GARNETITE-FORMING PROCESSES IN THE DEEP CRUST - THE VAL-FIORINA CASE-STUDY (IVREA-VERBANO ZONE, NW ALPS)

Garnetite (garnet > 60 %) is a prominent rock type in Val Fiorina (nor thern Ivrea Zone), where it occurs as layers up to 10 m thick between granulite-facies metapelites and mafic-ultramafic lithologies. The con tacts are sharp between garnetite and metapelite and gradational towar ds the mafic-ultramafic rocks. Biotite occurs together with garnet in streaky garnetite domains, while both hornblende and clinopyroxene inc rease in garnetite towards the mafic-ultramafic layers. The grossular content of garnet increases from the metasediment contact towards the mafic rocks, while almandine decreases, bulk-rock Y/Sc and Zr/Ti as we ll as initial Sr-87/Sr-86 decreases and K/Rb increases. Garnets have a negative Eu anomaly, but Eu/Eu increases smoothly in the garnets wit h increasing distance from the metasediment contact. Two hypotheses ar e discussed for garnetite petrogenesis: 1) metasomatism taking place b y interstitial melt retention in amphibolites and metapelites undergoi ng synchronous anatexis; 2) reaction between metapelite-derived anatec tic melt and phases segregated from a basaltic sill during intrusion w hich induced partial melting of the country rocks. An objection to hyp othesis 1 is the improbability of interstitial melts being retained un til completion of the metasomatism. The second hypothesis may explain the observed composition and structures and has been proposed for othe r Ivrea Zone sectors such as Val Sesia. However, this process only pro duced extensive garnetites in Val Fiorina. The special conditions requ ired in this case are tentatively ascribed to the flow characteristics and/or to differences in the anatectic component determined by a more fertile metapelite source in Val Fiorina with respect to Val Sesia. B oth processes led to geological scenarios consistent with events affec ting the Ivrea Zone.