HR: 14:05h
AN: V42F-02    [PDF]
TI: Modeling Peak T and Retrograde Evolution of Ultra-hot Granulites From Brazil
AU: * Baldwin, J A
EM: jbaldwin@geol.umd.edu
AF: University of Maryland, Lab for Crustal Petrology, Dept of Geology, College Park, MD 20742
AU: Brown, M
EM: mbrown@geol.umd.edu
AF: University of Maryland, Lab for Crustal Petrology, Dept of Geology, College Park, MD 20742
AU: Moraes, R
EM: remoraes01@yahoo.com
AF: UFRJ, Dept of Geology, Rio de Janeiro, Brazil
AU: Fuck, R A
EM: rfuck@unb.br
AF: UNB, Instituto de Geosciences, Brasilia, Brazil
AU: Piccoli, P M
EM: piccoli@geol.umd.edu
AF: University of Maryland, Lab for Crustal Petrology, Dept of Geology, College Park, MD 20742
AB: Granulite-facies rocks record extreme thermal perturbation of Earth's lithosphere. Characterizing these rocks yields information critical to our understanding of the P-T-X environment of formation and modification of continental crust. Ultrahigh-temperature (UHT) metamorphism was introduced for rocks that record extreme thermal conditions. Mineral associations characteristic of UHT conditions include Spr-Qtz and Sp-Qtz, and rare Opx-Sil-Qtz, which requires XMg $>$0.60-0.65, corresponding to minimum P and T of 8 kbar and 850$\deg$C. The migmatitic aspect of metapelites and metagreywackes suggests they were once melt-bearing, but survival of peak assemblages and depleted compositions suggest melt loss. Unless most melt is extracted, significant retrogression will occur during cooling as melt-consuming reactions are crossed. Where UHT assemblages are preserved, they occur within larger areas of common granulite. The restricted occurrence of UHT assemblages raises questions about the metamorphic processes involved. Did the extreme P-T conditions occur regionally? If so, did protolith composition restrict the record of these conditions or was retention of melt responsible for widespread retrogression? Given the clockwise P-T path inferred for many granulite terranes, how was the extreme thermal perturbation required by UHT assemblages achieved? We address these questions with new petrologic data from the An polis-Itau‡u Complex (AIC) of central Brazil, within the Neoproterozoic BrasĦlia Belt. The BrasĦlia Belt, including the Goi s arc, lies between the Sao Francisco and Amazon Cratons. Within the internal zone, the AIC comprises orthogranulite (metagabbro, charnockitic/enderbitic gneiss) and paragranulite (Grt-Sil/Opx-Sil gneiss, calc-silicate rock); magmatism and metamorphism occurred in the interval 650-630 Ma. Spr-Qtz occurs in Grt-Opx-Sil-Qtz assemblages at localities ~20 km apart, recording extreme T. At Fazenda Calif˘rnia, N of Goiƒnia, the post-peak evolution is constrained to begin at ~10 kbar by down T stability of Grt-Opx-Sil-Qtz and absence of Crd. To the N near Damolƒndia, the post-peak evolution is constrained by a succession of melt-present reactions, inferred from coronae/symplectites between Grt, Opx and Sil, crossed at P $<$9 kbar. We report new data from samples from Fazenda Calif˘rnia. Equant Opx (up to 10 mm; with exsolved Rt and Sil, and rare Spr and Bt inclusions) and sporadic elongate Grt (up to 15 mm; with Spr inclusions) occur as porphyroclasts in a mylonitic fabric defined by variable, but smaller (commonly ~3 mm) elongate Opx and laths of Sil set in a matrix of mm-size Qtz (with rare Spr inclusions) showing marked SPO and patchy CPO. Spr and Qtz are common as inclusions in Sil. In one sample, Crd occurs sporadically (together with Pl) separating Opx from Sil (rarely from Spr). These features are consistent with the peak assemblage Grt-Spr-Opx-Qtz-L, followed by reaction during cooling to produce Sil and, rarely, Crd; this implies that retrograde evolution was near the P-T stability of the Crd-producing reaction, with Crd controlled XMg. Al2O3 in equant Opx reaches 12.5-12.9 wt % in cores [y(Opx) of ~0.30], to suggest maximum T of 1175-1150$\deg$C. Rims are zoned to $<$8.0 wt % Al2O3 [y(Opx) of ~0.19]. Elongate Opx have lower Al2O3 in cores than porphyroclasts but similar rim compositions. Rim compositions indicate cooling to $<$975-950$\deg$C. We speculate that closure of an ocean basin inboard of the Goi s arc coeval with continuing subduction outboard enabled synchronous magmatism and crustal thickening to achieve UHT conditions.
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting