皖赣交界地区历口群岩石组成及其对造山后超大陆裂解的指示意义

历口群是一套沉积-火山岩地层,分布于皖赣交界地区祁门—潜口—歙县断裂带以北,是江南造山带东段新元古代上部浅变质基底。历口群邓家组夹层玄武岩与铺岭组玄武岩具有大陆弧型玄武岩的REE配分模式((La/Yb)N=2.97~4.47)和微量元素模式。邓家组夹层玄武岩与铺岭组玄武岩的里特曼指数分别为0.029~2.203和4.304~6.538,表明两者是同期不同阶段的产物。历口群邓家组、铺岭组、小安里组界面清晰,形成过程连续,应该合并为一个组,仍称为邓家组,铺岭组作为邓家组上段,小安里组作为邓家组顶段。历口群邓家组中段上部夹层玄武岩用LA-ICP-MS法测得的锆石U-Pb年龄为(804.2±7.4)Ma。原位锆石Lu-Hf同位素结果表明玄武岩锆石的初始176Hf/177Hf比值为0.281 837~0.282 076,εHf(t)为-15.9^-7.0,二阶段模式年龄(tDM2)为2.16~2.68 Ga,意味着岩浆源区受到了地壳物质的强烈混染,代表着裂解中期阶段结束,岩浆活动发展至地表。历口群的沉积特征表明,江南造山带东段的新元古代裂谷盆地属于被动型裂谷盆地,俯冲板块的回撤为其主导因素,Rodinia地幔柱活动对大地构造的演化有明显影响,裂解的形成是板块活动与地幔柱活动相互作用的结果。

Lithology of Likou Group from Around the Anhui-Jiangxi Boundary,South China: Implications for Post-orogenic Collapse of Supercontinent Breakup

The Likou Group comprises a volcanic-sedimentary succession distributed to the north of the Qimen-Qiankou-Shexian fault zone near the boundary between Anhui and Jiangxi, South China. It is the upper part of Neoproterozoic epimetamorphic basement in the eastern part of the Jiangnan orogen. Interbedded basalt from the Dengjia and Pulin formations have Continental Arc Basalt (CAB)-like REE distribution ((La/Yb)_N=2.97-4.47) and trace-element patterns. The Dengjia and Pulin Formation basalts have Rittmann Index of 0.029-2.203 and 4.304-6.538, respectively, suggesting comagmatic products of different stages. The Dengjia, Pulin and Xiao’anli formations of the Likou Group have clear boundary surfaces and probably continuous rock formation, and therefore can be merged into one formation (still named Dengjia Formation). The Pulin and Xiao’anli formations represent the upper and top parts of the newly-defined Dengjia Formation, respectively. Basalt from the Dengjia Formation is dated to be (804.2±7.4) Ma by high-precision LA-ICP-MS U-Pb zircon dating. The zircons from interbedded basalt have initial¹⁷⁶Hf/¹⁷⁷Hf ratios of 0.281,837 to 0.282,076, ε_Hf(t) of -15.9 to -7.0, and two-stage Hf model ages of 2.16 to 2.68 Ga, indicating a magmatic source with significant ancient crustal input. The results may reflect that the intermediate stage of rifting had ended, and the magmatism had progressed to the earth surface. Sedimentary characteristics of the Likou Group indicate that the Neoproterozoic rift basin in the eastern part of the Jiangnan orogen was a passive rift basin, which was controlled mainly by subduction rollback, and the Rodinia plume activities may have also made significant impact to the tectonic evolution. We suggest that the rifting was resulted from the interactions between plate subduction and Rodinia superplume.