北京十三陵地区中—新元古界碳酸盐岩Pb-Pb年龄研究

Pb-Pb Age Determination of Meso- to Neoproterozoic carbonates in the Ming Tombs District, Beijing

对北京十三陵地区中—新元古界碳酸盐岩进行了系统地采样和^(207)Pb/^(204)Pb-^(206)Pb/^(204)Pb年龄测定。共测定了63个样品(大部分样品采取两种溶解方式),得到中元古界雾迷山组第三段的^(207)Pb/^(204)Pb-^(206)Pb/^(204)Pb等时线年龄为1373±92Ma(95％置信度误差,下同,MSWD=4.7),杨庄组为1488±55Ma(MSWD=14),高于庄组第三段为1608±74Ma(MSWD=8.1)。这些年龄数据与目前已有地质记录相吻合。串岭沟组的碳酸盐相的^(206)Pb/^(204)Pb-^(207)Pb/^(204)Pb未构成等时线,但全岩的^(206)Pb/^(204)Pb-^(207)Pb/^(204)Pb比碳酸盐相具有更明显的线性趋势,其原因尚待进一步研究。另外,根据同一个样品用稀盐酸溶解和用氢氟酸加硝酸混合酸溶解的铅同位素比值的接近的现象,显示碳酸盐岩中的硅质没有陆源的特征,是沉积盆地中的产物。

63 carbonate samples, collected systematically from Proterozoic strata in the Ming Tombs District in Beijing, were determined by using the Pb-Pb method. The samples of pure or being mainly composed of carbonates were dissolved in 1M HCl, and the siliceous rocks (8 samples) in HF+HNO3. In order to compare with the results dissolved in 1M HCl, some of the carbonate samples were also dissolved in the HF + HNO3 solution. The chemical reagent was ultra-pure and the chemical procedures were treated in an ultra-pure laboratory. The Pb isotopes were analyzed on a MAT261 spectrometer. The decay constants of 238U and 235U are 1. 55130×10-11 a-1 and 9. 8485×10-11a-1, respectively, and the ratios of 238U/235U is 137.88, used in the calculation. The error of Pb isotope ratios is 20, and the age errors are at 95% confidence. The calculations and drawings of the Pb-Pb isochrons were done by using the model 2 of the Program ISOPLOT. ex2 designed by Dr. Ludwig. There is a little difference of a sample in Pb isotope ratios between being dissolved in 1M HCl and in HF + HNO3. The Pb-Pb isochron ages and the MSWD for some formations of the Proterozoic in the Ming Tombs District are as follows:Wumishan Fm. , (3rd Set) 1373 ± 92 Ma, MSWD = 4. 7; Yangzhuang Fm. , 1488±55 Ma, MSWD=14; Gaoyuzhuang Fm. , (3rd Set) 1608±74, MSWD=8.1.