Late Proterozoic High-pressure granulite facies meta-morphism in the north-east Ox inlier, north-west Ireland |
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Authors: | I S SANDERS J S DALY G R DAVIES |
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Institution: | Department of Geology, Trinity College, Dublin 2, Ireland; Department of Geology, University College Dublin, Belfield, Dublin 4, Ireland; Department of Earth Sciences, The University, Leeds LS2 9JT, UK |
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Abstract: | Abstract High-pressure granulite-facies gneisses in the NE Ox inlier in NW Ireland have undergone extensive Caledonian retrogression. In the local area of Slishwood, however, reworking was negligible and the gneisses (psammites, semipelites, pelites, metabasites and ultramafites) preserve evidence of P–T changes at high grade which mainly post-date pre-Caledonian polyphase deformation. Temperatures reached 850–900°C (based on garnet-clinopyroxene geothermometry and the presence of mesoperthite) during and after decompression from earlier eclogite-facies conditions (inferred from textural evidence of plagioclase release in sieve-textured augite). Subsequent cooling at high pressure is inferred from the unequivocal replacement of sillimanite by kyanite. A Sm–Nd mineral isochron (gt–cpx–plag–WR) of 605 ± 37 Ma is taken to date a point on the cooling path, and confirms the hitherto suspected pre-Caledonian age of the high-grade metamorphism. Geochemical and Sm–Nd isotopic data indicate that the protoliths were probably late Proterozoic arkosic sediments and tholeiites. Following metamorphism they apparently came to reside near the base of the crust where they slowly cooled. The eventual exhumation of these gneisses is attributed to Caledonian crustal imbrication, followed by rapid isostatic recovery. |
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Keywords: | Key-words: crustal imbrication eclogite geo-chemistry geothermobarometry high-pressure granulite facies Ireland isobaric cooling north-east Ox inlier Proterozoic P–T–t path Sm-Nd |
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