羌塘盆地胜利河海相油页岩地球化学特征及Re-Os定年

Organic Geochemistry and Re-Os Dating of Marine Oil Shale in Shenglihe Area, Northern Tibet, China

地球化学资料表明,羌塘盆地胜利河海相油页岩有机碳含量为15.05%～20.34%,平均为17.695%,灰分含量为55.23%,焦油含量为11.0%;干酪根类型为Ⅱ1或Ⅱ2型。利用Re-Os同位素对该油页岩层进行定年,得到的等时线年龄为101±24Ma。该等时线年龄比生物地层所获得的地层年龄年轻。

The Shenglihe oil shale, an Early Cretaceous marine oil shale mine site found recently in the northern part of the central uplift zone of Qiangtang Basin, is characterized by high TOC values ranging from 15.05% to 20.34% with an average value of 17.695%. Ash and tar content are 55.23% and 11.0% respectively in the shale. Kerogen determined is Type II, or II 2. Samples of the oil shale were dated by Re-Os technique using Carius Tube digestion, Os distillation, Re extraction by acetone and ICP-MS measurement. An isochron giving an age of 101±2.4 Ma with an initial 187Os/ 188Os ratio of 1.84±0.11 （n=11, MSWD = 3.5） is obtained. The Re-Os age obtained is younger than the geological age derived from biostraigraphic data. Omitting samples XP-13-2-A, B and C, regression of the scattered Re and Os data for the remaining samples gives a Re-Os date of 89±10 Ma, with an initial 187Os/188Os ratio of 1.890±0.046 （n=8, MSWD = 0.78）. When only the data points Omitted samples XP-13-3-A, B, C and D are regressed,the results are indistinguishable from the previous regression： the age being 113±29Ma and the initial 187Os/ 188Os ratio being 1.80±0.13 （n=7, MSWD = 3.1）, which indicates the former （89±10 Ma） is younger than the true depositional age, and these initial187Os/188Os ratios are more radiogenic than the initial 187Os/188Os ratio associated with regression of the full data set （11 samples）. It may be associated with the time of hydrocarbon formation while the older age（ 113±29 Ma） may be associated with the time of the true depositional age of the Shenglihe oil shale.