A method for modelling mass balance in partial melting and anatectic leucosome segregation

A method is proposed for adjusting the mass balance to characterize quantitatively the behaviour of minerals in anatexis. The method is based on an unconstrained simple mixing model that can be expressed as: where B , A ₀, and A _1-n, are compositional vectors of segregate, source rock and source minerals, respectively. The most important concepts are: (1) degree of partial fusion: F^MM= 1/a₀; (2) mineral fractionation index: and (3) plagioclase differentiation index: . For a given mineral, the MFI values have the following meaning: (a) MFI <0: residual phase originated, at least partly, as a product of incongruent melting; (b) 0 > MFI <1: preferential retention in the residue; (c) MFI= 1: identical modal fraction in source and melt; (d) a₀ > MFI > 1: preferential incorporation into the segregate, and (e) MFI > a₀: external contribution to the anatectic system defined by a₀ A ₀. To test the method and illustrate its use, it was applied to two real problems of partial melting in the Peña Negra Anatectic Complex (Central Spain). The first is a very simple case of segregation of a diktyonitic neosome from an orthogneiss through partial melting located in vertical shear zones. This process is characterized by: (1) F^MM= 0.51; (2) active incorporation of K-feldspar, plagioclase and biotite into the segregate; (3) disequilibrium melting of plagioclase; (4) residual behaviour of quartz and ilmenite. The second case concerns the formation of a cordierite-bearing granite from granodioritoid diatexites through an anatectic process, whose most salient characteristics are: (1) F^MM= 0.45; (2) incongruent melting of biotite; (3) residual behaviour of plagioclase, which melted with a PDI of 1.22; (4) preferential incorporation of quartz into the segregate; (5) total extraction of K-feldspar from the residue.