印度-亚洲大陆碰撞过程中新特提斯洋岩石圈的命运

印度-亚洲大陆碰撞之后的新特提斯洋板片的断离过程及其产生的岩浆作用一直是青藏高原南部地质研究中受到广泛关注但存在极大争议的问题.分析了青藏高原南部拉萨地块上新特提斯洋板片断离存在的问题，总结了目前用于限制板片断离过程的岩石学方法.对拉萨地块南部典型地区早新生代镁铁质岩石开展了详细的地质年代学、主微量元素和Sr-Nd-Hf同位素地球化学分析，厘定了~57 Ma和~50 Ma与新特提斯洋板片断离过程密切相关的两套岩石.~57 Ma的镁铁质岩石显示出高的Zr/Y和Ti/Y比值，不同于拉萨地块南部广泛分布的岛弧岩浆地球化学特征，表明它们形成于板内伸展背景下，很可能代表了新特提斯板片断离的开始.~50 Ma的镁铁质岩石为富闪深成岩，反映了印度-亚洲大陆碰撞后南拉萨地块岩石圈中的富水环境，暗示大洋板片断离后仍然持续释放流体至上覆岩石圈地幔中.结合拉萨地块上已有的镁铁质岩石的年代学和地球化学数据，重建了新特提斯洋在印度-亚洲大陆碰撞之后从初始撕裂至板片完全断离的全过程，即新特提斯板片在~57 Ma开始发生初始撕裂，随后以高角度俯冲并与印度大陆岩石圈脱离，导致中拉萨和南拉萨地块同时出现广泛的镁铁质岩浆作用，在~50 Ma大洋板片完全断离.拉萨地块内部岩石圈地幔地球化学组成存在极大的不均一性，中拉萨地块和南拉萨地块东部的局部地区存在古老的岩石圈物质组成，而南拉萨地块中部主要为亏损的岩石圈.拉萨地块内局部古老富集岩石圈可能受到新特提斯洋板片断离后深部地幔物质上涌的影响转变为新生的亏损岩石圈，这一过程很可能促进了拉萨地块的中酸性岩浆大爆发作用和大陆地壳生长. 

Destiny of Neo-Tethyan Lithosphere during India-Asia Collision

The breakoff of the Neo-Tethyan slab and related magmatism after India-Asia continental collision has been controversial topics in the geological study of the southern Tibetan Plateau. In this study, it reviews the unsolved problems and petrological methods for exploring the process on the Neo-Tethyan slab breakoff. Based on the systematic geochronology, major and trace elements and Sr-Nd-Hf isotope geochemical analyses of the Early Cenozoic mafic rocks in the typical areas of the Lhasa Terrane, it is found that two suites of mafic rocks with ages of ~57 Ma and ~50 Ma have close relationship with the Neo-Tethyan slab breakoff. The ~57 Ma ma?c rocks are characterized by high Zr/Y and Ti/Y ratios, and their geochemistry indicates an intraplate af?nity rather than arc magmas, indicating that they likely correspond to the magmatic expression of the initial stage of Neo-Tethyan slab breakoff. The ~50 Ma ma?c rocks are appinites, reflecting the water-rich environment in the lithospheric mantle of the southern Lhasa Terrane after the India-Asia collision, suggesting the ?ux of slab ?uids through the lithospheric mantle during breakoff of the Neo-Tethyan slab. Integrating the geochronological and geochemical data of the Early Cenozoic mafic magmatism in the Lhasa Terrane, we have reconstructed the processes from initial tear to completely breakoff of the Neo-Tethyan lithosphere after India-Asia continent collision. The initial tear of the Neo-Tethyan lithosphere occurred at ~57 Ma, then the slab detached from the India lithosphere with a high angle subduction, which resulted in the simultaneous occurrence of extensive mafic magmatism in the central and southern Lhasa Terrane. The complete slab breakoff happened at ~50 Ma. The isotopic compositions of Early Cenozoic magmatic rocks reveal that there was great geochemical heterogeneity of lithospheric mantle beneath the Lhasa Terrane. There were ancient lithospheric materials in the central Lhasa Terrane and eastern part of the southern Lhasa Terrane, while there was mainly depleted juvenile lithosphere in the central part of the southern Lhasa Terrane. The local ancient enriched lithosphere in the Lhasa Terrane is likely to transform into a juvenile depleted lithosphere by the upwelling of the deep mantle material, which may promote the eruption of the felsic magmatism and the growth of continental crust in the Lhasa Terrane.