Extending the time range of apatite (U–Th)/He thermochronometry in slowly cooled terranes: Palaeozoic to Cenozoic exhumation history of southeast Sweden

详细信息	查看全文 | 推荐本文 |

• 作者：Pia Sö ; derlund ; Joaquim Juez-Larré ; Laurence M. Page and Tibor J. Dunai
• 关键词：apatite ; (U– ; Th)/He thermochronology ; Caledonian foreland basin ; Sweden ; Baltic Shield
• 刊名：Earth and Planetary Science Letters
• 出版年：2005
• 期刊代码：18_0012821x
• 类别：ph
• 出版时间：2005
• 卷：239
• 期：3-4
• 页码：266-275
• 文件大小：573 K

摘要

The use of (U–Th)/He thermochronometry in old slowly cooled terranes has long been debated. Hence, studies of natural setting are needed to verify the expected diffusivity behaviour of helium. Our results demonstrate that the (U–Th)/He method can be applicable to samples older than 200 Ma, and that available helium diffusion models can reproduce the results obtained from the age-vs.-depth trends. In addition, the results shed some light on the cross-validation with the apatite fission-track thermochronometer.

(U–Th)/He thermochronometry on apatites has been applied to samples taken from drill cores in the Precambrian basement in southeast Sweden to unravel the time of exhumation of the uppermost 1–3 km of the crust. Much of the Phanerozoic exhumation history in Sweden is poorly constrained due to a large hiatus in the sediment record. Results from the drill cores yield He ages decreasing with increasing depth from 270 Ma at the surface down to 120 Ma at 1700 m depth. The age-vs.-depth trend shows an inflection point at 1400 m depth, which coincides with the upper boundary of the present Helium Partial Retention Zone. A period of slow exhumation (17 m/my), probably linked to the isostatic uplift of the Caledonian foreland basin, is revealed by the upper section of the trend in the data. The results demonstrate that the area has not been reheated since the mid-Permian. This argues against the existence of a thick Mesozoic sedimentary cover in this region, which, if present at all, cannot have exceeded 100 m in thickness. Furthermore, the age-vs.-depth trend below the inflection point allows dating the cessation of exhumation at approximately 100 Ma. The area has had no significant amount of sedimentation, erosion or uplift since this time.