Contrasting behaviour of boron during crustal anatexis

The study of volatile elements in crustal anatectic terranes may provide in formation about the role of fluid flow and fluid pathways during high tempe rature metamorphism. We have studied the distribution of Li and B in two mi gmatitic areas: the Pena Negra Anatectic Complex (central Spain) and the mi gmatites and leucogranites associated with the Ronda peridotites (southern Spain). They represent two contrasting cases in the behaviour of volatiles, particularly B, during anatexis. The Pena Negra migmatites are dominated b y diatexites where it is possible to distinguish granitic leucosomes, melan osomes, metric to decimetric leucogranitic segregates, and decimetric to me tric sillimanite-rich restitic enclaves. All lithologies are characterized by low B concentrations (mean values < 30 ppm), and B is enriched in melano somes/restites with respect to leucosomes/leucogranites. This may be due to the fact that a notable proportion of B can reside in restitic sillimanite . B concentrations in leucosomes and leucogranites can be explained by clos ed system anatexis. At Ronda, metatexitic and diatexitic migmatites underlie the Ronda peridoti tes, and diatexites and leucogranite dikes intrude the peridotites. Diatexi tes underlying the peridotites are characterized by cordierite + biotite +/ - garnet as ferromagnesian phases, with rare or absent tourmaline, low B co ntents(< 10 ppm), and cordierite +/- garnet leucosomes. Leucogranite dikes have high B concentrations (mean value approximate to 400 ppm), with tourma line being a common and, sometimes, the most abundant ferromagnesian phase. A generic relationship between diatexites and leucogranites is strongly su ggested by the presence of dikes of diatexites intruding the bottom of the peridotites. We conclude that B in the leucogranites may stem from the diss olution of tourmaline originally present in the metasediments, or infiltrat ion of B-rich aqueous fluids into the anatectic zone. According to recent experimental work, textures in some tourmaline-rich Ron da leucogranites suggest initial B2O3 contents in the melt higher than the present measured concentrations. Preliminary experimental results presented in this paper propose that this difference is not due to water activities in the melt. Rapid crystallization of the melts following undercooling may explain the existence of abundant euhedral tourmaline in leucogranites with low aluminium saturation index (ASI = molar Al2O3/(CaO + Na2O + K2O)) and relatively low B contents. (C) 2001 Elsevier Science B.V. All rights reserv ed.