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Journal of the Geological Society; 1990; v. 147; issue.6; p. 1061-1069;
DOI: 10.1144/gsjgs.147.6.1061
© 1990 Geological Society of London

Article

Late Caledonian dolerite sills from SW Connacht, Ireland

PAUL MOHR

Department of Geology, University College Galway, Galway, Ireland

Bimodal Silurian igneous activity in SW Connacht occurred during basin subsidence and sedimentation, and before basin deformation. Mafic sills averaging 3 m thickness were preferentially intruded into and supported by a thin horizon of fine-grained siliceous sediment (Tonalee Fm.). Multiple injection produced the thicker sills. A primary igneous lithology of augite–plagioclase–olivine–phyric dolerite has, in some sills, been obliterated by pervasive hydration and carbonation expressed in a biotite-calcite–quartz–albite mineralogy. This represents volatile influx into magma at a sub-intrusive level, and contrasts with syn-intrusive assimilation and post-intrusive alteration that introduced silica and potassium into, and removed magnesium from marginal dolerite. The SW Connacht magmatism occurred in an extensional tectonic setting, probably in a NNE–SSW structurally controlled basin, and was coeval with bimodal magmatism in other parts of Ireland and Britain on both sides of the Iapetus Sutures.




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D. M. WILLIAMS, P. D. O'CONNOR, and J. MENUGE
Silurian turbidite provenance and the closure of Iapetus
Journal of the Geological Society, 1992; 149: 349 - 357.
[Abstract] [PDF]