皖南浅变质岩和沉积岩的钕同位素特点及其大地构造意义

根据Nd同位素模式年龄通常保留源区大陆地块的平均年龄的原理,作者研究了皖南上溪群千枚岩和震旦系到二叠系沉积岩的Nd同位素组成和模式年龄。发现千枚岩和沉积岩具有不同的Nd同位素组成和模式年龄,它们明显地分成二组。千枚岩的~(147)Sm/~(144)Nd=0.1220~0.1290,T_(DM)~(Nd)＝1.63~1.69Ga;沉积岩的~(147)Sm/~144Nd＝0.1100~0.1182,T_(DM)~(Nd)=1.92~2.14Ga。这表明,它们来自不同的物源区:上溪群可能来自附近的古岛弧双桥山群;沉积岩可能来自大别古陆和华北地台。

ND ISOTOPIC CHARACTERISTICS OF LOW GRADE METAMORPHICAND SEDIMENTARY ROCKS FROM SOUTHERN ANHUI PROVINCEAND THEIR SIGNIFICANCE IN TECTONICS

We have determined Nd isotopic compositions for the middle Proterozoic Shangxi group phyl-lites and Sinian to Permian sedimentary rocks from southern Anhui province and calculated their Nd model ages assuming that the depleted mantle sources with a linear evolution or a non-linear evolution respectively. Results are shown in the table. J. Calculated assuming a depleted mantle source with a linear evolution (Goldstein et al. 1984). 2. Calculated assuming a depleted mantle described by: Nd (T) =0. 25T-3T + 8. 5 (Nelson and DePaolo, 1985) . These results show that Nd isotope ratios and model ages for the studied rocks can be divided into two significantly different groups with remarkable internal consistency within each group. Stratigraphically older phyllite shows younger Nd model ages ranging from 1. 6 to 1. 7 Ga, while Stratigraphically younger sedimentary rocks show older mode! ages ranging from 1. 9 to 2. 0 Ga. Nd isotopic model ages of clastic sedimentary rocks usually preserve the model ages of the continental crust of the provenance area. Therefore the Shangxi group phyllite and the Sinian to Permiansedimentary rocks came from two different provenances. Phyllite material probably came from an ancient island arc (Shuangqiaoshan group) at the south. Provenance terrain for studied sedimentary rocks could be large including Dabie old micro- continent and possibly Sino- Korea plat-form. Determined sedimentary rocks with different stratigraphic ages and lithologies show consistent Nd isotopic ratios and relatively uniform model ages which indicate that the Yangtze and Sino- Korea plates were possibly accreted since Sinian period and that this united plate was a stable terrigenous provenance of the studied Sinian to Permian sedimentary rocks. Table Nd isotopic compositions and Nd model ages of phyllites and sedimentary rocks from southern Anhui province