MODELING ULTRA-HOT BEAUTIES FROM BRAZIL: PEAK TEMPERATURE AND P-T EVOLUTION

The Brasília Belt, which includes the Goiás magmatic arc, lies between the São Francisco and Amazon Cratons. Within the internal zone, the Anápolis-Itauçu Complex comprises NW-SE oriented units of orthogranulite (metagabbro and charnockitic/enderbitic gneiss) and paragranulite (Grt-Sil/Opx-Sil gneiss and calc-silicate rock). Magmatism and metamorphism occurred in the interval 650-630 Ma (Piuzana et al. Precambrian Res 2003). Spr-Qtz occurs in Grt-Opx-Sil-Qtz assemblages at localities ~20 km apart, recording extreme T (Moraes et al. J Pet 2002). At Fazenda Califórnia, N of Goiânia, the post-peak evolution is constrained to begin at ~10 kbar by down T stability of Grt-Opx-Sil-Qtz and the apparent absence of Crd. To the N near Damolândia the post-peak evolution is constrained by a succession of melt-present reactions, inferred from coronae/symplectites between Grt, Opx and Sil, that were crossed at P < 9 kbar. We report new data from a suite of samples from the Fazenda Califórnia locality. Equant Opx (up to 10 mm across; with exsolved Rt and Sil, and rare Spr and Bt inclusions) and sporadic elongate Grt (up to 15 mm in length, with Spr inclusions) occur as porphyroclasts in a mylonitic fabric defined by variable, but smaller (commonly ~3 mm long) elongate Opx and laths of Sil set in a matrix of mm-size Qtz (with very 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 the retrograde evolution was near the P-T stability of the Crd-producing reaction, with Crd controlled by differences in X_Mg. In all samples, Al₂O₃ in equant Opx reaches 12.5-12.9 wt. % in cores [y(Opx) of ~0.30], to suggest maximum T of 1175-1150°C (Harley & Motoyoshi CMP 2000; Kelsey et al. JMG 2003). 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°C. We speculate that closure of an ocean basin inboard of the Goiás magmatic arc coeval with continuing subduction outboard enabled synchronous magmatism and crustal thickening to achieve UHT conditions.