Geochemistry of metasomatism adjacent to amphibole-bearing veins in the Lherz peridotite massif

McPherson, E.; Thirlwall, M. F.; Parkinson, I. J.; Menzies, M. A.; Bodinier, J. L.; Woodland, A. and Bussod, G. (1996). Geochemistry of metasomatism adjacent to amphibole-bearing veins in the Lherz peridotite massif. Chemical Geology, 134 pp. 135–157.

DOI: https://doi.org/10.1016/S0009-2541(96)00084-8

Abstract

The Lherz peridotite massif, in the French Pyrenees, is intruded by a number of hornblendite and garnet-amphibole-pyroxenite (GAP) veins. New, high quality, elemental and isotopic data are presented for veins and their adjacent harzburgite wallrocks in order to evaluate the extent of reaction and the ability of fluids to permeate clinopyroxene-poor peridotites. Hornblendite and GAP veins have convex upward rare earth element (REE) profiles consistent with an origin as crystal segregates from alkali basalts. In all of the traverses Mg# increases away from the veins and MnO, TiO₂, Zr, and the REE decrease away from the veins within a zone < 50 cm from the veins. Calculated melts in equilibrium with hornblendite veins have strong LREE enrichment similar to many alkali basalts. Metasomatism adjacent to these veins is consistent with small-scale outward movement of alkali basalt, equilibration with wallrock minerals and precipitation of amphibole and phlogopite. Increase in Ce/Sm ratios and decrease in Ce contents away from the veins is consistent with equilibration of the calculated melt from the vein with the preexisting harzburgites. A region with high Ce/Sm on the right of one vein may be the result of chromatographic fractionation of melt during percolation from the amphibole-bearing veins. However, this is not observed on the opposite side of the vein and ratios are variable within the zone, so an asymmetrical and irregular chromatographic front would be required. The high Ce/Sm ratios, therefore, most likely reflect pre-vein REE heterogeneity in the harzburgites. Data for the Lherz massif suggest that dramatic variations in incompatible element concentrations can develop metasomatically in the continental lithospheric mantle over a relatively short length scale.

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