Behavior of accessory phases and redistribution of Zr, REE, Y, Th, and U during metamorphism and partial melting of metapelites in the lower crust: An example from the Kinzigite Formation of Ivrea-Verbano, NW Italy

This study is aimed at understanding the behavior of monazite, xenotime, ap atite and zircon, and the redistribution of Zr, REE, Y, Th, and U among mel t, rock-forming and accessory phases in a prograde metamorphic sequence, th e Kinzigite Formation of Ivrea-Verbano, NW Italy, that may represent a sect ion from the middle to lower continental crust. Metamorphism ranges from mi ddle amphibolite to granulite facies and metapelites show evidence of inten se partial melting and melt extraction. The appearance of melt controls the grain size, fraction of inclusions and redistribution of REE, Y, Th, and U among accessories and major minerals. The textural evolution of zircon and monazite follows, in general, the model of Watson et al. (1989). Apatite i s extracted from the system dissolved into partial melts. Xenotime is consu med in garnet-forming reactions and is the first source for the elevated Y and HREE contents of garnet. Once xenotime is exhausted, monazite, apatite, zircon, K-feldspar, and plagioclase are progressively depleted in Y, HREE, and MREE as the modal abundance of garnet increases. Monazite is severely affected by two retrograde reactions, which may have consequences for U-Pb dating of this mineral. Granulite-grade metapelites (stronalites) are signi ficantly richer in Ti, Al, Fe, Mg, Sc, V, Cr, Zn, Y, and HREE, and poorer i n Li, Na, K, Rb, Cs, Tl, U, and P, but have roughly the same average concen tration of Cu, Sr, Pb, Zr, Ba, LREE, and Th as amphibolite-grade metapelite s (kinzigites). The kinzigite-stronalite transition is marked by the sudden change of Th/U from 5-6 to 14-15, the progressive increase of Nb/Ta, and t he decoupling of Ho from Y. Leucosomes were saturated in zircon, apatite, a nd (except at the lowest degree of partial melting) monazite. Their REE pat terns, especially the magnitude of the Eu anomaly, depend on the relative p roportion of feldspars and monazite incorporated into the melt. The presenc e of monazite in the source causes an excellent correlation of LREE and Th, with nearly constant Nd/Th approximate to 2.5-3. The U depletion and incre ase in Th/U characteristic of granulite facies only happens in monazite-bea ring rocks. It is attributed to enhancement of the U partitioning in the me lt due to elevated Cl activity followed by the release of a Cl-rich F-poor aqueous fluid at the end of the crystallization of leucosomes. Halide activ ity in partial melts was buffered by monazite and apatite. Since the U (and K) depletion does not substantially affect the heat-production of metapeli tes, and mafic granulites maintain similar Th/U and abundance of U and Th a s their unmetamorphosed equivalents, it seems that geochemical changes asso ciated to granulitization have only a minor influence on heat-production in the lower crust. Copyright (C) 1999 Elsevier Science Ltd.