the geology of ireland part one. some useful terms basement [to a particular sequence] – rocks...
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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”
Main Tectonic Zones
Main Tectonic Zones
A history of two halves!
Highly Deformed Rocks!
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.
Late Neoproterozoic(circa 600 Ma)
(from Dalziel, 1997)
Meanwhile, a long way away
• What has SE Ireland been up to?
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
Newfoundland
Nova Scotia
New England
Distribution of Avalonian Basement
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
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?
The Iapetus ocean
Main Tectonic Zones
Iapetus:Father of Atlas
Atlantis in Greek, Ἀτλαντὶς νῆσος, "island of Atlas“
So the Iapetus Ocean is a precursor of the Atlantic
Additional Reading