扬子地块东南缘沉积岩的Nd同位素研究
Nd Isotope Ratios of Sedimentary Rocks from the Southeastern Margin of Yangtze Block, China
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摘要: 扬子地块东南缘上溪群分布区及其周边沉积岩的Ｎｄ同位素研究结果，支持存在一条苏浙皖古生代裂陷槽（或江南深断裂）的观点。上溪群以北直至长江边所分布的震旦系－古生代的盖层沉积岩，其Ｎｄ模式年龄有两组，表明物源区不同。裂陷槽以北，沉积岩的物源区为Ｎｄ模式年龄约２.０～２.１Ｇａ的扬子物源区；以南的沉积岩表现出明显的幔源物质混染，显示出元古代岩浆活动的影响，而上溪群分布区以南直到江绍断裂附近主要表现上溪物源区的影响，华夏地块古老基底岩石则无显著贡献。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.
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