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The Geology of Ireland

The Geology of Ireland. Part One. Some useful terms. Basement [to a particular sequence] – rocks with a previous orogenic history Terrane – “A fault-bounded geological entity of regional extent characterized by a geological history that is different from the history of contiguous terranes”.

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The Geology of Ireland

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  1. The Geology of Ireland Part One

  2. Some useful terms • Basement [to a particular sequence] – rocks with a previous orogenic history • Terrane – “A fault-bounded geological entity of regional extent characterized by a geological history that is different from the history of contiguous terranes”

  3. Main Tectonic Zones

  4. Main Tectonic Zones

  5. A history of two halves!

  6. Highly Deformed Rocks!

  7. A Grenvillian Supercontinent (Rodinia) • At the end of the c. 1 Ga Grenvillian orogeny, all landmasses formed one supercontinent, called Rodinia. • From about 750 Ma to 550 Ma, Rodinia rifts and breaks up. The two largest continents produced by this rifting episode are Gondwana and Laurentia. • The Irish Palaeoproterozoic (c. 1.75 Ga) gneiss complex – the Annagh Gneisses – form part of Laurentia.

  8. Late Neoproterozoic (circa 600 Ma) (from Dalziel, 1997)

  9. Meanwhile, a long way away • What has SE Ireland been up to?

  10. Avalonia • Unlike Laurentia, Avalonia is a small crustal block. Avalonia basement is restricted to Nova Scotia, Newfoundland, SE Ireland, S England and Brittany • The majority of the Avalonian basement is comprised of c. 630 to 590 Ma calc-alkaline arc complexes • The only fragment of Avalonian basement in SE Ireland is the Rosslare Complex

  11. Distribution of Avalonian Basement Newfoundland Nova Scotia New England

  12. Rosslare Complex • Two units – Kilmore Quay Group and the Greenore Point Group • The two units were metamorphosed and then intruded by a gabbro, the St. Helen’s Gabbro • Radiometric dating of metamorphic minerals suggest that the deformation is is c. 620 Ma, i.e. latest Precambrian

  13. Recognising the Iapetus Suture • How do we recognize that a substantial ocean was formerly present between two blocks of crust (i.e. NW and SE Ireland)? • Although the crust on the opposite sides of Ireland have different geological histories - they have undergone different orogenic episodes, this is not necessarily evidence they are separated by a large ocean. • Two principal methods: palaeomagnetism, and faunal provinciality • What is the main prerequisite for faunal provinciality?

  14. The Iapetus ocean

  15. Main Tectonic Zones

  16. Iapetus:Father of Atlas Atlantis in Greek, Ἀτλαντὶς νῆσος, "island of Atlas“ So the Iapetus Ocean is a precursor of the Atlantic

  17. Additional Reading

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