Geochemistry And Petrogenesis Of The Migmatite In Cuona,Tibet

The collision between the Eurasia and the Indian plate formed the Himalaya orogenic belt,which is the youngest and the largest orogen in the world and led to pervasive deformation,metamorphism and magmatism.The melt from the continental crust will significantly affect the tectonic evolution of orogenic belts and can connect the metamorphism of the lower crust with the tectonic deformation of the upper crust.The migmatite formed by anatexis is a good probe to investigate these processes.The migmatite is considered to be a transitional rock formed by the melt which has not been segregated from its source.The study of migmatite can help the study of the leucogranite and Himalaya orogenic belt.Therefore,in order to explore the origin of the migmatite and the relationship between the migmatite and the leucogranite,which is exposed on a large area,in The Greater Himalayan Crystalline Sequence,21 rock samples were collected in the eastern part of the belt in Cuona,Tibet.There are 9 pairs of leucosomes and melanosomes of migmatites,1 leucogranite sample and 2 metasedimentary rock samples.This paper systematically studied the petrology,zircon U-Pb geochronology and geochemistry of the investigated samples and compared them with previously reported migmatite and leucogranite data in The Greater Himalayan Crystalline Sequence.The zircon U-Pb geochronology data show that the age of the migmatite was 19.5 Ma to 14.5 Ma and the age of the leucogranite was 14 Ma.The partial melting lasted for about 5 Ma.These results were in coincidence with literature data.,and it is also consistent with the ages of pervasive leucogranites in The Greater Himalayan Crystalline Sequence.The characteristics of Ti-in-zircon thermometer,petrology and geochemistry show that the protolith of the migmatite is the metasedimentary rock in the lower parts of The Greater Himalayan Crystalline Sequence,and the diagenesis temperature is 600 ?-700 ?.Although the mechanisms for these migmatite samples are not exactly the same,they were formed by dehydration partial melting of hydrous minerals such as biotite and hornblende in the protolith,and the melt from protolith are accumulated and extracted to form the migmatite.Some melanosomes of the migmatites were from the residues after the segregation of melt,and others were formed by the reaction between the residues and the melt.Only one sample was not formed by partial melting,which was formed by the injection of exogenous melt.The leucogranite and the leucosome have similar mineral assemblage,structure and texture.Moreover,the leucosome and the leucogranite show similar major and trace elemental compositions,demonstrating that the leucogranite is extracted from the migmatite.Based on previous data,the leucosomes were suggested to be produced by partial melting of the metasedimentary rocks in The Greater Himalayan Crystalline Sequence during high-temperature metamorphism.And the Himalayan leucogranite were derived from these melts which were segregated from their sources.