青藏新生代钾质火山活动的时空迁移及向东部玄武岩省的过渡:壳幔深部物质流的暗示

Migration of the Tibetan Cenozoic Potassic Volcanism and Its Transition to Eastern Basaltic Province:Implications for Crustal and Mantle Flow

中国西部与东部在构造作用、岩浆活动、岩石圈结构、沉积盆地、地貌等方面存在着巨大的差异。印度-亚洲大陆碰撞以来青藏高原火山岩的时空分布，显示了从高原腹地发端向周缘迁移的趋势。在同碰撞阶段（65—40Ma）的末期，火成活动背离雅鲁藏布碰撞缝合带向北迁移。在45—6Ma期间，火山活动从高原腹部开始，然后向外迁移，暗示软流圈上隆呈波状向外扩展。最后（6Ma-近代），火山活动向着高原的北西、北东-东、南东三隅迁移。这种醒目的火山活动时空迁移型式，可以解释为由印度和欧亚两个较厚大陆板块的靠近、碰撞诱发的软流圈地幔流及深部地壳物质流动。推测可能存在两个地幔流“通道”，一条“通道”可能通过西秦岭礼县-宕昌一带伸向中国东部；另一条“通道”可能沿“三江”带向东南经云南马关-屏边一带与中国东南部和越南的新生代玄武岩省相接。在礼县-宕昌、马关-屏边一带，表现出从青藏高原钾质火山岩省向着中国东部玄武岩省过渡的特征。大量地球物理资料也支持关于软流圈地幔与深部地壳层次上的横向物质流动的认识。为了进一步检验这种认识是否正确，需要在青藏高原钾质岩省与东部玄武岩省的过渡带选择若干短剖面，进行系统的研究工作，鉴别岩石源区组成和形成深度，反演岩石圈和软流圈地幔性质，揭示两个岩省之间的关系，为鉴别大陆碰撞引起的岩石圈或软流圈层次的物质流动与块体之间相互作用提供岩浆作用约束，同时也为东亚大背景之下，以新生代印度-亚洲碰撞为主导的中国西部与以太平洋板块俯冲为主导的中国东部的构造交接关系提供岩浆作用限制。

There are tremendous contrasts between eastern and western China in tectonics, magmatism, lithospheric structure, sedimentary basins and geomorphology. The spatial and temporal distribution of volcanic rocks in the Tibetan Plateau since India-Asia continental collision shows a highly distinctive migration pattern. Initially, igneous activity migrated at ca. 45 Ma, around the end of syn-collisional period, away from the Tsangpo collision suture. Between ca. 45 and 6 Ma, volcanic activity implying wavelike outward propagation of upwelling asthenosphere. migrated outward from the plateau interior, A third stage, still in progress, is marked by the migration of activity to northwestern, northeast-eastern, and southeastern peripheral regions of the plateau between 6 Ma and the present. Overall, such a highly distinctive pattern of activity can be interpreted to reflect lateral asthenospheric mantle flow and deep crustal flow induced by the approach, and ensuing collision, of relatively thick （India and Eurasia） continental plates. There seem two deduced channels of mantle flow, that is, one channel eastward to the East China via Lixian-Dangchang along western Qinling Range, and another one southeastward to SE China and Vietnam Cenozoic basalt province via Maquan-Pinbian along the Sanjiang orogenic belt. Volcanic rocks in both Lixian-Dangchang and Maquan-Pinbian show transitional characteristics between the Tibetan potassic volcanic province and the eastern China basalt province. Many geophysical data are also in support of the idea about lateral flow at the asthenospheric and lithospheric levels. To testify this hypothesis, intensivestudies along several selected profiles across the transitional region are required for determining the source region and depth at which magmas generated, and revealing the characteristics of the lithosphere and the asthenosphere beneath the two volcanic provinces mentioned above. These studies will no doubt be favorable either for providing igneous constraints on the deduced lateral flow at the asthenospheric and lithospheric levels and the interaction between the above-mentioned two provinces, or for understanding the tectonic relationship between western China and eastern China, which are mainly controlled by the India-Asia collision and by the subduction of the Pacific plate, respectively.