扬子克拉通北缘大洪山地区打鼓石群与神农架地区神农架群的对比:锆石SHRIMP U-Pb年龄及Hf同位素证据

出露于扬子克拉通北缘大洪山地区的打鼓石群一直被认为是一套中元古代沉积地层，其上被花山群不整合覆盖。但是由于缺少可靠的年龄依据，打鼓石群与同属扬子克拉通北缘的神农架群以及其他相关地层的对比关系一直存有争议。本文作者利用高精度高分辨率离子探针质谱仪(SHRIMP)测定了打鼓石群中凝灰岩的锆石U-Pb年龄，并利用激光烧蚀多接收器电感耦合等离子体质谱仪(LA-MC-ICPMS)测定了打鼓石群凝灰岩和基本同时代的神农架地区神农架群上亚群凝灰岩的锆石Lu-Hf同位素。这是迄今为止第一次测得了打鼓石群罗汉岭组凝灰岩年龄(1 225±19) Ma和(1 239±23) Ma；锆石Lu-Hf同位素结果显示打鼓石群罗汉岭组凝灰岩与神农架群上亚群野马河组凝灰岩具有相似的Lu-Hf同位素特征：176Hf/177Hf初始值介于0.282 172~0.282 397，εHf(t)值为4.9~12.0， TCDM=1 150~1 943 Ma。结合现有区域地质资料，我们认为打鼓石群与神农架群上亚群属于同时期沉积物，形成于超级地幔柱主导的哥伦比亚超大陆裂解背景。

Correlation between the Dagushi Group in the Dahongshan Area and the Shennongjia Group in the Shennongjia Area on the northern margin of the Yangtze Craton:Constraints from zircon U-Pb ages and Lu-Hf isotopic systematics

The Dagushi Group in the Dahongshan Area, at the northern margin of the Yangtze Craton, has long been considered as a Mesoproterozoic sedimentary succession unconformably overlain by the Huashan Group. Owing to lacking of accurate and reliable isotopic ages, the correlation between the Dagushi Group and the Shennongjia Group that also occurred on the northern margin of the Yangtze Craton about 260 km to the west of the Dahongshan Area and other relevant strata in China, still remains as an enigmatic issue. In this study, using Sensitive and High Resolution Ion Micro Probe (SHRIMP) and Laser Ablasion coupled with Multi Collector Inductively Coupled Plasma Mass Spectrometer (LA MC ICPMS), we analyzed U-Pb ages and Lu Hf isotopic compositions for zircons from the tuff beds in the Dagushi and Shennongjia groups, respectively. Two precise zircon U Pb ages, 1225±19 Ma and 1239±23 Ma, from the tuff beds in the Luohanling Formation of the Dagushi Group were obtained for the first time. On the other hand, zircons from the two tuff samples of the Luohanling Formation show comparable Lu Hf isotopic compositions with (176Hf/177Hf)i varying from 0.282172 to 0.282397, corresponding to εHf(t) from 4.9 to 12.0 and TCDM from 1150 Ma to 1943 Ma. The Lu Hf results of the Dagushi Group are consistent with those of the zircons from a tuff sample from the Yemahe Formation of the Shennongjia Group. Integrating the data obtained in this study with the available regional geological data, we suggest that the Dagushi Group and the Upper Shennongjia Subgroup are coeval sedimentary succesion formed during the breakup of the Columbia Supercontinent, possibly geodynamically related to a super plume.