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Mournes - Gruggandoo Quartz Porphyry SheetDown
Summary Full report
Site Type: Inland exposure
Site Status: PASSI
District: Newry & Mourne District Council
Grid Reference: J202258
Rock Age: Tertiary (Eocene)
Rock Name: Granite
Rock Type: Granite, Porphyry
Other interest: cone-sheet, Contact, Intrusion, Rough moorland, sheep grazing.

Summary of site:

This locality is unique in the Mourne Mountains because it shows a cone sheet cut by the fourth of the Mourne granite intrusions. For a general account of the cone sheet see site 1111.

Following the intrusion of the first three granites in the eastern Mournes, the centre of activity shifted westwards and the final two granites, G4 and G5, were derived from there. In the general understanding of the evolution of granite batholiths, cone sheets precede the upward migration of the main mass of molten rock but in the Mournes this has been difficult to prove since the outcrops of the cone sheet fall in the surrounding country rock, outside the granite outcrop. In the early 1950s while mapping discontinuous sections of inclined cone sheets to the west of the Mournes, at Gruggandoo 3 km south and slightly west of Hilltown, Emeleus traced a porphyritic granophyre cone sheet right up to the contact with G4 where it abruptly terminated against the granite. At the margin the granite was finer grained showing that it had chilled against the country rock with its cone sheet intrusion, clear evidence that the cone sheet was solid and cold by the time the granite was emplaced. This confirmed a normal sequence of events, at least for the G4 granite and, consequently, for G5 which can be shown to be later still. The only threat to this site is caused by bracken growth in summer months but careful management could do much to mitigate the problem.

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