Nd Isotope Ratios of Sedimentary Rocks from the Southeastern Margin of Yangtze Block, China

Key words: 
• Nd isotopic model age /
• sedimentary provenances /
• Southeastern margin of Yangtze Block

Abstract: From the study of Nd isotope geochemistry for low grade metamorphic rocks, covering strata sedimentary rochs and associated volcanic rocks occured in the distribution area of Proterozoic Shangxi Group (traditionally called the Jiangnan old land) and surroundings in the southeastern margin of Yangtze Block, it is shown that the Nd isotopic model ages of the Sinian to Permian sedimentary rocks in the north side and south side of Jiangnan Deep Fault (or called Su-Zhe-Wan Palaeozoic Aulacogrn), as well as low-grade metamorphic rocks of Shangxi Group are 1.9-2.IGa, 1.2-1.3Ga and 1.7Ga, respectivelly. Therefore, the provenances of the Sinian to Palaeozoic sedimentary rocks on hath sides of the Jiangnan hop Fault were different and the idea that the Jiangnan hop Fault was harrier for edment transportation is proved by Nd isotopic geochemical evidence.There is a distribution area of the Palaeozoic strata between the Shangxi Group area and Jiangshan-Shaoxing Fault adjacent to Cathaysia Block. The Nd model ages of 2 samples for the Sinian to Silurian sedimentary rocks are 1. 8- 1. 7Ga, but the Nd model age of the basement rocks in cathaysia Block is older, for example, the model age of 2.1-2.5Ga for Chencai Group. It is indicated that the sediment supplment for the area to the south of the Shangxi Group distribution area mainly comes from the low metamorphic rock of Shangxi Group, and the basement rocks in Cathaysia Block are not an important contributors. So that the provenance of sedimentary rocks over the area between Jiangnan Deep Fault and Jiangshao Fault is the distribution area of Shangxi Group and named "Shanxi Provenance". The sediment provenance for the north side of Jiangnan Deep Fault is a particular block in the north and named "Yangtze provenance".The Nd model ages of the Sinian eedimentary rocks in the Shangxi Group distribution area and its northern margin are 1.2-1.3Ga, lower than that of Shangxi Group. The andesitic volcanic rock of Puling Formation in the northern margin of the Shangxi Group area and its xenoliths also give lower Nd model age of 1.3-1.4Ga. The intermediate and acidic volcanic rocks of Jingtan Formation in the southern margin of Shangxi Group area give Nd model ages of 1.6-2.2 Ga.From the variation of Nd model age in space and time, it is deduced that there were an orogenic movement in late Proterozoic Era there and the effect of mantle materials on sediment supplement at that time.