Abstract

C. S. Jones & P. T. Leat write: In a recent paper Winchester et al. 1987 presented geochemical data for Caledonized metavolcanic rocks from the Western Ox Mountains and NW Mayo Inlier, Ireland. They found geochemical differences between two metabasalt units that occur within a thick metasedimentary sequence of semi-pelite and psammite, and are separated by a syn-metamorphic dislocation, (the Glennawoo Slide; Taylor 1969). Winchester et al. demonstrated that the metabasalts south of the slide have trace element compositions similar to mid ocean ridge basalt (MORB), whereas metabasalts north of the slide, have compositions similar to continental tholeiites and tholeiites of the ocean island basalt (OIB) association. They concluded from these data that the two groups of basalts were erupted in different crustal settings and proposed that the Glennawoo Slide is a major terrane boundary which subsequently brought the two units together. If accepted, this conclusion would have major implications for the tectonic history of the Irish Caledonides.

The psammitic nature of the metasedimentary rocks closely associated with both groups of metabasalts indicates that neither was erupted in a truly oceanic setting. We argue here that both groups of metabasalts could have been erupted in the same continental margin environment, and we summarize structural, stratigraphic and metamorphic evidence which precludes the identification of the Glennawoo Slide as a terrane boundary as defined by Jones et al. 1983.

Geochemical evidence. A recent trend in basalt geochemistry has been the increasing awareness that degree of partial melting of mantle has a major

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