滇西地区深部构造特征及其对成岩-成矿的控制作用

滇西地区地处扬子板块与印度板块之间特殊地带,是西南三江复合造山带的重要组成部分。地球物理资料揭示该区深构造特征复杂:地壳呈相对稳定的三层结构,其内部存在不规则低速透镜体;莫霍面自北而南呈近东西向阶梯式逐渐抬升,形态上隆坳相间;壳-幔界面存在明显的壳-幔过渡带,软流层地幔具有多层次隆起现象。深浅部构造呈现明显的立交桥式多层架结构。深部构造与区域成岩成矿关系密切:空间上,岩体与成矿的集中区分布体现出受由幔坡带所推断的深大断构造所控制,并与壳内低速体和壳-幔混合带存在垂向上的对应关系。岩浆岩岩石组合、岩石学和地球化学特征反映岩浆形成于壳-幔混合带物质的部分熔融,而地幔流体上涌是激发壳-幔混合带发生部分熔融的重要因素。研究认为,滇西地区燕山晚期-喜马拉雅期所处的特殊构造部位及经历的区域构造动力体制时空转换是形成深部特殊地质结构的动力学条件,并是激发强烈壳-幔相互作用的重要前提。在壳幔相互作用过程中,大量地幔流体携带成矿物质经由深大断裂上升至壳-幔混合带,激发其部分熔融,形成原始岩浆;随岩浆上升和分异演化,在构造有利部位成岩成矿,由此形成区域成岩成矿的一体化特点。而软流层幔的多期次脉动式上涌则是导致成矿多期性的内在因素。

Characteristics of deep-seated structure and its control action for magmatic activity and mineralization in western Yunnan Province

The western Yunnan area, an important part of the Sanjiang composite orogenic belt in the southwestern part of China, is located in the special position between the Yangtze plate and the India plate. Geological data revealed a complex structure in this area. The crust could be divided into three layers bearing irregular lens showing low velocity. The Moho interface shows upstairs from north to south and alternated between hunch and hollow. The transition belt occurred between the crust and the mantle. The lithosphere mantle shows multilevel hunches. The lower structure and the upper structure compose multilevel flyover-like structure. The magmatism and mineralization events are supposed to be closely related to the deep structures. Spatially, the areas hosting magmatic rocks and deposits are controlled by the deep faults indicated by the mantle slope, and corresponded to the inner crust low velocity bodies and crust-mantle transition belt. The magmatic rock diversities, petrology and geochemical data show that the magma responsible for these rocks would be originated from the transition belt between the crust and mantle by partial melting for which the upwelling of mantle fluid should be one of the important inducements. Composite analysis indicated that the special structural position and the regional evolution history of dynamic transition of the western Yunnan area in Late Yanshanian to Himalayan epochs should be responsible for the formation of the structures in the crust and mantle. They were also should be a triggering to the extensive crust-mantle interaction. In such a process, huge amount of metallic element be transported to the transition belt along the important faults by the mantle fluid which triggered the partial melting to form original magma. With the evolution and separation of magma, the magmatic rocks and mineral deposits formed in suitable structural locations to constitute the characteristic uniform of regional magmatism and mineralization. The upwelling pulses of lithosphere mantle would be the fundamental control to the multi-stage mineralization events.