Records of Crustal Metasomatism in the Garnet Peridotites of the Ulten Zone (Upper Austroalpine, Eastern Alps) |
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Authors: | RAMPONE E; MORTEN L |
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Institution: | 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 |
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Abstract: | Peridotites in the Ulten Zone (Upper Austroalpine, Eastern Alps),occur as small bodies within lower-crustal rocks (gneisses andmigmatites) subducted at eclogite-facies conditions during theVariscan orogeny. They record a complex metamorphic and deformationevolution as indicated by the transition from coarse-grainedspinel-bearing peridotites to fine-grained garnet + amphibole-bearingperidotites, and are interpreted as portions of mantle wedgethat were incorporated in a downgoing slab of cold continentalcrust. The transition from spinel- to garnet-bearing assemblagewas accompanied by significant input of metasomatic agents,as shown by the crystallization of abundant amphibole. Herewe present trace-element mineral chemistry data for selectedUlten peridotites, with the aim of unravelling the nature ofthe metasomatic processes. Amphiboles display significant lightrare earth element (LREE) enrichment CeN/YbN = 3·9011·50;LREE up to (2050) x C1], high Sr (150250 ppm),K (19107280 ppm) and Ba (280800 ppm) contents,and low concentrations of high field strength elements (HFSE)(Zr = 1425 ppm, Y = 6·716 ppm, Ti = 11502500ppm, Nb = 27 ppm). On the basis of (1) the evidence formodal orthopyroxene decrease as a result of the garnet-formingreaction rather than abundant orthopyroxene crystallization,(2) the high modal amounts of amphibole (up to 23%) in the mostmetasomatized peridotites and (3) the strong large ion lithophileelement (LILE)/HFSE fractionation in amphiboles, we infer thatthe metasomatic agent was an H2OCO2 fluid with a lowCO2/H2O ratio. Petrological investigations and geochronologicaldata indicate that the host metapelites experienced in situpartial melting and migmatization concomitantly with the garnet+ amphibole-facies recrystallization in the enclosed peridotites.We infer that the metasomatizing hydrous fluids could representthe residual fluids left after the crystallization of leucosomes,starting from water-undersaturated melts produced during migmatizationof the host gneisses. KEY WORDS: garnet peridotite; crustal metasomatism; amphibole; hydrous fluids |
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