P-T PATH OF A KYANITE- K-FELDSPAR (HP) GRANULITE OF THE ANDRELÂNDIA SEQUENCE, BRASÍLIA FOLD BELT, BRAZIL

The southern portion of the Neoproterozoic Brasília Fold Belt, Brazil, is dominated by the Andrelândia Sequence, which is composed of six different lithofacies associations, A1 through A6, including mainly pelitic and psammitic rocks. These lithofacies associations represent an evolution from opening of a rift, through several marine stages, including turbidite flows, to marine flooding events and sedimentation in the deep sea. Amphibolite bodies are common in the A1, A2 and A6 associations and represent evidence of the tholeiitic magmatism related to the basin evolution. Metamorphic conditions vary from middle greenschist in the autochthonous domain on the craton to the east to HP granulite facies in the upper nappes to the west, with kyanite-K-feldspar in metapelitic rocks and clinopyroxene-garnet-quartz-plagioclase in mafic rocks. Eclogites occur as rare lenses, reinforcing the HP character of metamorphism. The metamorphic grade increases towards the top of the upper nappes both as a gradational inverted metamorphism within a single nappe and by thrusting of higher grade nappes over the lower grade nappes. Anatectic granites and voluminous leucosome veins are a common feature in the upper nappes, suggesting that widespread melting occurred during or close to metamorphic peak. Near the city of Três Pontas, within the uppermost nappe, an association of felsic and mafic granulites may be studied in detail in a quarry. Here, we report results from felsic granulites composed of Qtz, Grt, Kfs, Ky, Rt and minor Bt. St occurs as inclusions in Grt, which sometimes have irregular rims; leucosome occurs concordant with and cross cutting the layering. No Opx has been recorded from the felsic granulite, and although plagioclase is rare orthoclase has oligoclase exsolution. We infer that the felsic granulite crossed the St stability field during the prograde segment (increasing P and T) of the P-T evolution to reach the Bt + Pl + Qtz melting reaction at T ~ 950 oC and P > 17 kbar (based on experimental data by Vielzeuf & Montel, CMP, 1994) to produce Grt + melt + ternary feldspar (but without Opx); a clockwise P-T path is implied. Calculations using THERMOCALC with the Holland & Powell dataset (JMG 1998) suggest lower P of 12.5 ± 1.9 kbar but similar T of 920 ± 75 ^oC, or using the average P-T mode P of 20 ± 5 kbar but similar T of 1200 ± 250 ^oC.