扬子东南缘新元古代地质演化

Geological Evolution of the Southeastern Yangtze Block during the Neoproterozoic

华南陆块在新元古时期经历了复杂的地质演化过程,其中以江南褶皱带的形成和南华盆地的裂解为标志。江南褶皱带是一套浅变质强变形的新元古代早-中期沉积-火山岩组合,之后经历了820Ma酸性岩浆岩侵入。新元古代晚期地层沉积在南华盆地中,以不整合上覆于江南褶皱带和过铝质花岗岩。过去几十年,科学家们对这一系列地质过程提出了多种演化模型,如地幔柱模型,裂解模型和岛弧模型。笔者通过该区沉积岩和岩浆岩的详细研究,结合已有模式,建立了全新的演化模型——俯冲-弧后伸展模型,该模型认为:扬子东南缘新元古代俯冲作用开始于850Ma左右,四堡群及其相应地层为弧前盆地沉积;扬子与华夏板块的拼接发生在830Ma左右;扬子西北缘持续的俯冲作用引发弧后拉张,导致了南华盆地的形成和大规模花岗质岩石侵位。

South China underwent complicated geological evolutions during the Neoproterozoic. It is characterized by amalgamation of the Yangtze and Cathaysia blocks along the Jiangnan Fold Belt and the subsequent formation of the Nanhua intracontinental rift basin. The Jiangnan Fold Belt is composed of sedimentary and volcanic rocks that experienced strong deformation and greenschist facies metamorphism. This belt was intruded by numerous 820 Ma peraluminous granitic plutons. The Nanhua rift basin is filled with the Late Neoproterozoic strata which are overlaid unconformably on the granitic intrusions and the Jiangnan Fold Belt. In the past few decades, three models had been proposed, including the plume model, the plate-rift model and the arc model. Alternatively, based on systematic studies of sedimentary and igneous rocks in the region, we proposed a new model, the subduction-back-arc rift model （SBAR）. This model can be summarized as follows： subduction of oceanic slab beneath the southeastern margin of the Yangtze Block began at 850 Ma. The Early Neoproterozoic strata were deposited in the fore-arc setting; the assemblage of Yangtze and Cathaysia blocks occurred at 830 Ma; the long term subduction beneath the northwestern margin of the Yangtze Block resulted in asthenosphere up- welling and back-arc spreading at its southeastern part, which is the key factor controlling generation of granite and formation of the Nanhua rift basin.