下扬子及周边地区深部泊松比结构及深部动力过程约束研究

Study on Poisson′s ratio structure beneath the lower Yangtze block and its surrounding region and constraints on the deep dynamic process

下扬子及周边地区存在着丰富的地质构造和多金属矿产资源,其深部结构和动力学过程已成为地学界研究的热点.为了更好地讨论下扬子及周边地区的深部动力学过程和岩浆活动机制,本文基于均匀网格层析成像方法提出了非均匀网格远震层析成像方法,利用大量的天然地震相对走时残差数据反演获得了下扬子及周边地区深至700 km范围内的三维S波和P波速度结构,并根据纵横波的比值关系计算出泊松比异常.由于S波速度比P波对流体的反应更加敏感,所以泊松比异常反映了物质是否包含流体或者物质的软硬、冷热程度.本研究结果显示:(1)长江中下游成矿带下方的上地幔内存在明显的高泊松比异常,而地幔过渡带内则存在明显的低泊松比异常;(2)大别造山带及其南侧的中扬子地块的上地幔中下部及地幔过渡带内都存在明显低泊松比异常,且呈现东深西浅的空间分布特征.结合已有的地质、地球物理和地球化学等资料,我们认为长江中下游成矿带下方的地幔过渡带内滞留着古太平洋俯冲板块,其上地幔内则赋存着软的上地幔热物质,为深部成矿提供了热量或幔源物质.因此,古太平洋板块的俯冲对长江中下游成矿带的形成发挥了至关重要的作用.

There are abundant mineral resources in the lower Yangtze block and its surrounding region,the deep dynamic process of the area becomes a focus for many geoscientists.To further study the deep dynamic process in lower Yangtze block and its surrounding region,we have developed a teleseismic non-uniform grid tomography method based on the uniform grid tomography method.We obtained 3-D S-wave and P-wave velocity models down to 700 km depth of the study area with the method.The Poisson′s ratio models were then established according to the ratio between S-wave and P-wave velocity models.Since the S-wave velocity is more sensitive to the fluid than the P-wave velocity,the Poisson′s ratio can reflect whether the substance contains the fluid or the softness and temperature of the substance better.Our tomography results show that:(1)there are low Poisson′s ratio anomalies in the mantle transition zone and high Poisson′s ratio anomalies in the upper mantle beneath the middle-lower Yangtze metallogenic belt;(2)There are obvious lower Poisson′s ratio anomalies within the middle and lower part of the upper mantle and the mantle transition zone beneath the Dabie orogenic belt and the middle Yangtze Craton,and the lower Poisson′s ratio anomalies extend more deeply to the east than to the west.Combining with a large number of geological,geophysical and geochemical results,we illustrate that there are paleo-Pacific slab staying in the mantle transition zone of the middle-lower Yangtze metallogenic belt.The hot mantle materials in the upper mantle right beneath the lower Yangtze metallogenic belt supply the heat and the mantle-derived material for the mineralization.In summary,the subduction of the paleo-Pacific slab played a crucial role in the formation of the metallogenic belt in the middle-lower Yangtze block.