Records of crustal metasomatism in the garnet peridotites of the Ulten Zone (Upper Austroalpine, Eastern Alps)

Abstract

Peridotites in the Ulten Zone (Upper Austroalpine, Eastern Alps), occur as small bodies within lower-crustal rocks (gneisses and migmatites) subducted at eclogite-facies conditions during the Variscan orogeny. They record a complex metamorphic and deformation evolution as indicated by the transition from coarse-grained spinel-bearing peridotites to fine-grained garnet + amphibole-bearing peridotites, and are interpreted as portions of mantle wedge that were incorporated in a downgoing slab of cold continental crust. The transition from spinel- to garnet-bearing assemblage was accompanied by significant input of metasomatic agents, as shown by the crystallization of abundant amphibole. Here we present shown by the trace-element mineral chemistry data for selected Ulten peridotites, with the aim of unravelling the nature of the metasomatic processes. Amphiboles display significant light rare earth element (LREE) enrichment [CeN/YbN = 3.90-11.50; LREE up to (20-50) x CI], high Sr (150-250 ppm), K (1910-7280 ppm) and Ba (280-800 ppm) contents, and low concentrations of high field strength elements (HFSE) (Zr = 14-25 ppm, Y = 6.7-16 ppm, Ti = 1150-2500 ppm, Nb = 2-7 ppm). On the basis of (1) the evidence for modal orthopyroxene decrease as a result of the garnet-forming reaction rather than abundant orthopyroxene crystallization, (2) the high modal amounts of amphibole (up to 23%) in the most metasomatized peridotites and (3) the strong large ion lithophile element (LILE)/HFSE fractionation in amphiboles, we infer that the metasomatic agent was an H2O-CO2 fluid with a low CO2/H2O ratio. Petrological investigations and geochronological data indicate that the host metapelites experienced in situ partial melting and migmatization concomitantly with the garnet + amphibole-facies recrystallization in the enclosed peridotites. We infer that the metasomatizing hydrous fluids could represent the residual fluids left after the crystallization of leucosomes, starting from water-undersaturated melts produced during migmatization of the host gneisses.