Determination of Eocene–Oligocene (30–40 Ma) deformational time by zircon U–Pb SHRIMP dating from leucocratic rocks in the Ailao Shan–Red River shear zone, southeast Tibet, China

Abstract

The Ailao Shan–Red River (ASRR) shear zone, a long, narrow metamorphic belt that strikes NW–SE, is the continuation of the southeastern margin of the Tibetan Plateau. It mainly comprises amphibolite-facies mylonitic gneiss. In this study, we report zircon concordant U–Pb SHRIMP ages of 37.52 Ma ± 0.67 Ma [mean square weighted deviation (MSWD) = 1.4] and 30.8 Ma ± 0.3 Ma (MSWD = 3.2) from a granitic gneiss in the northern part of the Ailao Shan metamorphic belt, and from a leucogranitic dike in the middle of the zone, respectively. Zircon U–Pb ages between 35 Ma and 41 Ma were also obtained from a biotite gneissic granite in the southern part of the belt. Contrasting the internal structure of the zircon crystals with our new U–Pb dating suggests that their nucleation and growth initiated during a late Eocene tectonic uplift event between ca. 40 and 35 Ma. Detailed field observations and zircon U–Pb dating results suggest that the deformation time of mylonitic gneiss and granitic gneiss in the ASRR was 40–30 Ma, the formation age of the gneissic schistosity of the ASRR metamorphic belt was as early as ca. 40 Ma.