Water-present melting in the middle crust: The case of the Ollo de Sapo gneiss in the Iberian Massif (Spain)

• Published in: Chemical geology 2015-12, Vol.419, p.176-191
• Main Authors: García-Arias, Marcos, Corretgé, Luis Guillermo, Fernández, Carlos, Castro, Antonio
• Format: Article
• Language: English
• Subjects: Boundaries; Fluid-present melting; Geochemistry; Gneiss; Leucosome; Massifs; Mathematical models; Melting; Melts; Migmatite; Ollo de Sapo gneiss; Residuum entrainment; Trace elements
• ISSN: 0009-2541; 1872-6836
• DOI: 10.1016/j.chemgeo.2015.10.040

The Variscan migmatitic region of Sanabria, in the inner zone of the Iberian Massif (Spain), provides a unique place to study the geochemistry and rheology of anatectic melts formed by water-present conditions. In Sanabria, the Ollo de Sapo gneiss underwent partial melting at 0.5–0.66GPa and 690–770°C. Leucosomes, leucogranitic bodies within the migmatite and leucogranites are widespread throughout the migmatite, with the leucogranitic bodies being concordant to discordant to the foliation. Mass balance calculations indicate that the partial melting of the gneiss produced 62.5wt.% of melt. Experiments and pseudosections made at the peak metamorphic P–T conditions under fluid-absent conditions yield much lower volumes of melt and therefore a fluid-present scenario is proposed. Major and trace element composition and structures of leucosomes and leucogranites support this scenario. Trace element composition of leucosomes is variable, with low to high REE, Zr, Y and Th content and positive to negative Eu anomalies. The composition of the most felsic, REE-depleted, Eu-rich leucosomes is closely similar to that of experimental melts with added water at T