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Regional metasomatism and the geochemistry ofthe Dalradian measediments of Connemara, Western Ireland.

Senior, Antony and Leake, Bernard E. 1978. Regional metasomatism and the geochemistry ofthe Dalradian measediments of Connemara, Western Ireland. Journal of Petrology 19 (3) , pp. 585-625. 10.1093/petrology/19.3.585

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Based on 225 analyses of quartzites, siliceous granoblastites, calc silicate rocks, calcite and dolomite marbles, including 120 analyses of pelites and semipelites, sedimentary trends of chemical variation are identified in staurolite and sillimanite grade rocks. The correlation of original clay mineral content with Ti, Fe, K, Rb, Y, Nb, Ca, Ni, Ga, Zn and probably Ba and Mn is shown. A similar clay mineral (whose composition is calculated) was added to all the sediments except the quartz-rich sandstones, now quartzites. This pattern appears to be general for most sediments, based on crustal averages. The form of the original addition of Sr in the sediments might be identified as either carbonate or feldspar by a Ca vs. Sr plot. The southern pelites in a 2–4 km peripheral zone to the Connemara orthogneisses and migmatites have been metasomatized. The crude order of elemental enrichment from the elements increased the most to those increased the least relative to the same stratigraphical horizons in the north is: Mn, Ba, Th, Cu, Ca, Sr, Y, Pb, Zn, Pr, Ge, Nd, La, Mg, S, Ce, Rb, Sm, Ti, Na, K and Ga while Si, Al, Cr, Ni, Co, Fe and P are unchanged or removed. The source of the material added is postulated to be the water-rich residual fraction of the migmatitic quartz diorite gneiss, the transport being by movement of a water-rich fluid out of the migmatites, the fixation being mainly in biotite and new, more calcic, plagioclase porphyroblasts, there being a positive correlation between element enrichment and ionic radius.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Earth and Environmental Sciences
Publisher: Oxford University Press
ISSN: 0022-3530
Last Modified: 16 Jul 2020 14:15

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