青藏高原东南缘碰撞造山结构与物质组成:来自岩石地球化学和地球物理的联合约束

青藏高原东南缘位于印度板块与欧亚板块侧向汇聚部位,是检验碰撞造山动力学模型的理想场所。尽可能全面收集该区已有地球物理和新生代岩浆岩数据,探讨这些资料对碰撞造山带结构和物质组成的指示。结果表明:青藏高原东南缘不同部位的壳幔结构和组成存在较大差异。兰坪—思茅地块、保山地块和腾冲地块等的中地壳(15~30 km深度)普遍发育低速层,表明富水层或者部分熔融物质的存在,为青藏高原物质向东南流动提供了可能。部分熔融产物以大型剪切带内具有高Sr、低Nd同位素特征的淡色花岗岩脉为代表。但是,扬子板块同等深度下却发育高速层,其组成很可能是峨眉山玄武岩,它的存在阻隔了碰撞带物质向东流动。扬子板块和兰坪—思茅地块下地壳底部均出现呈条带状展布的高速体。根据新生代具有高Sr、低Y的岩石显示的下地壳源区特征,结合该区地质演化历史,将上述两套呈条带状展布的高速体分别解释为新元古代铁镁质弧岩浆岩和二叠纪—三叠纪铁镁质弧岩浆岩。青藏高原东南缘地幔各向异性存在明显南、北分区特征,在26°N以北表现为SN向,在26°N以南表现为近EW向。这一差异跟俯冲的印度板片撕裂有密切关系。该撕裂在综合地球物理剖面上显示为突变的印度板片俯冲角度,在地表表现为苦橄岩、煌斑岩、埃达克岩以及淡色花岗岩等的集中出露。这一新模型明显区别于前人的岩石圈拆沉和对流减薄等作用。

Structure and Composition of Collisional Orogen in the Southeastern Margin of Tibetan Plateau,China: Constraints fromPetrogeochemistry and Geophysics

The southeastern margin of Tibetan Plateau is located at the lateral convergence between Indian plate and Eurasian plate,which is an ideal place to test the geodynamic model of the collision orogen.The present geophysical and Cenozoic magmatic rock data in this area were collected as comprehensively as possible,and the indication of these data on the structure and composition of the collision orogen was discussed.It is suggested that there are great differences in the structure and composition of crust and mantle in different parts of the southeastern margin of Tibetan Plateau.Low-velocity layers are generally developed in the middle crust(the depth is 15-30 km)of Lanping-Simao,Baoshan and Tengchong blocks,indicating the existence of water-rich or partial melting material layers,which makes it possible for the material of Tibetan Plateau to flow to the southeast.The high-Sr and low-Nd leucogranite dykes in the large shear zone are derived from above partial melting layer.However,Yangtze plate develops a high-velocity body at the same depth,and its composition is likely to be Emeishan basalt,which blocks the material flow in the collision zone.High-velocity bodies with banded distribution appear at the bottom of the lower crust of both Yangtze plate and Lanping-Simao block.According to the characteristics of the lower crust source area shown by Cenozoic high-Sr and low-Y rocks,and combined with the geological evolution history of this area,these two high-velocity bodies are interpreted as Neoproterozoic mafic arc magmatic rocks and Permian-Triassic mafic arc magmatic rocks,respectively.The mantle anisotropy in the southeastern margin of Tibetan Plateau shows SN to the north of 26°N and nearly EW to the south of 26°N.This difference is closely related to the tearing of the subducted Indian slab.The slab tearing shows an abrupt change of the Indian slab subduction angle on the comprehensive geophysical profiles,and the concentrated exposure of picrite,lamprophyre,adakite and leucogranite on the surface.This new model is obviously different from the previous lithosphere delamination and convective thinning.