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Cushendun Bay and Rock PortAntrim
Cushendun Bay, Co. Antrim.
Summary Full report
Site Type: Coastal section
Site Status: PASSI
Grid Reference: D252335
Rock Age: Precambrian (Dalradian)
Rock Name: Cushendun Granite, Glendun Formation, Granite
Rock Type: Feldspar porphyry, Metabasite, Porphyry, Quartz-feldspar porphyry, Schist
Minerals: Albite
Other interest: No data, No Data

Summary of site:

This site has many facets of interest and one that justifies international recognition.

The Glendun Formation is the topmost and final formation of the Southern Highland Group. It is the last division of the ancient Dalradian rocks, with an end date around 595 million years ago. The rocks are well exposed at Rock Port where they consist of grey and green tinged schists containing the minerals albite, chlorite, biotite and tourmaline. Well formed crystals of albite are conspicuous in some bands, readily recognised by their pink colouration on weathering. Towards the close of the Dalradian there were substantial submarine basins or hollows on the margin of a supercontinent very close to the south pole of the time. Sandstones and similar silica rich sediments regularly slid off the shallow coastal margins in the form of turbidity currents and settled across the beds of these basins in thick blankets. This was the original condition of the Glendun Formation. Sometime later, after solidifying, molten magma opened and filled cracks in the sediments which then solidified into dolerite dykes. The South Pole was ice-free at the time and the supercontinental surfaces were not only barren but also being relentlessly eroded. About 130 million years later, during the Ordovician period, the collision of a continental margin across the island arc of the Tyrone igneous complex folded and deeply buried the Dalradian rocks, including the Glendun Formation (an event called the Grampian orogeny). They were metamorphosed by the high temperatures and pressures - the sandy sediments became schists and the dolerite dykes became metabasites, so clearly exposed in Glendun Bay (particularly at Rock Port). At least three stages of deformation of the schists can be identified in the immediate area, all related to varying stresses during the regional metamorphism. At roughly the same time as the metamorphism, two miles to the north west, a mass of molten rock was migrating upwards through the collision zone where it became trapped and slowly cooled and solidified. This mass, the Cushendun Granodiorite, had other local effects as its late stage molten material intruded the newly formed schists. These intrusions can be seen to the north and south of Rock Port where they take the form of sheets of a distinctive rock called porphyry. It is pinkish brown and contains large oligoclase crystals (phenocrysts) showing zoning (reflecting compositional changes while the crystals were growing) in a quartz-rich, fine grained groundmass. Related to the porphyry intrusions are veins of a coarsely crystalline rock called a pegmatite. It formed in the closing stages of granite cooling when fluids, rich in volatiles, invaded voids in the schists and slowly cooled allowing crystals more than 2.5cm long to grow, in this case of albite (a feldspar mineral). The area is also famous because its rocks were an essential part of a serious academic controversy about the origins of albite schists. The original view, argued by Bailey and McCallien in 1934, was that the schists merely reflected the chemical composition of the original sediments, probably albite-rich muds, altered during metamorphism. However, in 1942 Doris Reynolds took a radically different view and proposed that hot fluids percolated the rocks during metamorphism and introduced sodium (essential for albite formation) into the quartz-rich rocks as well as potassium and possibly iron. Reynold’s work was a fundamental contribution to the understanding of these rocks worldwide, making this a vital reference site in international geology. This is a site of national and international importance and should be designated and fully protected. There are currently no obvious threats.

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