-5.87542
County: DownCouncil area:Newry, Mourne & Down District Council
Period:Tertiary
Stages:Eocene
Lithostrat:Granite
Site Description
image: 1112b.jpg The macrovesicular member of Dyke No.55, within northern margin of intrusion; stretched vesicles vary from 1cm to 4cm in size.
The macrovesicular member of Dyke No.55, within northern margin of intrusion; stretched vesicles vary from 1cm to 4cm in size.
Highlights:
Occurrence of a multiple and composite dyke.

Introduction:
Basic and intermediate dyke rocks occupy the same fissure; the latter shows a variolitic (chill) texture, which is common among the Mourne dykes, an amygdaloidal structure, and carries xenocrysts derived from a deeper acid igneous rock.

Description:
Tomkeieff and Marshall (1935) give a detailed description and geological map of this multiple and composite dyke which they labelled No.55.

The first member emplaced in the fissure was an olivine dolerite; it is some 3-8 m wide and can be seen along the northern contact which shows an offset to the north lower down the beach. Olivine is sparse and mostly altered, plagioclase laths are labradoritic to basic andesine in composition and in ophitic intergrowth with a somewhat titaniferous augite. Chlorite is an alteration product of pyroxene, and ilmenite and magnetite are accessory minerals.

A macrovesicular variolite,
image: 1112c.jpg Final intrusion into Dyke No.55 was a xenoporphyritic variolite which transgresses earlier members at head of beach exposure; characteristic texture visible in foreground; earlier microvesicular variolite occurs above (with rusty-weathered crush veins).
Final intrusion into Dyke No.55 was a xenoporphyritic variolite which transgresses earlier members at head of beach exposure; characteristic texture visible in foreground; earlier microvesicular variolite occurs above (with rusty-weathered crush veins).
some 3 m thick, is chilled against it. The vesicles (now filled with secondary minerals and more properly called amygdales), up to 4 cm in length are usually elongated and aligned parallel to the dyke wall; they are bordered by a leucocratic zone of quartz and alkali feldspar and are filled with fascicular aggregates of hornblende. The specimen of hornblende which Thomson (1836) analysed and called 'Kirwanite' came from this dyke. This rock grades into a microvesicular variolite of comparable thickness which alone defines the southern contact against the hornfelsed Silurian slates. Thin apophyses of this variolite penetrate the country rock.

A final intrusion of a xenoporphyritic variolite,
image: 1112d.jpg Fine-grained basic igneous rock in steep contact with finely-banded Silurian hornfels, 20m N of Dyke No.55; northerly trend, narrow width and association with felsite makes this probable exposure of the solitary cone-sheet which encircles Mourne Granites.
Fine-grained basic igneous rock in steep contact with finely-banded Silurian hornfels, 20m N of Dyke No.55; northerly trend, narrow width and association with felsite makes this probable exposure of the solitary cone-sheet which encircles Mourne Granites.
about 3 m in thickness cross-cuts all earlier members of the dyke but is not markedly chilled against the earlier variolites. These variolites are andesitic in composition with tachylitic chilled edges and mesostatial glass crowded with hornblende between the variolitic growths of plagioclase and augite. The youngest member carries abundant xenocrysts of quartz and alkali feldspar as well as small fragments of granitic rock often showing a granophyric texture. (N.b. Tomkeieff and Marshall's description is somewhat confused since in the text-figure caption and in the catalogue of dykes this final intrusion is named a porphyritic variolite yet on p.265 and again in the order of intrusion listed on p.277 it is called a xenoporphyritic variolite! In the last paragraph of the same page the xenocrysts are ascribed to the vesicular variolite.)

A crush zone traverses Dyke No.55 and makes offsets in both its outer contacts. Northwards the crush affects a fine-grained basic igneous rock carrying amygdales and showing possible internal contacts. The crushed rock truncates thin veins of dolerite and to the north becomes associated with a massive light-coloured felsite. Some 80-100 m north of Dyke No.55 this felsite shows fine-grained basic margins on either side; both felsite/dolerite and dolerite/hornfels contacts are vertical and trend NNE. This intrusion is probably another expression of the cone sheet seen at Glasdrumman Port and Bloody Bridge. It is not listed by Tomkeieff and Marshall (1935), nor is it shown on the one-inch Geological Map of the Mourne Mountains, but Cole (1894) mentions such an occurrence of andesite and eurite at Green Harbour which is probably the same intrusion.
Importance:
A fine example of repeated intrusion of basic and intermediate magma into the one fissure. The rocks are representative of many other members of the Mourne dyke swarm. The nature and sequence of intrusion offers some insight into deeper seated vulcanism beneath the Mourne area.
Interpretation:
Reopening of the same fissure has brought into juxtaposition early and probably undifferentiated basic rock, and somewhat more differentiated rocks of slightly lower specific gravity which carry hornblende as a primary ferromagnesian mineral. The variolitic texture is probably a result of rapid cooling of the magma. The vesicular nature points to free degassing of the rock-melt as it moved towards the Tertiary surface. The xenocrysts and xenoliths would appear to be derived from an acid igneous rock; one can only speculate on its nature. Corrosion features around these foreign crystal and rock fragments could indicate their incorporation at greater depth.

Notes:


The dyke is well exposed at the head of the beach below HWM. Further down the beach cobbles and boulders obscure the outcrop. Landslip from the moraine above the beach may temporarily cover the exposure but the sea would eventually remove all but the largest boulders. Storm and/or spring tides may redistribute the beach sediment but it is unlikely to cover the whole outcrop of the dyke.

For Mourne Granites general information and reference list see;

Key Site 1129 - The Granites - Overview.

For general information on the cone-sheet intrusions associated with the Mourne Granites, and the Mourne Mountain dyke swarm, see;

Key Site 1131 - Cone-sheet and Dyke Swarm - Overview.
Keywords
Rocks:Dolerite, Variolite
Structures:amygdales, dyke
Relations:Intrusion
Measurements
Length:Width:~9-14mHeight:
Depth:Area:  
Access
Approach:Access to the beach is made from the Newcastle-Annalong road. At Irish Grid ref. J384246 a grassy path (National Trust) starts at a small conifer plantation and leads directly to the beach at Dunmore Head; traverse the beach north to Green Harbour. Dyke No.55 occurs just south of the Crock Horn Stream that flows under Ballagh Bridge.
Threats:Temporary burial
Map(s): Sketch maps from Plate XIX, and Fig. 7 in Tomkeieff & Marshall (1935), will
suffice to locate the dyke.
Record type: ESCR report    
Entered by: E M Porter
 Enlander, I., Dempster, M. & Doughty, P., 2026. Mournes - Green Harbour Dyke No. 55, County Down, site details. [In] Earth Science Conservation Review.
https://www.habitas.org.uk/escr/site.php?item=1112. Accessed on 2026-11-06.

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