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