Abstract

Two sets of U-Pb zircon data from widely spaced specimen sets in the oldest migmatites in the Annagh Division gneisses yield upper intercept ages of 1297 +164-101 and 1224 +192-64 Ma and lower intercept ages of 869+69–118 and 813+166269 Ma. Zircons from the metamorphosed alkaline Doolough Granite (gneiss) yielded an upper intercept age of 1093+8–7 Ma; thus the intrusion is chronologically related, as an immediate precursor, to the widespread granitization and migmatization previously dated by U-Pb at 1070 ±30 Ma to 1000 ±30 Ma and by Rb-Sr whole-rock at 1069 ±72 Ma to 985 ± 186 Ma. Much of the Annagh Division is composed of pre-Grenville (Elsonian?) or very early Grenville gneisses which were intensively reworked during the main Grenville orogeny. The lower intercept ages from the older gneisses may possibly define a separate post-Grenville and pre-Caledonian tectono-thermal event.

The Termon Granite at the southern tip of the Mullet Peninsula in NW Country Mayo was emplaced at 404+2-3 Ma but the U-Pb upper intercept age of 1956+127–121Ma indicates that basement of Lewisian age may well underlie the exposed pre-Caledonian basement and its Caledonian (Dalradian) cover in the NW County Mayo metamorphic inlier.

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