Granulite-facies garnet-bearing metapelites, metabasics and calc-silicate rocks from the lower metamorphic complex (Kharta Gneiss) of the Greater Himalayan Sequence (GHS) in the Kharta region of S Tibet, E Himalaya, preserve textural and chemical evidence for prograde equilibration at temperatures of at least 700-720°C and pressures in the vicinity of 8 kbar during the main Himalayan metamorphism. Post-deformational reaction textures including clinopyroxene (± orthopyroxene) - plagioclase symplectites after garnet in calcsilicate rocks, and cordierite ± spinel coronas on sillimanite and garnet in metapelite granulites indicate a decompressional pressure-temperature evolution which is confirmed by geo-thermobarometry of zoned and symplectite minerals, and by modeled phase equilibria. A pervasive decompression through c. 3 kbar occurred at temperatures of about 700°C, and was followed by static recrystallization at temperatures of c. 650°C and low metamorphic pressures (~ 2.5 kbar).This type of trajectory, with most of the exhumation occurring at relatively constant metamorphic temperatures, requires a high-exhumation rate and/or a high heat flow, both typical conditions of post-collisional exhumation of thickened continental crust, with exhumation rates controlled by tectonic processes which operate on shorter time scales than erosion.
Thermal and baric evolution of garnet granulites from the Kharta region of S Tibet, E Himalaya .
2003
Abstract
Granulite-facies garnet-bearing metapelites, metabasics and calc-silicate rocks from the lower metamorphic complex (Kharta Gneiss) of the Greater Himalayan Sequence (GHS) in the Kharta region of S Tibet, E Himalaya, preserve textural and chemical evidence for prograde equilibration at temperatures of at least 700-720°C and pressures in the vicinity of 8 kbar during the main Himalayan metamorphism. Post-deformational reaction textures including clinopyroxene (± orthopyroxene) - plagioclase symplectites after garnet in calcsilicate rocks, and cordierite ± spinel coronas on sillimanite and garnet in metapelite granulites indicate a decompressional pressure-temperature evolution which is confirmed by geo-thermobarometry of zoned and symplectite minerals, and by modeled phase equilibria. A pervasive decompression through c. 3 kbar occurred at temperatures of about 700°C, and was followed by static recrystallization at temperatures of c. 650°C and low metamorphic pressures (~ 2.5 kbar).This type of trajectory, with most of the exhumation occurring at relatively constant metamorphic temperatures, requires a high-exhumation rate and/or a high heat flow, both typical conditions of post-collisional exhumation of thickened continental crust, with exhumation rates controlled by tectonic processes which operate on shorter time scales than erosion.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.