Abstract

Similarity of the Carlsbad-albite twin extinction angles indicates that the composition of the basic plagioclase occurring in a gabbroic layer in the Dawros peridotite is similar to that in the nearby Currywongaun-Doughruagh intrusion, in the ultrabasic Cashel-Lough Wheelaun intrusion, and in the Roundstone intrusion, all in Connemara, western Ireland. Although the feldspars are highly saussuritized, a fresh specimen of anorthite was found in an anorthosite and has been analyzed. The chemical composition is An<91.5) Ab<6.8) Or<1.7) showing good agreement with the refractive index and 2V measurements. It is postulated that the ultrabasic bodies in Connemara all belong to one magma series which differentiated to give basic rocks with calcic plagioclase whose composition is quite similar to plagioclase found in such layered complexes as the Bushveld and Stillwater. Tentative suggestions are put forth that there may be both primary igneous layering and secondary metamorphic banding present, with the possibility of more than one period of folding. The comments by Rothstein concern the possibility that the peridotite layer may be a caught-up raft in the gabbro, the role of hypersthene-bearing pyroxenites in the igneous sequence, and the relation of the folding to the variation in the mineral assemblages.

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