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

• Published in: International geology review 2014-01, Vol.56 (1), p.74-87
• Main Authors: Li, Baolong, Ji, Jianqing, Wang, Dandan, Gong, Junfeng, Ma, Zongjin
• Format: Article
• Language: English
• Subjects: amphibolite-facies peak metamorphism; Eocene-Oligocene; the Ailao Shan-Red River shear zone; zircon U-Pb ages
• ISSN: 0020-6814; 1938-2839
• DOI: 10.1080/01431161.2013.819960

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.