下扬子地区晚中生代花岗质岩石的源区物质组成反演

The Source Composition Inversion of the Late Mesozoic Granitoids in Lower Yangtze Region

扬子陆块东段包括两个重要的地质块体,分别是大别造山带和下扬子地区。本研究选择下扬子地区中沿江带内出露的洪镇、月山岩体和皖南地区的太平—黄山杂岩体为研究对象,根据这些岩浆岩的性质,反演下扬子地区地壳物质组成和结构,并与大别造山带进行对比。Nd同位素组成是研究地壳组成和结构的重要手段。这些岩体同位素组成有明显差异,洪镇岩体中除二长岩外,花岗岩的εNd(t)范围是-11.6^-23.6,Pb同位素组成与大别造山带晚中生代岩浆岩的范围一致;月山、太平岩体的εNd(t)范围分别是-6.5^-8.2和-6.1^-8.3,黄山岩体的εNd(t)值较高,为-2.6^-3.3,它们均有较高的放射成因Pb同位素组成。地球化学性质表明,这些岩浆岩在形成过程中没有地幔物质的贡献,可以用来反演所研究地区的地壳结构。Nd同位素组成表明,下扬子地区地壳物质组成可能包括晚太古代—古元古代古老地壳、中元古代早期增生陆壳和中元古代晚期—新元古代新生地壳,与大别造山带地壳增生时代有很好的相似性。但两者间地壳结构存在明显差异,其中,最重要的差异是下扬子地区中元古代晚期新生地壳物质可能底垫于古老地壳物质之下,不同于大别造山带从下到上由老到新的结构特征。此外,中元古代早期增生地壳物质在沿江地区可能没有分布。

The eastern part of Yangtze block is composed of two geological units, Dabie orogen and Lower Yangtze region. In this paper, four intrusions, Hongzhen, Yueshan and Taiping-Huangshan, from Lower Yangtze region are studied for understanding the crustal compositions and architectures in the area, and comparing with those of the Dabie orogen. Nd isotope is a powerful tool for crustal architecture studies. The isotopic compositions of these intrusions are significantly different. The eNd（t） of the granitic rocks of Hongzhen are in range of -11.6--23.6, which are similar to those of the radiogenic late Mesozoic intrusions in the Dabie orogen as well as Pb isotope compositions. The ~Nd（t） of Yueshan and Taiping intrusions are in range of -6.5^-8.2 and -6.1 ^-8.3, respectively, and those of Huangshan are higher and in range of -2.6^-3.3. The radiogenic Pb isotope compositions of the three intrusions are all higher than those of Hongzhen. The geochemical data suggest that the sources of these intrusions have no contributions from mantle materials, and can be used to inverse the crustal architecture in these region. The results indicate that crustal compositions in Lower Yangtze region were accreted by multistages during the Neoarchean-Paleoproterozoic,early Mesoproterozoic and late Mesoproterozoic-Neoproterozoic, which are similar to the eras of the Dabie crustal accretion. However, the architectures between Lower Yangtze region and Dabie orogen are different. The most important difference is that the late Mesoproterozoic-Neoproterozoic juvenile crustal material in the Lower Yangtze region is underplated to the old crustal, but the crustal architecture in the Dabie orogen from the bottom to the top are assembled from old to young. In addition, the early Mesoproterozoic accretion crust may not exist in the area along Yangtze River..