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Journal of Petrology Volume 42 Number 1 Pages 207-219 2001
© Oxford University Press 2001

Records of Crustal Metasomatism in the Garnet Peridotites of the Ulten Zone (Upper Austroalpine, Eastern Alps)

E. RAMPONE1,* and L. MORTEN2

1DIPARTIMENTO DI SCIENZE DELLA TERRA, UNIVERSITÀ DEGLI STUDI DI MILANO, VIA BOTTICELLI 23, 20133 MILANO, ITALY
2DIPARTIMENTO DI SCIENZE DELLA TERRA E GEOLOGICO-AMBIENTALI, PIAZZA DI PORTA S. DONATO 1, 40126 BOLOGNA, ITALY

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 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 C1], 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.

KEY WORDS: garnet peridotite; crustal metasomatism; amphibole; hydrous fluids


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