Presentation on theme: "The Geology of Ireland Part One. Some useful terms Basement [to a particular sequence] – rocks with a previous orogenic history Terrane – “A fault-bounded."— Presentation transcript:
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”
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)
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?