Irish Journal of Earth Sciences

Abstract:

The northern part of the ~470 Ma syntectonic D2 basic and ultrabasic Roundstone intrusion into the Connemara Dalradian metasediments has been mapped in detail for the first time and consists of metagabbros containing a 180m peridotite xenolith and later injections of Quartz diorite gneiss which silicified and agmatised some of the metagabbros. With the already studied southern (Errisbeg) part, plus over 300 chemical analyses of rocks and over 400 probe analyses of minerals, this enables a synthesis of the geology of the whole intrusion. A tholeiitic magma crystallised olivine Fo83-76, orthopyroxene En84-59, diopside to salite and plagioclase from An96-42, and late stage cumulus magnetite. The exceptionally high An% is indicative of high pH2O with copious water (derived from dewatering of a subduction zone?) as is the abundant hornblende that replaced most of the magmatic minerals, the magma being comparable with that of the Lesser Antilles Island arc. The intrusion was formed by numerous already fractionated magma pulses intruded in upward sequence, closely (~467 Ma) followed by pulses of Quartz diorite to granite gneisses with magmatic hornblende, not by closed system fractionation of a single magma injection. The peridotites were mostly carried in as already solidified xenoliths by pulses of gabbro magma. Only the larger bodies in the Errisbeg part of the intrusion were intruded as magma.

Igneous fractionation as measured by molecular MgO/(MgO+FeO+Fe2O3+MnO), Cr and Ni contents plus normative An/(An+Ab), because of heavy saussuritisation of the plagioclase, shows the intrusion is inverted from early metaperidotites and metagabbros from around the 300m Errisbeg summit downwards through the sideways injected meta-SE Gabbros to the later northern metagabbros, which last are virtually all <30m in altitude, to the underlying most fractionated metagabbros and fine-grained metagabbros, called Epidiorites. Parts of the last two together with a little included gneiss were converted into a mylonitic amphibolite, the Ballyconneely Amphibolite, immediately above the 447 ± 4 Ma (Tanner et al. 1989) D5 Mannin Thrust and form part of the Delaney Dome, west of the intrusion. The thrust underlies the whole of the Roundstone intrusion and Connemara but would have been deformed by D6 (~440 Ma) and D7 (~426 Ma). The N-S axial planar folding (most unusual in Connemara) which completed the formation of the Delaney Dome and also formed the Errisbeg antiform in the centre of the intrusion is shown to be D6 for the first time. The inversion of the metagabbros, west of the Delaney Dome, and over the Dome and to the east in the Roundstone intrusion and still further east in Gowla demonstrates that the Errismore-Roundstone-Gowla body and practically all of the 80 x >20km Grampian metagabbro-gneiss complex from Slyne Head to Galway is inverted except the Cashel-Lough Wheelaun-Loughaunanny intrusion and its envelope.