Fractionation of elements in the pelitic hornfelses of the Cashel-Lough Wheelaun intrusion, Connemara, Eire

Mathematics – Logic

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Scientific paper

The fractionation of major and minor elements in the pelitic hornfelses of the Cashel-Lough Wheelaun basic and ultrabasic intrusion is examined in the light of the observed mineralogical changes. Comparison with trace element fractionation in silicate magmas shows excellent agreement, except in the cases of Ga: Al and Co:Ni. The available evidence favours the hypothesis of extensive fractional melting of the aureole rocks, with the bulk chemical changes taking place as a result of the extraction of the interstitial liquid. The absence of any additive metasomatism is suggested by the lack of change in ratios among the apparently enriched elements, e.g. Mg:Mg + Fe and the ratios of Cr, Ni, Vc and Co to Mg + Fe. The progressive losses of Si, Na and K from the pelites were accompanied by important losses of Ba, Rb, Li, La and Cs. Sc, Mn, Sr and Zr losses were small at lower, but more profound at higher temperatures of alteration. Cr, V, Co, Ga and Ni, like total Fe, Mg and Ti, were enriched roughly threefold in the more intensely altered rocks, although in the extreme stages (formation of emery rocks) there is evidence of removal of Mg, Ti and Ni, together with proportionally smaller amounts of Fe. The compositions of the successive fluid or liquid fractions are estimated, using Cr content as an indicator of the absolute decrease in total number of cations in the hornfelses. The envelope hornfelses first yielded a K-rich granitic liquid and, at slightly higher temperatures, a more normal granitic or granodioritic liquid. The liquid lost by the xenoliths to the enclosing magma was rich in Si and contained appreciable Mg, Ti, Fe, Mn, Ca and K.

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