摘要:
江南过渡带位于下扬子地区江南造山带北缘,为了探讨江南过渡带的基底性质,对贵池地区化芝里村中生代闪长玢岩脉中的捕获锆石进行了U-Pb测年和Hf同位素分析。结果显示: 贵池地区闪长玢岩脉锆石年龄分布较为分散,其中5颗原生锆石的加权平均年龄为(147.7±2.2) Ma,指示了岩脉的形成年龄; 捕获锆石中除一颗古太古代锆石年龄为(3 263±44) Ma外,其余锆石年龄主要分布在850~712 Ma、1 346~1 139 Ma、2 283~1 828 Ma 及2 944~2 302 Ma,与长江中下游平原发育的董岭岩群碎屑锆石的年龄分布一致。贵池地区闪长玢岩脉锆石的Hf同位素组成也与董岭岩群、崆岭岩群的岩石类似,与江南造山带基底岩石存在较大差别,指示了本区存在“江北式”基底,同时表明常州—崇阳断裂不适合作为下扬子地区“江北式”和“江南式”基底的界线,推测界线位于更靠南侧的江南断裂。研究有助于加深对下扬子地区构造格局的理解。
Abstract:
Jiangnan transitional zone is located in the northern margin of Jiangnan Orogenic belt of the lower Yangtze area. In order to investigate the basal nature of Jiangnan transitional zone, the authors conducted U-Pb chronology and Hf isotope analysis on captured zircons developed in the Mesozoic diorite porphyrite vein of Huazhili village in Guizhi area. The results show that these zircons have a relatively scattered age distribution, and the age-weighted average of the five primary zircons is (147.7±2.2) Ma, indicating the age of the vein formation. Except for the oldest one with the age of (3 263±44) Ma, the captured zircon ages are mainly distributed in 850~712 Ma, 1 346~1 139 Ma, 2 283~1 828 Ma and 2 944~2 302 Ma, which are consistent with the distribution ages of detrital zircons in Dongling Group of middle and lower reaches of Changjiang River. The zircon Hf isotopic compositions are similar to those in Dongling Group and Kongling Group rocks, which are different from the basement rocks of Jiangnan orogenic belt, indicating the existence of “Jiangbei” basement in this area. At the same time, Changzhou-Chongyang Fault cannot be used as the boundary between “Jiangbei” and “Jiangnan” basement in Lower Yangtze area, and it is predicted that the boundary is located in Jiangnan Fault, which is more to the south. This new understanding helps us to deepen our understanding of the tectonic pattern of Lower Yangtze area.
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