The farnacht formation along the south side of the clew bay fault zone,Western Ireland: Its chemistry and age of metamorphism

The tectonically isolated Farnacht Formation consists of calc-alkaline dacitic-andesitic lavas of volcanic arc affinity. It is situated immediately to the south of the Clew Bay Fault Zone (western continuation of the Highland Boundary Fault Zone of Scotland) in the northeast corner of the Lower Palaeozoic South Mayo Trough in northwest Ireland. It has been metamorphosed to biotite grade greenschist facies following the development of a pervasive, c-s composite muscovite, quartz, and feldspar schistosity. The Farnacht Formation may comprise a terrane that is directly unrelated to nearby Ordovician and Silurian rocks; its present position was fixed largely by Wenlock times. The age of the Farnacht Formation and the deformational event(s) that produced the schistosity are not known. ⁴⁰Ar/³⁹ Ar step heating from four specimens have dated the crystallization of biotite at from 422 ± 2 to 405 ± 14 Ma with a mean age of 413 Ma. These ages date either the post-D2, pre-D3 metamorphic peak, or a hornfelsing of the same structural age related to an unseen thermal source, and provide a minimum age for the end Silurian - early Devonian Caledonian tectonothermal activity in the northeast part of the South Mayo Trough.