Computer Science
Scientific paper
Jul 1991
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991gecoa..55.1859b&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 55, Issue 7, pp.1859-1874
Computer Science
4
Scientific paper
A study was made of the chemical fractionation associated with four cases of anatectic segregation of low melt-fraction cordieritic granites from migmatized meta-greywackes. The aims of the study were to (1) reveal the fractionation patterns of major and trace elements, (2) compare the major element chemistry of leucogranites and the quantitative behavior of source minerals during anatexis--inferred by mass-balance adjustment--with available experimental data for peraluminous systems, and (3) discuss the behavior of trace elements in crustal melting by comparing the chemically determined composition of leucogranites with the results of three fractionation models. Two of these assume a perfect diffusive behavior of trace elements within residual solids, but they use a different set of distribution coefficients. The third assumes a perfect nondiffusive behavior. In relation to their source rocks, the leucogranites are strongly depleted in Li, Transition Elements, and Light Rare Earth Elements, but enriched in K 2 O, SiO 2 , and Ba. The K / Rb ratio in leucogranites is higher than in sources and reaches values typical of high-grade metamorphism. Mass balance analysis using the Anatexis Mixing Model shows that the chemistry of cordierite leucogranites is compatible with its having originated by closed-system, water-undersaturated anatexis on previously migmatized meta-greywackes, leaving a residue enriched in cordierite plus biotite and exhausted in K-feldspar. Biotite melts congruently unless important amounts of sillimanite were also present in the source. Compared with experimental melts obtained from sources with the same chemical composition but with a different femic mineralogy (biotite + sillimanite, instead of cordierite + biotite), the Pena Negra leucogranites are richer in K 2 O and MgO with a lower Fe/(Fe + Mg) ratio. The differences in magnesium are believed to result from the changes in the mineral assemblage of the source rocks. However, the differences in potassium contents are still not well understood. The behavior of trace elements during anatexis cannot be modeled by trace element fractionation equations. Because of the sluggish diffusion kinetics of trace elements at the temperature of anatexis, making the melting rate of minerals greater than the diffusion rate of trace elements within minerals, the behavior of trace elements essentially consists in following that of the minerals containing them.
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