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Mournes - Glasdrumman Port Cone-sheetDown
PHOTO TO BE ADDED Glassdrumman Port, Mournes, Co. Down.
Summary Full report
Site Type: Coastal section
Site Status: PASSI
District: Newry & Mourne District Council
Grid Reference: J382232, J380223
Rock Age: Tertiary (Eocene)
Rock Name: Granite
Rock Type: Basalt, Porphyry
Other interest: cone-sheet, magma mixing, Intrusion

Summary of site:

On the shore between Glasdrumman Port in the south, to just north of George's Quay in the north, there is a segment of a dyke inclined steeply inland. It is part of a much larger dyke structure which rings the Mourne Mountains to the east and south in the Silurian country rock surrounding the granites. In all exposures it is inclined inwards towards the roots of the Mourne granites.

The interest of this inclined sheet of rock lies in the form of the intrusion and in its composition which reveals an incongruous mixture of rock types. The rock that is chilled against the walls of Silurian grits is hornblende-olivine basalt of the kind found in mobile lavas but here it encloses a rock in the middle of the dyke of granitic composition, a quartz-feldspar porphyry of larger grainsize containing feldspar phenocrysts (larger crystals in a finer-grained matrix) up to 25 mm across. There is a sharp boundary between the two but no evidence that the porphyry chilled against the basalt suggesting that the basalt was still hot when the porphyry was injected. The basalt incorporates angular fragments of the Silurian grits (xenoliths) presumably detached and incorporated as the fracture opened but it also contains alien crystals (called xenocrysts) of quartz and alkali feldspar not normally found in basalts. The porphyry encloses many basalt fragments near its junction with the basalt forming a kind of hybrid rock with minerals typical of basalts, such as hornblende. This extensive arc structure around the Mourne granites is a cone sheet which takes the form of an inverted cone or funnel, more than 14 km in diameter around the eastern Mourne granites. Pressure exerted on the roof of the magma chamber (the mass of molten rock at depth) initiated a circular fracture in the country rock above which propagated upwards and outwards in all directions to create the cone form. Molten rock was forced into it at high pressure to create the dyke-like cone sheet seen at Glasdrumman Port. The slope of this cone suggests a focus about 5 km below the centre of the granites. Cone sheets are commonly associated with deeply buried igneous intrusions and they form before the main molten mass begins its upward migration. Two explanations have been offered for this cone sheet. The earlier believed that the basalt had been breached by the porphyry and blocks of basalt were remelted and remained molten after the porphyry had solidified. The more recent explanation sees the original basalt magma incorporating crystals from another, co-existing, magma at depth (not necessarily the porphyry of the cone sheet) to be followed by the final pulse of quartz feldspar porphyry magma. Obviously the two magmas chilled against each other in some places while still liquid. This site shows a rare state of affairs in the development of the Mournes cone sheet and its good exposure makes it an ideal teaching and demonstration venue that should be protected from unsympathetic or obscuring coastal developments.

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