Abstract:

In this study, we present detailed petrographic study and LA-ICP-MS zircon U-Pb dating for a garnetbearing amphibolite from the Nyingchi Complex in the southeastern Lhasa terrane. Petrographic study indicates that the garnetbearing amphibolite had undergone two stages of peak granulitefacies and retrograde amphibolefacies metamorphism. The peak mineral assemblage is characterized by Grt+highTi Amp + Hy+Pl+Qtz+Rt. There are a large number of rutile exsolutions in the garnet, quartz and amphiboles, suggesting that the primary composition of these minerals have high Ti contents. The retrograde amphibolefacies assemblage is characterized by lowTi Amp+Pl+Czo+Qtz+Rt. Based on the Tiinquartz (TitaniQ) thermometers, the peak and retrograde metamorphic temperatures are 803924 ℃ and 555732 ℃, respectively. The zircons in the garnetbearing amphibolite show corerim structure in CL images. The cores are characterized by high Th/U ratios, enrichment in HREE, a positive Ce anomaly and a marked negative Eu anomaly, suggesting that they are typical of magmatic zircon. They yielded a weighted 206Pb/238U mean age of 89.3±0.6 Ma, which is interpreted as the crystallization age of the protolith of garnetbearing amphibolite. By contrast, the rims exhibit no zoning. They have low Th/U ratios and MHREE contents, and show weak or no Eu anomaly, indicating that they formed in the peak metamorphic stage. The 206Pb/238U age of 81.1±0.8 Ma obtained on these rims is interpreted as the peak metamorphic age. Combined with regional studies, we suggest that the about 81 Ma hightemperature metamorphism resulted from the upwelling of asthenosphere through the slab window opened as a result of ridge subduction.

Key words:  Lhasa terrane, high-temperature metamorphism, rutile exsolution, TitaniQ geothermometer, zircon U-Pb dating