The geology of SW Connemara, Ireland: a fold and thrust Dalradian and metagabbroic-gneiss complex

Abstract

Sillimanite grade Dalradian schists composed of pelites, impure psammites, quartzites and calcite marbles, all containing amphibolites, were intruded and hornfelsed (giving hornfels sillimanite, garnet and cordierite) by a plutonic basic to acid igneous rock series now metamorphosed and folded. Important folding (denoted D 2 , D 3 , D 4 , and D 5 ) as been recognized with a major southward facing D 2 , anticline, the Cashel antiform, being repeatedly folded by D 3 , which was a very strong phase, and accompanied and followed by D 3 -D 4 , major ductile slides and thrusting. Although the meta-igneous suite includes rare peridotites (olivine Fo 75-87 ,) and anorthosites, it is mostly hornblende-augite-labradorite or bytownite-metagabbro and later quartz diorite or trondhjemite gneiss, which has injected the earlier more basic rocks. Variably preserved igneous layering in the basic rocks indicates that the whole intrusive basic sheet is flat-lying and inverted, lying on the inverted limb of the Cashel antiform. About 460 Ma ago the D 3 -D 4 , Mannin thrust carried the igneous complex, including the largely crystallized gneisses and the cover of Dalradian metasediment, over a now largely mylonitized set of acid volcanic rocks (rhyolites or ignimbrites) which enclose a separate suite of massive amphibolites intruded or extruded into the acid volcanic rocks before the thrusting. The base of the overthrust metagabbro sheet, which had cooled to albite-epidote-amphibolite facies conditions, was ground down into a fine-grained NNW lineated amphibolite. The volcanic rocks, the Mannin thrust, and the overlying basic complex, were folded by D 4 , and D 5 , into a domed structure. About 400 Ma the aplopegmatitic facies of the Inish Granite intruded and caused hornfelsing and folding while the Galway Granite was intruded in the south. Lamprophyric sills and dykes and porphyry dykes were followed by the final phase of the Inish Granite. The southward facing folding and thrusting of the Connemara Dalradian block and the gabbro-gneiss emplacement are major elements in the Caledonide history of the British Isles. It is postulated that there was a tectonic and genetic connection between the gabbro-gneiss intrusion into these Dalradian rocks, the unusually rapid decrease in metamorphic pressure already documented for Connemara and the exceptionally southern position of the Connemara block.