Nappes in the southern sector of the Southern Brasília Belt record suturing of the Paranapanema Block and Socorro–Guaxupé Arc with a subducted passive margin on the western side of the São Francisco Craton. We report secondary ion mass spectrometry U–Pb zircon ages that for the first time constrain the age of: (1) retrograded eclogite from a block along the tectonic contact beneath the uppermost nappe in a stack of passive margin-derived nappes; (2) high-pressure granulite-facies metamorphism in the uppermost passive margin-derived nappe; (3) high-pressure granulite-facies metamorphism in the overlying arc-derived nappe. Rare zircons from a retrograded eclogite yield a 206 Pb/ 238 U age of 678 ± 29 Ma, which we interpret as most likely to date close-to-peak- P metamorphism and to provide a minimum age for detachment of the overlying passive margin-derived nappe from the subducting plate. Zircon associated with ilmenite in samples from two structural levels in the passive margin-derived high-pressure granulite nappe yields 206 Pb/ 238 U ages of 648 ± 12 and 647 ± 11 Ma, and Ti-in-zircon crystallization temperatures from c . 860 °C down to c . 785 °C, but skewed toward the lower part of the range. These data indicate zircon formation during cooling from around peak T to the solidus, consistent with the high- T retrograde P – T path deduced from microstructures linked to phase assemblage fields in isochemical phase diagrams. Rb–Sr multi-mineral–whole-rock isochrons from two samples from close to the bottom of this nappe date formation of a retrograde sillimanite-bearing penetrative fabric to c . 590 Ma at temperatures of c . 750 °C (based on Ti-in-quartz thermometry). Rare zircons from leucosome in high-pressure granulite from the overlying arc-derived nappe yield a 206 Pb/ 238 U age of 622 ± 28 Ma and Ti-in-zircon crystallization temperatures from c . 970 °C down to c . 820 °C, which we interpret to record formation of zircon during cooling from peak high-pressure granulite-facies conditions. These ages indicate that the first stage of craton amalgamation in West Gondwana may have occurred earlier than previously inferred. Supplementary material: Geochemical and geochronological data are available at http://www.geolsoc.org.uk/SUP18377 .

In the southern sector of the Southern Brası´lia Belt, late Neoproterozoic arc-passive margin collision resulted in juxtaposition of an arc-derived nappe (the Socorro-Guaxupe´Nappe) over a stack of passive margin-derived nappes (the Andrelaˆndia Nappe Complex) that lies on top of autochthonous basement of the Sa˜o Francisco Craton. (U-Th)-Pb monazite ages are reported from the high-grade nappes of the Andrelaˆndia Nappe Complex to better constrain the high-temperature retrograde evolution. For residual HP granulites from the uppermost Treˆs Pontas-Varginha Nappe, (U-Th)-Pb ages of c. 662 and 655 Ma from low yttrium monazite inclusions in the rims of, or associated with garnet are interpreted to date the late-stage close-to-peak prograde evolution, whereas an age of c. 648 Ma from a similar low yttrium monazite inclusion is interpreted to record post-peak recrystallization with melt via factures in garnet. For the same nappe, ages of 640-631 Ma retrieved from higher yttrium areas or cores in monazite grains that occur both as inclusions in garnet and in the matrix are interpreted to record growth of monazite either by local breakdown of garnet (±older monazite) and mass exchange with a matrix melt reservoir along cracks or growth from residual melt in the matrix as it crystallized during high-pressure, closeto-isobaric cooling close to the solidus, the temperature of which, at a given pressure, varies with bulk composition of the residual granulites. (U-Th)-Pb ages in the range 620-588 Ma from lower yttrium areas in these monazite grains and from matrix-hosted patchy monazite are interpreted to date exhumation, as recorded by close-to-isothermal decompression and subsequent close-to-isobaric cooling. Older monazite ages in this group are interpreted to record late-stage interaction with melt close to the solidus whereas younger monazite ages are interpreted to record recrystallization of monazite by dissolution-reprecipitation owing to ingress of alkali fluid from the Carmo da Cachoeira Nappe beneath as fluid was released by crystallization of in-source melt at the solidus. In the underlying Carmo da Cachoeira Nappe, higher yttrium areas in monazite and one single domain monazite yield chemical ages of 619-616 Ma, which are interpreted to date growth as in-source melt crystallized close to the solidus along the high-pressure, close-to-isobaric segment of the retrograde P-T evolution. Younger (U-Th)-Pb ages of 600-595 Ma retrieved from lower yttrium areas and one single domain monazite are interpreted to record recrystallization of monazite by dissolution-reprecipitation owing to release of fluid at the solidus during exhumation of this nappe. Monazite from the Carvalhos Klippe, interpreted to be correlative with the uppermost nappe, yields a wide range of (U-Th)-Pb ages: for two zoned grains, c. 619 and c. 614 Ma from higher yttrium cores, and c. 583 and c. 595 Ma from lower yttrium rims; and, 592-580 Ma from single domain grains in one sample, and ages of c. 593 and c. 563 Ma from monazite in a second sa...

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The incorporation of hydrogen into the coesite structure was investigated at pressures ranging from 4.0-9.0 GPa and temperatures from 750-1300 ∞C using Al and B doped SiO 2 starting materials. The spectra show four sharp bands (n 1 , n 2a , n 2b , and n 3 ) in the energy range of 3450-3580 cm -1 , consistent with the hydrogarnet substitution [Si 4+(T2) + 4O 2-= va T2 + 4OH -], two weak sharp bands at 3537 and 3500 cm -1 (n 6a and n 6b ) attributed to B-based point defects, and two weaker and broad bands at 3300 and 3210 cm -1 (n 4 and n 5 ) attributed to substitution of Si 4+ by Al 3+ + H. More than 80% of the dissolved water is incorporated via the hydrogarnet substitution mechanism. The hydrogen solubility in coesite increases with pressure and temperature. At 7.5 GPa and 1100 ∞C, 1335 H/10 6 Si is incorporated into the coesite structure. At 8.5 GPa and 1200 ∞C, the incorporation mechanism changes: in the IR spectra four new sharp bands appear in the energy range of 3380-3460 cm -1 (n 7 -n 10 ) and the n 1 -n 3 bands disappear. Single crystal X-ray diffraction, Raman spectroscopy, polarized single-crystal and in situ high-pressure FTIR spectroscopy confirm that the new bands are due to OH -in coesite. The polarization and high-pressure behavior of the n 7 -n 10 OH bands is quite different from that of the n 1 -n 3 bands, indicating that the H incorporation in coesite changes dramatically at these P and T conditions. Quantitative determination of hydrogen solubility in synthetic coesite as a function of pressure, temperature, and chemical impurity allow us to interpret observations in natural coesite. Hydrogen has not previously been detected in natural coesite samples from ultra high-pressure metamorphic rocks. In this study, we report the first FTIR spectrum of a natural OH-bearing coesite. The dominant substitution mechanism in this sample is the hydrogarnet substitution and the calculated hydrogen content is about 900 z ± 300 H/10 6 Si. The coesite occurs as an inclusion in diamond together with an OH-bearing omphacite. The shift of the OH-bands of coesite and omphacite to lower energies indicates that the minerals are still under confining pressure.