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The origin of the Connemara migmatites of the Cashel district, Connemara, Ireland

Leake, Bernard Elgey 1969. The origin of the Connemara migmatites of the Cashel district, Connemara, Ireland. Quarterly Journal of the Geological Society 125 (1-4) , pp. 219-276. 10.1144/gsjgs.125.1.0219

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After the Connemara schists, of Dalradian age, had been folded (F1) and metamorphosed to sillimanite grade, they were strongly hornfelsed by the intrusion of basic magma. From this magma crystallized peridotites, norites, pyroxenites and gabbros, all with An80–87 plagioclase. Movements then caused the magma to break up these ultrabasic rocks and crystallization of basic rocks occurred. They are labradorite (An55–65)-hornblende-relic clinopyroxene rocks that grade into quartz-hornblende-labradorite (An55)-gneiss, quartz-hornblende-labradorite (An55)- and andesine (An40)-gneiss and quartz-andesine (An40)-hornblende-biotite-gneiss. These gneisses formed during strong F2 and F3 movements, and probably during amphibolite-facies metamorphism, and they broke up, injected, metasomatized and amphibolized the earlier basic and ultrabasic rocks, giving a heterogeneous migmatite that commonly contains two plagioclases, An40 and An55. Quartzo-feldspath-ization of metasediment occurred, possibly up to a mile from the migmatites. This, and recrystallization, formed quartz-andesine (An40)-biotite-gneiss that merges into the magmatically crystallized gneisses as do gneisses formed by silicification of basic and ultrabasic rocks. Last to crystallize was K-feldspar gneiss, possibly formed by segregation of residual magma into one zone. This migmatite complex contains evidence of both magmatic and metamorphic crystallization of hornblende and the relative importance of each process remains to be determined. Tight F3folding formed the Cashel syncline, and a series of complementary folds. Later retrogressive metamorphism caused widespread chloritization, sericitization and saussuritization. Last of all was the intrusion of the Galway and Roundstone granites followed by their associated dykes. Twenty-four chemical analyses and nearly five hundred plagioclase determinations are utilized.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Earth and Environmental Sciences
Subjects: Q Science > QE Geology
Publisher: Geological Society of London
ISSN: 0370-291X
Last Modified: 04 Jun 2017 08:16

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