Studies on Rb-Sr and Sm-Nd Isotope of Yadong Leucogranite in Tibet: Constraint on Its Age and Source Material

The detailed Rb-Sr and Sm-Nd isotopic studies of Yadong leucogranite show that it is characterized by large Rb-Sr and Sm-Nd isotopic variations. Its initial Sr isotopic ratios and eNd, calculated at its age of 13 Ma, are in the range 0. 756 ~0. 775 and -11. 6~ -16. 3, respectively. Because these values match the isotropic composition of the paragneisses in Nielamu Group distributed along High Himalayas, they are regarded as the most probable parental rocks of Yadong leucogranite. The Rb-Sr and Sm-Nd isotopic variations make it difficult to obtain a whole-rock Rb-Sr isochron. However, a mineral-whole rock Rb-Sr isochron age of 12. 9±0. 95 Ma with initial Sr isotopic ration of 0. 7744±0. 0008 was obtained, which is comparable well with that of other granitic plutons within Himalayan leucogranite zone. That the formation of Himalayan leucogranite is statistically consistent with the age of the crustal detachment and the rapid uplift of Tibetan Plateau implies leucogranites is an important sign of crustal movement and rapid uplift in post-orogenic period.