NEW LIGHT ON THE GEOLOGY OF THE ROUNDSTONE INTRUSION, ITS INVERSION AND THAT OF THE GRAMPIAN METAGABBRO-GNEISS COMPLEX, CONNEMARA,WESTERN IRELAND

摘要

The northern part of the ?470 Ma syntectonic D2 basic and ultrabasic Roundstone intrusion into the Connemara Dal radian 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 wer