Zircon U–Pb geochronology of the Nyainqentanglha Group from the Lhasa terrane: New constraints on the Triassic orogeny of the south Tibet

Abstract

The studied amphibolite-facies metamorphic rocks, typical of the Nyainqentanglha Group, occur as a tectonic block in the central Lhasa terrane, south Tibet, and consist mainly of gneiss, amphibolite, quartz schist and marble. The amphibolite has a similar mineral paragenesis of plagioclase + garnet + biotite + amphibole + quartz ± muscovite, and were metamorphosed under P– T conditions of 0.63–0.66 GPa and 588–622 °C. Zircons from three dated samples include two types. First type of zircons has an inherited magmatic core and a metamorphic rim; whereas the other type of zircons is of metamorphic origin. LA-ICP-MS in situ U–Pb analysis indicates that the inherited magmatic cores of zircons yield widely variable 206Pb/ 238U ages ranging from 1857 Ma to 743 Ma, recording the magmatic-thermal events of the material source of the metamorphic rock protoliths. In contrast, the metamorphic zircons yield concordant 206Pb/ 238U ages of 225–213 Ma. The results indicate that the Nyainqentanglha Group has been subjected to Triassic metamorphism, instead of the Precambrian metamorphic basement of the Lhasa terrane as considered traditionally. These rocks, together with the coeval Sumdo eclogite and Pana blueschist, form a Permian to Triassic metamorphic belt with an E–W extension of up to 300 km. Therefore, this work provides new constraints on the late Paleozoic to early Mesozoic orogeny in the central Lhasa terrane, south Tibet.