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Journal of the Geological Society

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A Grenville age for pre-Caledonian rocks in NW Co. Mayo, Ireland

Pink Gneisses at Erris Head in NW Co. Mayo, Ireland have yielded a Rb-Sr whole rock age of 1070±70Ma (⁸⁷Rb = 1.42x10^–11 year^–1). Interbanded Grey Gneisses show a somewhat younger but probably related age with a slightly higher ⁸⁷Sr/⁸⁶Sr initial ratio. Isotopic homogenization appears to have been locally controlled by lithology, with the mineralogically more stable suite of tectosilicate-rich gneisses giving the older age. The ages of these 2 main components of the Annagh Gneiss unit at Erris Head may reflect a reworking event rather than the formation of the gneisses as there is evidence locally for a lengthy, tectono-thermal history.

At both Erris Head and nearby Cross Point, K-Ar whole rock analyses of the same specimens confirm a metamorphic overprint which is consistent with an early Caledonian metamorphic history seen in the adjacent Moine and Dalradian rocks.

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