庐山变质核杂岩伸展拆离和岩浆作用的年代学约束

Extensional Detachment and Magmatism of the Lushan Metamorphic Core Complex:Constraints From ^(40)Ar/^(39)Ar and U-Pb Geochronology

庐山变质核杂岩是华南中生代岩石圈减薄事件的浅层响应。研究伸展拆离和岩浆作用在其形成过程中的相互关系,对于认识地壳浅层的伸展减薄过程和变质核杂岩的形成机制具有重要的意义。在宏观和显微构造研究的基础上,分别对基底拆离断层和玉京山岩体进行了40Ar/39Ar年代学和锆石U-Pb年代学研究,为伸展拆离和岩浆作用提供了年代学约束,主要获得了以下认识:(1)变质核杂岩核部玉京山花岗岩体的锆石LA-ICP-MSU-Pb年龄为(133.0±2.1)Ma,代表了变质核杂岩核部岩浆活动的启动时间。(2)基底拆离断层下盘韧性剪切带内的白云母40Ar/39Ar年龄(140.4±1.7)Ma代表了庐山变质核杂岩的形成时间,也代表了庐山地区岩石圈伸展减薄的时间,即岩石圈伸展减薄始于早白垩世。(3)庐山变质核杂岩是中生代不同尺度的伸展拆离和岩浆活动共同作用的结果,在地壳浅层的具体形成过程中伸展拆离早于岩浆侵位,基底拆离断层的减压效应为深部岩浆的上升侵位创造了条件,岩浆侵位造成山体的隆升,又引发了盖层的重力滑脱。

The Lushan metamorphic core complex（MCC） is the shallow response of lithospheric extending and thinning during the Mesozoic in South China.Studying the relationship between extensional detachment and magmatism during the forming process of MCC,is of great significance in understanding the extension-thinning process of shallow crust as well as the forming mechanism of MCC.Based on the mesoscopic and microscopic investigation,^40Ar/^39Ar geochronological studies on the basement detachment fault and U-Pb zircon geochronology studies on the Yujing granite were carried out.These geochronology data provide new constrains for extensional detachment and magmatism.It is concluded that：（1） the Yujing granite was formed at（133.0±2.1） Ma（LA-ICP-MS U-Pb zircon dating）,and the intrusion of the granite signifies the beginning of magmatism of MCC;（2） the muscovite ^40Ar/^39Ar dating of ductile shear zone on the foot wall of basement detachment fault was（140.4±1.7） Ma,which represents both the forming time of Lushan MCC and the time of lithospheric extension-thinning in the Lushan region.Regional lithospheric extension-thinning began in late Jurassic;（3） the Lushan MCC was the result of different scale extensional detachment and magmatism during the Mesozoic.The forming processes of shallow crust are as follows：Extensional detachment took place earlier than magmatic emplacement.Decompression of the basement detachment fault led to the ascending and emplacing of magma at depth.Then magmatic emplacement led to the Lushan massif uplifting which provided the motivity for the gravitational detachment of the cover.