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 (northern Ivrea Zone), where it occurs as layers up to 10 m thick between granulite-facies metapelites and mafic-ultramafic lithologies. The contacts are sharp between garnetite and metapelite and gradational towards the mafic-ultramafic rocks. Biotite occurs together with garnet in streaky garnetite domains, while both hornblende and clinopyroxene increase 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 well 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 with increasing distance from the metasediment contact. Two hypotheses are discussed for garnetite petrogenesis: 1) metasomatism taking place by interstitial melt retention in amphibolites and metapelites undergoing synchronous anatexis; 2) reaction between metapelite-derived anatectic melt and phases segregated from a basaltic sill during intrusion which induced partial melting of the country rocks. An objection to hypothesis 1 is the improbability of interstitial melts being retained until completion of the metasomatism. The second hypothesis may explain the observed composition and structures and has been proposed for other Ivrea Zone sectors such as Val Sesia. However, this process only produced extensive garnetites in Val Fiorina. The special conditions required 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. Both processes led to geological scenarios consistent with events affecting the Ivrea Zone.