Contrasting behaviour of boron during crustal anatexis

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 Li and B in two migmatitic areas: the Peña Negra Anatectic Complex (central Spain) and the migmatites and leucogranites associated with the Ronda peridotites (southern Spain). They represent two contrasting cases in the behaviour of volatiles, particularly B, during anatexis. The Peña Negra migmatites are dominated by diatexites where it is possible to distinguish granitic leucosomes, melanosomes, metric to decimetric leucogranitic segregates, and decimetric to metric sillimanite-rich restitic enclaves. All lithologies are characterized by low B concentrations (mean values <30 ppm), and B is enriched in melanosomes/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 closed system anatexis. At Ronda, metatexitic and diatexitic migmatites underlie the Ronda peridotites, and diatexite and leucogranite dikes intrude the peridotites. Diatexites underlying the peridotites are characterized by cordierite+biotite±garnet as ferromagnesian phases, with rare or absent tourmaline, low B contents (<10 ppm), and cordierite±garnet leucosomes. Leucogranite dikes have high B concentrations (mean value≈400 ppm), with tourmaline being a common and, sometimes, the most abundant ferromagnesian phase. A genetic relationship between diatexites and leucogranites is strongly suggested by the presence of dikes of diatexites intruding the bottom of the peridotites. We conclude that B in the leucogranites may stem from the dissolution of tourmaline originally present in the metasediments, or infiltration of B-rich aqueous fluids into the anatectic zone. According to recent experimental work, textures in some tourmaline-rich Ronda leucogranites suggest initial B 2O 3 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 Al 2O 3/(CaO+Na 2O+K 2O)) and relatively low B contents.