Stratigraphy, structure and metamorphism of Dalradian rocks of the Maumturk Mountains, Connemara, Ireland

Abstract

Appin (lower) and Argyll (middle) Dalradian metasediments and associated igneous rocks are described and a formal stratigraphy proposed. The deformation history has four main phases, D 1 to D 4 . Major "nappe" structures, formed during D 1 and D 2 , were refolded by two phases of approximately coaxial crossfolding developed during D 3 and D 4 . Two of the major F 4 cross-folds, the Connemara Antiform and Joyce's Synform, superficially dominate the structure of the area. The main penetrative fabric in the rocks, an axial planar schistosity, was developed during D 2 . The deformations were accompanied by metamorphism that reached its peak (in the Maumturks) between D 2 and D 3 (MP 2 ). The rocks were metamorphosed in the amphibolite facies and show an increase in grade from north to south across the area. It is suggested that the prograde MP 2 metamorphism, with coeval growth of staurolite and sillimanite, was partly caused by heat emanating from the complex of basic, ultrabasic and migmatitic rocks located during MP 2 in the upper limb of the F 1 Lissoughter Anticline, structurally above the present day outcrop in the Maumturks and now occurring mainly in south Connemara.