Influence of volatiles in the generation of crustal anatectic melts

Abstract

The study of volatile elements in crustal anatectic terranes may provide information about the role of fluid flow and fluid pathways during high-temperature metamorphism. We have studied the distribution of B in migmatites and leucogranites associated with the Ronda peridotites, Betic Cordilleras (SE Spain). The migmatites underlie the Ronda peridotites and constitute the upper part of an inverted metamorphic sequence. Their B contents are low (≈5ppm) and their leucosomes are characterised by cordierite and garnet as ferromagnesian phases. The leucogranites crop out as dikes intruding the peridotites. They show high B concentrations (mean ≈400ppm), with tourmaline being frequent and, sometimes, the most abundant ferromagnesian phase. We conclude that B-rich leucogranites are generated during open system anatexis associated with crustal emplacement of the peridotites, with partial melting being aided by B-rich fluids channelled through the thrust plane. B-poor leucosomes, however, are interpreted as related to partial melting in a closed system poor in B, during the same Alpine anatectic event or, as suggested by recent geochronological data, during the Hercynian.