通过对江南造山带中段冷家溪群和板溪群凝灰岩及碎屑岩锆石U-Pb定年及系统的Hf同位素分析表明,冷家溪群沉积时代大致在860～820 Ma,不整合在其上的板溪群沉积下限在820Ma左右,这表明在江南造山带,晋宁造山运动之后,很快进入伸展裂解阶段.锆石的Hf同位素组成显示,在850～800Ma之间,锆石Hf初始值表现出快速上升趋势,表明这一时期有显著的地幔物质加入.造成这一现象的原因,一方面是早期的岛弧岩浆岩(850～820Ma,尤其是Ⅰ型花岗岩)是碎屑物的主要来源,另一方面造山后快速裂解形成的壳幔混合岩浆物质(如凝灰岩)形成板溪群部分物质来源.结合本研究和前人的研究成果,表明新元古代板块俯冲、拼合以及拼合后由于板片的拆沉引起的快速裂解(plate-rift model)是江南造山带形成的比较合理的地球动力学模型.%Zircon LA -ICPMS U -Pb ages and Hf isotope compositions of the tuff and clastic rocks from Lengjiaxi Group and Banxi Group in the middle section of the Jiangnan orogenic belt were determined. The results reveal that the sedimentary time of the Lengjiaxi Group is 860-820Ma. On the other hand, the lower limit of sedimentation of the Banxi Group which overlies the Lengjiaxi Group in angular unconformity is ca.820Ma, suggesting the quick transformation from the assemblage between Cathysia and Yangtze to the extension of the Jiangnan orogenic belt. Zircon Hf isotope compositions exhibit a rapid increase of the initial Hf values during 850 -820 Ma, indicating significant addition of the depleted mantle materials during this period. The quick increase of εHf (t) could be ascribed to the following factors: (1) most clastic materials were derived from arc igneous rocks (e.g., Ⅰ-type granites) and (2) some materials were derived from the mixed substances (such as tuff) between crust -derived magma and mantle -derived magma during the quick collapse after the orogeny). In combination with previous studies, the authors hold that the plate-rift model is a most plausible geodynamic model for the Neoproterozoic tectonic evolution of South China.
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