Element concentrations in granitic magmas: ghosts of textures past?
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The geochemistry of granitic rocks appears to contain information about some aspects of the pre-anatectic textures of the protoliths. There are marked contrasts in the covariations between Ba (a proxy for melting proportion in the protolith) and the P2O5, TiO2, Y, Zr and Ce concentrations in I- and S-type granitic magmas (rocks). These contrasts are interpreted as features resulting from the interplay between differences in the locations of accessory mineral grains in the protoliths, their solubilities in the granitic melts and the degree of foliation developed in the protoliths. It is inferred that S-type protoliths are more highly foliated, with accessory minerals concentrated into the biotite-rich layers that form the foci for partial melting. I-type protoliths appear to have less pronounced foliation and a more random distribution of both hydrous mafic silicates and accessory grains.