Constraints on the development of magmatic layering in a syntectonic mafic–ultramafic suite, NW Connemara, Ireland

Abstract

Synopsis The Dawros–Currywongaun–Doughruagh Complex of NW Connemara comprises a deformed and metamorphosed suite of syntectonic mafic to ultramafic intrusions within folded, lower amphibolite grade metasediments of the Dalradian Supergroup. Field observations and thin-section petrography have identified examples of exceptionally well-preserved modal layering and primary igneous (cumulus) textures in central areas of two of the intrusions (Dawros and Currywongaun), together with macroscopic graded bedding, slumping, and scour structures that allow layer formation to be attributed to magmatic sedimentation. Layering is occasionally chaotically folded and deformed, and as evidence of associated solid state or plastic deformation is absent, this is thought to be due to synmagmatic deformation. A magmatic fabric that is observed almost everywhere throughout the intrusions generally increases in intensity towards their margins, giving a poorly defined internal strain zonation in both. The presence of layering in these relatively small syntectonic intrusives offers insights into the way that magma chamber processes operate in tectonically active environments by highlighting the structural and petrological constraints these environments impose on layer development. This has significant implications for perceived restrictions on layer development in terms of the types of intrusions they develop in.