Early Jurassic high-pressure metamorphism of the Amdo terrane, Tibet: Constraints from zircon U–Pb geochronology of mafic granulites

Abstract

The Amdo terrane holds one of the important keys to understand the tectonic evolution of Central Tibet. Here we report the results from a combined study of Cathodoluminescence imaging, LA–ICPMS U–Pb dating, and trace element analysis of zircons from high-pressure (HP) mafic granulites from the Amdo terrane. Zircons from the mafic granulite are rounded or anhedral and show sector or planar domains with low Th/U ratios, low REE contents, and flat HREE patterns, and can be divided into two types. The first type of zircons shows negative Eu anomalies, indicating that they coexisted with garnet and symplectitic plagioclase during the retrograde stage of metamorphism, whereas the second type possesses positive Eu anomalies, implying that they formed during the peak-metamorphic stage of the HP granulite. U–Pb data yielded a weighted mean 206Pb/238U age of 190.7±3.0Ma (MSWD=2.8, n=8) for zircons with positive Eu anomalies, interpreted as the time of the peak metamorphism, and a mean age of 181.4±1.8Ma (MSWD=2.2, n=16) for zircons with negative Eu anomalies corresponding to the amphibolite-facies retrogression. Combining with previous data, we suggest that the Amdo terrane became a microcontinent in the Tethys Ocean during the Permian–Triassic rifting between the Lhasa and Qiangtang terranes, and then the Amdo terrane subducted to about 50-km-depth beneath the Qiangtang terrane experiencing HP granulite-facies metamorphism in the Early Jurassic at 190Ma. The region was then uplifted to the mid-crustal levels (~20km) with an exhumation rate of ca. 3mm/year. The presence of Early Jurassic HP granulite shows that the Amdo terrane is related to the Bangong–Nujiang Suture zone instead of with the Mid-Qiangtang Suture zone.