Abstract
Liquid immiscibility has been used for explaining several modern situations either in effusive or in intrusive rocks. Liquid immiscibility is also proposed for explaining the variolitic structures in the Archean rocks of the Abitibi Greenstone belt, Ontario. The volcanic sequence of the lower portion of the Greenstone belt of Piumhi (Minas Gerais, Brazil) shows the occurrence of variolitic structure, that may have been formed by liquid immiscibility. This possibility is supported by the following points: (a) the contact between varioles and matrix is very sharp; (b) spinifex texture occurs both in the varioles and in the matrix and the single crystal may cross the varioles; (c) the varioles and the matrix show contrasting compositions: the latter has the composition of a basaltic komatiite and the former varies form that of an alkali-basalt to andesite; and (d) a test by a thermodynamic model from literature shows that, although with some doubt, the free energy of the possible parent liquid is higher than that of the two unmixed liquids.
Published Version
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