西藏长青温池－波弄贡变质带片麻岩锆石ELA-ICP-MS U-Pb年龄及地质意义

The metamorphic gneisses in the Lizhi-Tongmai region, Xizang (Tibet) host aciculate zircon grains. The morphologic features such as euhedral crystal form of zircon grains, inclusion minerals such as mica, potassic feldspar, albite, quartz and glass in zircon grains, together with cathodoluminescence (CL) photos showing the oscillatory zoning and high Th/U ratios indicate that the aciculate zircon grains are not formed by metamorphic processes but derived from volcanic rocks. The zircon grains are affected by metamorphic fluids. ELA-ICP-MS U-Pb ages of the aciculate zircon grains from the gneisses are (40.7±0.3) Ma with MSWD=2.35. The rim, which were affected by the metamorphic fluids, and the core of the zircon grains have almost the same ages. The features of the zircon grains together with their U-Pb age suggest that the protolith of the metamorphic rocks are Cenozoic volcanic sedimentary rocks, rather than the Proterozoic base.