Abstract:

In this study, internal structures of zircons from the Baimashan and Wawutang biotite monzogranite plutons are analyzed by the CL technology, and reliable ages of zircons are dated by the SHRIMP U-Pb method. Two samples from the Baimashan Longtan super-unit yield weighted mean ²⁰⁶Pb/²³⁸U ages of (215.9±1.9) Ma and (212.2±2.1) Ma, respectively. Moreover, several zircons exhibit core-rim textures, in which the zircon cores yield older ages of 230.3-227.0 Ma, and the zircon rims yield younger mean ²⁰⁶Pb/²³⁸U age of (201.0±2.8)Ma. These data suggest that the Baibashan biotite monzogranite was generated by multi-phase magmatic intrusions. One biotite monzogranite sample from the Wawutang granite yields a weighted mean ²⁰⁶Pb/²³⁸U age of (217.7±1.8) Ma. These geochronological data provide new evidences for the conclusion that there occurred a peak magmatism (210-225 Ma) in South China during the late stage of the Indosinian orogeny. Geochemical studies indicate that the Baimashan and Wawutang plutons are typical post-orogenic crust-sourced peraluminous granites, resulted from partial melting of the Paleoproterozic metamorphic greywacke. These data allow us to infer that the two plutons were formed after the Early Triassic collision along the Songma and Qingling-Dabie sutures, associated with the parting melting of the thickened crust in the post orogenic extensional setting.

Key words: Baimashan, Wawutang, SHRIMP U-Pb dating of zircon, Indosinian granite, crustal extension, Hunan Province