FLUID EVOLUTION FROM METAMORPHIC PEAK TO EXHUMATION IN HIMALAYAN GRANULITISED ECLOGITES, AMA DRIME RANGE, SOUTHERN TIBET

Abstract

A fluid inclusion study has been carried out on granulitised eclogites and associated amphibolite-facies veins from the eastern Himalaya (Ama Drime range, southern Tibet) in order to better characterise the fluid evolution of continental crust involved in Cenozoic subduction and continent-continent collision processes. Six distinct events of fluid influx have been characterised on the basis of petrographic observations and microthermometric measurements: 1) a high-density - medium-salinity aqueous fluid with CO2, trapped at the eclogitic peak (metamorphic stage Ml); 2) pure CO2, present at the granulitic stage M2; 3) a low-salinity - low-density aqueous fluid with minor CO2 in equilibrium with amphibole-bearing LP-M/HT mineral assemblage (stage M4); 4) a low-density aqueo-carbonic fluid responsible of vein formation (stage M5); 5) a subsequent influx of a low-density CO 2-rich fluid; 6) a late influx of very-low salinity aqueous fluid. Comparing these data with those obtained from other localities of the Himalayas and from other collisional orogens, an internal origin is proposed for eclogitic and granulitic fluids, whereas an origin from the underlying metasediments of the Lesser Himalaya is suggested for the subsequent types of fluid, safe for the meteoric origin proposed for the latest fluid influx. © 2007 E. Schweizerbart'schc Verlagsbuchhandlung.