Element concentrations in granitic magmas: ghosts of textures past?

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.

花岗岩类岩石的地球化学组成，似乎蕴含着原岩深熔前结构的相关信息。在I型（I-type）与S型（S-type）花岗岩岩浆（岩石）中，作为原岩部分熔融比例替代指标的钡（Ba），与五氧化二磷（P₂O₅）、二氧化钛（TiO₂）、钇（Y）、锆（Zr）及铈（Ce）的浓度之间的协变关系存在显著差异。这类差异被认为是原岩中副矿物颗粒的赋存位置差异、其在花岗岩熔体中的溶解度差异，以及原岩所发育面理程度三者共同作用的产物。研究推断，S型原岩的面理发育程度更高，副矿物富集于构成部分熔融核心区域的富黑云母层中；与之相对，I型原岩的面理发育程度相对较弱，含水镁铁质硅酸盐矿物与副矿物颗粒的分布也更为随机。