Metamorphic P-T evolution of granulites in the central Ribeira Fold Belt, SE Brazil

被引:0
|
作者
Telmo M. Bento dos Santos
José M. Munhá
Colombo C. G. Tassinari
Paulo E. Fonseca
Coriolano Dias Neto
机构
[1] Estrada da Portela - Zambujal - Alfragide,Laboratório Nacional de Energia e Geologia (LNEG)
[2] Universidade de Lisboa,Centro de Geologia
[3] Universidade de São Paulo,Instituto de Geociências
来源
Geosciences Journal | 2011年 / 15卷
关键词
Ribeira Fold Belt; granulite; geothermobarometry; pseudosection; P-T path;
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学科分类号
摘要
Pseudosections, geothermobarometric estimates and careful petrographic observations of gneissic migmatites and granulites from Neoproterozoic central Ribeira Fold Belt (SE Brazil) were performed in order to quantify the metamorphic P-T conditions during prograde and retrograde evolution of the Brasiliano Orogeny. Results establish a prograde metamorphic trajectory from amphibolite facies conditions to metamorphic peak (T = 850 ± 50 °C; P = 8 ± 1 kbar) that promoted widespread dehydrationmelting of 30 to 40% of the gneisses and high-grade granitization. After the metamorphic peak, migmatites evolved with cooling and decompression to T ≈ 500 °C and P ≈ 5 kbar coupled with aH2O increase, replacing the high-grade paragenesis plagioclase-quartz-K-feldspar-garnet by quartz-biotite-sillimanite-(muscovite). Cordierite absence, microtextural observations and P-T results constrain the migmatite metamorphic evolution in the pseudosections as a clockwise P-T path with retrograde cooling and decompression. High-temperature conditions further dehydrated the lower crust with biotite and amphibole-dehydration melting and granulite formation coupled with 10% melt generation. Granulites can thus be envisaged as middle to lower crust dehydrated restites. Granulites were slowly (nearly isobarically) cooled, followed by late exhumation/retrograde rapid decompression and cooling, reflecting a two step P-T path. This retrograde evolution, coupled with water influx, chemically reequilibrated the rocks from granulite to amphibolite/greenschist facies, promoting the replacement of the plagioclase-quartz-garnet-hypersthene peak assemblage by quartz-biotite- K-feldspar symplectites.
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页码:27 / 51
页数:24
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