湖南白马山龙潭超单元、瓦屋塘花岗岩锆石SHRIMP U-Pb年龄及其地质意义

运用阴极发光技术，对湖南白马山龙潭超单元2个样品和瓦屋塘花岗岩1个样品的锆石进行了内部结构分析，在此基础上利用锆石SHRIMP U-Pb定年方法进行了同位素年代学测定。其中，白马山龙潭超单元2个黑云母二长花岗岩样品分别给出了(215.9±1.9) Ma和(212.2±2.1) Ma的主体谐和年龄。同时还测得了一组较年轻的谐和年龄((201.0±2.8) Ma)和一组较老的锆石核部年龄(230.3~227.0 Ma)，表明研究区印支晚期存在多期花岗质岩浆的侵入活动。瓦屋塘岩体黑云母二长花岗岩1个样品给出了(217.7±1.8) Ma的谐和年龄。这2个岩体的形成进一步佐证了华南大陆印支晚期岩浆活动于210~225 Ma，达到岩浆活动的峰期。地球化学测试结果显示，白马山和瓦屋塘岩体均为弱过铝-强过铝质花岗岩，具壳源型花岗岩的特征，形成于后碰撞期或碰撞晚期的构造环境，源于早元古代变质杂砂岩的部分熔融。结合区域大地构造背景认为，这2个岩体形成于秦岭-大别和松马2条印支期缝合带碰撞结束后的印支晚期伸展构造背景下，为热-应力松弛阶段，板内挤压加厚的地壳减压熔融作用的产物。

SHRIMP U-Pb Dating of Zircons from the Baimashan Longtan Super-Unit and Wawutang Granites in Hunan Province and Its Geological Implication

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.