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St. John's PointDown
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Summary Full report
Site Type: Coastal section
Site Status: PASSI
District: Down District Council
Grid Reference: J527333
Rocks
Rock Age: Tertiary (Eocene, Palaeocene)
Rock Type: Basalt, Dolerite, Spilite, Trachybasalt
Interest
Other interest: dyke, phenocrysts, Intrusion

Summary of site:

During the early Tertiary period about 60 million years ago, tectonic plate movement stretched the crustal rocks in the area of the north Irish Sea resulting in a series of north west/south east fractures that were injected with molten, mostly basaltic, magma from below. This episode broadly coincided with the major eruption from fissures of similar orientation further north that created the Antrim Basalts.

From Guns Island in the north east to Ringsallin Point 4 km south east of Ballykinler, large numbers of dykes can be seen along the shore, injected into the local Silurian sediments of the Hawick Group. The highest density occurs on the east coast of the peninsula from just south of Killough to the lighthouse at the southern tip of St Johnís Point. The dykes show variations, some resulting from a single intrusion, others with two or more intrusions of the same magma and yet others with multiple intrusions of different magmas. The dykes all show variations of basaltic composition, usually with some olivine and they range in thickness from centimetres to over 8 m, averaging about 3 m. The dykes intersect the intensely folded Silurian rocks without deviation showing them to be later than the folding. The combined thickness of all the dykes indicates an extension of the crust of over 2% showing the magnitude of the stretching in the area. Microscopic inspection of the dolerites confirms them as olivine dolerites with labradorite as the dominant feldspar with augite, iron ore and a few subsidiary minerals. Occasionally the olivines appear as large crystals (phenocrysts) in a fine groundmass giving a porphyritic texture and a few of the dykes with multiple intrusions of different magmas have oligoclase-albite feldspar phenocrysts. The geochemistry of this dyke swarm shows it to have closer affinities with the alkali-enriched plateau magma of the Skye Lava Series than the main basalts of the Antrim Plateau. This is only one of several dyke swarms in Northern Ireland and it shows interesting differences in comparison with the Antrim lavas. There is some evidence that the magma feeding the dykes began to separate into two types (fractions) but no great difference results, certainly not the range of types seen around the Mournes to the south west.


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