Coupled Deformation and Reaction Softening Processes: Retrograde Shear Zones in the Rosslare Complex, South-East Ireland

Abstract:East-north-east trending amphibolite facies metasedimentary and metaigneous gneisses of the Rosslare Complex, south-east Ireland, have been reconstituted into north-east trending greenschist facies mylonitic shear zones composed mainly of chlorite, sericite and opaque minerals. Modal abundances of feldspar and amphibole in metadiorite decrease and the abundances of sericite and chlorite increase with increasing retrogression. Garnet was altered to chlorite and sheared parallel to the foliation. Plagioclase feldspar underwent brittle deformation whereas quartz experienced syntectonic recrystallisation to finer strain-free grains. Following brittle deformation, feldspar was chloritised during retrogression. Published whole rock chemical analyses indicate that altered metadiorite at Greenore Point gained Mg and lost alkalis (Na, K, Rb), Sr, and Si. Some trace elements (such as Ti, Zr, P) and major elements (Fe, Al and Ca) were immobile. Aqueous fluid inclusions in syntectonic quartz veins have salinities of 5-21 wt% NaCl equivalent, and their first melting temperatures indicate two distinct compositional fluid types are present: KCl and MgCl₂-rich compositions. Balanced retrograde reactions indicate an influx of Mg-rich fluids that exchanged with alkalis in the rock. A simple petrological model involving hydration of periclase to form brucite displays several features analogous to the natural retrograde process, namely, buffering of the fluid salinity and composition by retrograde reaction, reaction-enhanced weakening resulting in retrograde shear zone formation and episodic open and closed system behaviour involving pulsed fluid flow.