The distribution of REE minerals in metasedimentary rocks was investigated to gain insight into the stability of allanite, monazite and xenotime in metapelites. Samples were collected in the central Swiss Alps, along a well-established metamorphic field gradient that record conditions from very low grade metamorphism (250°C) to the lower amphibolite facies ($600°C). In the Alpine metapelites investigated, mass balance calculations show that LREE are mainly transferred between monazite and allanite during the course of prograde metamorphism. At very low grade metamorphism, detrital monazite grains (mostly Variscan in age) have two distinct populations in terms of LREE and MREE compositions. Newly formed monazite crystallized during low-grade metamorphism (<440°C); these are enriched in La, but depleted in Th and Y, compared with inherited grains. Upon the appearance of chloritoid ($440-450°C, thermometry based on chlorite-choritoid and carbonaceous material), monazite is consumed, and MREE and LREE are taken up preferentially in two distinct zones of allanite distinguishable by EMPA and X-ray mapping. Prior to garnet growth, allanite acquires two growth zones of clinozoisite: a first one rich in HREE + Y and a second one containing low REE contents. Following garnet growth, close to the chloritoid-out zone boundary ($556-580°C, based on phase equilibrium calculations), allanite and its rims are partially to totally replaced by monazite and xenotime, both associated with plagioclase (± biotite ± staurolite ± kyanite ± quartz). In these samples, epidote relics are located in the matrix or as inclusions in garnet, and these preserve their characteristic chemical and textural growth zoning, indicating that they did not experience reequilibration following their prograde formation. Hence, the partial breakdown of allanite to monazite offers the attractive possibility to obtain in situ ages, representing two distinct crystallization stages. In addition, the complex REE + Y and Th zoning pattern of allanite and monazite are essential monitors of crystallization conditions at relatively low metamorphic grade.

Exposed high-pressure (HP) crustal rocks in Phanerozoic orogens represent the result of a sequence of processes that

The timing and dynamics of fluid-induced melting in the typical Barrovian sequence of the Central Alps has been investigated using zircon chronology and trace element composition. Multiple zircon domains in leucosomes and country rocks yield U-Pb ages spanning from *32 to 22 Ma. The zircon formed during Alpine melting can be distinguished from the inherited and detrital cores on the basis of their age, Th/U (\0.1) and trace element composition. Ti-in-zircon thermometry indicates crystallization temperatures around 620-700°C. Their composition allows discriminating between (1) zircon formation in the presence of early garnet, (2) zircon in equilibrium with abundant L-MREE-rich accessory phases (allanite, titanite and apatite) typical of metatonalites, and (3) zircon formed during melting of metasediments in feldspar-dominated assemblages. The distribution of zircon overgrowths and ages indicate that repeated melting events occurred within a single Barrovian metamorphic cycle at roughly constant temperature; that in the country rocks zircon formation was limited to the initial stages of melting, whereas further melting concentrated in the segregated leucosomes; that melting occurred at different times in samples a few meters apart because of the local rock composition and localized influx of the fluids; and that leucosomes were repeatedly melted when fluids became available. The geochronological data force a revision of the temperature-time path of the migmatite belt in the Central Alps. Protracted melting over 10 My followed the fast exhumation of Alpine eclogites contained within the same region and preceded fast cooling in the order of 100°C/Ma to upper crustal levels. Communicated by J. Hoefs.

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A B S TR A CT Metamorphic field gradients were determined across the entire amphibolite grade Central Alps (c. 50×100 km). P-T were calculated from 116 samples acquired from our own field work, from samples provided to us by others, and from rocks with mineral compositions described in the literature. Only fluid-conserved equilibria were used to determine P-T . The use of an internally consistent thermodynamic database and mineral solid solution models makes the results robust and reduces relative errors. The results are presented in contour maps. Temperature increases from 500 to 550°C along the limit of amphibolite grade metamorphism in the north and west, to c. 675°C toward the south at the Insubric line near the town of Bellinzona. Maximum recorded pressures of c. 7 kbar are in a central region c. 20 km north of the Insubric line, and decrease both to the north (5.5 kbar) and south (4.5 kbar). The P-T results indicate that there is a relatively large area that reached conditions in the sillimanite stability field but developed neither sillimanite nor fibrolite; this is interpreted as a result of kinetic constraints on nucleation and growth because of the small amounts of thermal overstep (<40°C) of the kyanitesillimanite phase boundary. Comparison of P-T conditions with carbonate isograds in the region indicate that fluids present during metamorphism were not dominated by a homogeneous external source. Examination of the two-dimensional distribution of pressure and temperature in the context of thermal and tectonic models indicates that two thermal pulses aÂected the Central Alps during the Tertiary. In the second, heat aÂected only the southern parts of the area and overprinted the previously established P-T gradients. Pl: 0.745 0.254 0.001 Rock type: mpel=metapelite; marl=calcareous metasediment; amph=amphibolite. X-and Y-coordinates: Swiss national grid in km. Mineral abbreviations according to Kretz (1983 ). (Sil) means that sillimanite was used in P-T calculations, although it does not exist in thin section (see text). Mineral compositions: Bt: X Mg X Fe X Ti XVI AI X K X OH -Chl: X Mg X Fe -Grt: X Grs X Prp X Alm X Sps -Ilm: X Ilm X Mg X Mn X Hem -Ms: X K X Na XVI AI -Pl: X An X Ab X