The Geology of Ireland -1
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 Deformend 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.
• Our two Irish Palaeoproterozoic (c. 1.75 Ga)
gneiss complexes (the Annagh Gneiss and
Rhinn’s Complexes) 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 calcalkaline arc complexes
• The only fragment of Avalonian
basement in SE Ireland is the Rosslare
Complex
Distribution of Avalonian Basement
Newfoundland
Nova Scotia
New England
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