江南中、新元古代岛弧的运动学和动力学

ON THE MESO-NEOPROTEROZOIC JIANGNAN ISLAND ARC: ITS KINEMATICS AND DYNAMICS

自经７０年代作者等发表江南中－新元古代岛弧构造观点以来，这一地区已经在８０年代后期和９０年代成为我国大地构造研究热点之一。大量新资料支持以下的古板块构造演化模式，即古华南洋壳中元古代１３（１７）～９．８亿年时向北（西北）俯冲于扬子板块东南边缘之下，形成江南火山岛弧和弧后盆地，在东北段是皖－浙－赣火山岛弧和樟树墩－伏川弧后盆地；新元古代９．８～７．７亿年时发生了陈蔡弧（浙东地体）与皖－浙－赣弧的弧－弧碰撞造山作用，并导致樟树墩－伏川边缘海的崩塌和陆－弧弧后碰撞造山过程。江南古岛弧带经历了多次的后期构造变形。

The present authors rose the idea of Meso-Neoproterozoic Jiangnan island arc caused by a northwestward subduction of the paleo-South-China oceanic plate in 1970s.Since that the studies on the Jiangnan island arc has become a hot point of tectonic research on east Asia in 1980s and early 1990s.Represented by the dismembered ophiolite suite having Sm - Nd ages of 1295Ma and 1734Ma and the uppermost sedimentary age of 988Ma of 'Chencai Group' to the south of Jiangshan-Shaoxing fault zone, the oceanic crust of paleo-South-China plate subducted northwestward under the southeast margin of the Yangtze plate, resulting in the formation of the Anhui-Zhijiang-Jiangxi Meso-Neoproterozoic island arc (Huiyu island arc), northeastern segment of the Jiangnan island arc. The island arc mobilized in the age range of 9801200Ma, which implied that the oceanic crust subduction had lasted until 980Ma. TheFuchuan ophiolite to the north of the island arc represented the oceanic crust of a marginal sea, which ranged from 935Ma to 1024Ma roughly coincided with island arc in time. In fact.the northeast Jiangxi ophiolite belt (930- 1150Ma) trending in northeast is not located on theJiangshan Shaoxing fault zone but tends to be extended to the Fuchuan ophiolite, southern Anhui. Not only the northeast Jiangxi ophiolite is identified as the marginal sea type by petrochemistry and REE analysis, but also the matrix graywacke of ophiolite blocks isproved as the back arc basin sediment. Therefore, taking the Jiangshan-Shaoxing fault zone as the subduction zone, Anhui-Zhejiang-Jiangxi volcanic island arc located to the north, and the Zhangshudun-Fuchuan as the back arc basin to the north of the arc in turn, a trench-arcback arc basin system with the northward/northwestward polarity of plate subduction is reasonably disposed in spatial relationship. This is the typical model of the Jiangnan Meso-Neoproterozoic trench-arc-back arc basin system.An arc-arc collision event between the Chencai arc and Anhui-Zhejiang-Jiangxi arc occurred during 980～770 Ma, which resulted in the intensive deforn tion of island arc rocks,the formation of collision type granites in the Anhui-Zhejiang-Jiangxi arc, and the deposits of late Mesoproterozoic, Stenian-Early Neoproterozoic, Tonian flysch and molasse which overlaid the arc rock assemblage unconformably (Shengong movement) and of the Sinian which overlaid the Tonian strata unconformably (Xuefeng movement). The collision also resulted in the collapse of the Zhangshudun-Fuchuan back arc basin, proved by the blueschists withthe K-Ar age of 866±14Ma. The oceanic rust subducted northward and that finally led to the processes of orogeny of back arc continent-arc collision with a series of thrusts in the southeastward tectonic transportation direction. On tile southern flank of the back arc basin,there possibly existed the southward subduction which related with the formation of a short life local volcanic rock assemblage (Shangshu formation of Tonian).The late Neoproterozolc-early Paleozoic sinistral strike slip faulting and the late Paleozoic-early Mesezoic southward thrusting and gravitational nappe occurred in the southern flank of the Anhui-Zhejiang-Jiangxi paleo-island arc. The later deformation processes involveCarboniferous Permian radiolaria siliceous shales into the melange along northeast Jiangxi fault zone, the remains of Zhangshudun-Fuchuan marginal sea.The komatlite in the Jiangnan Meso-Neoproterozoic trench-arc-back arc system implies that the processes of Isn collision model displayed there, which occurred in the situation of high heat flow.The Shengong movement in eastern Zhejiang occurred around 980Ma implied the collision between the Chencai arc and Anhui-Zhejiang-Jiangxi arc. This arc-arc collision event lasted until 770Ma, which is the early stage of Sinian. The Xuefeng movement recorded the late effect of the collision event. The time of unconformity surfaces vary from place to place,bacause the spatial and temporal variation of the collision events. That implies that the crust movement represented by the