Re-Os Geochronology of Bitumen from the Northern Longmen Shan Thrust Belt, Southwest China
Zeyang Liu1,2, Chuanbo Shen
3,4, David S Selby
5, Xiaoyan Ruan
6, Shouzhi Hu
1 and Jiayi Du
1, (1). Key Laboratory of Tectonics and Petroleum Resources, China University of Geosciences, Ministry of Education, Wuhan, China, (2)Faculty of Earth Resources, China University of Geosciences, Wuhan, China, (3)China University of Geosciences, Key Laboratory of Tectonics and Petroleum Resources of the Ministry of Education, Wuhan, China, (4)Faculty of Earth Resources, China University of Geosciences, Wuhan, Wuhan, China, (5)Durham University, Department of Earth Sciences, Durham, United Kingdom, (6)Key Laboratory of Tectonics and Petroleum Resources of Ministry of Education, China University of Geosciences, Wuhan, China
Contact First Author: Zeyang Liu; zeyang_92@163.com
English Abstract:
The bitumen Re-Os geochronology as a novel technique has the good potential for wider use to constrain hydrocarbon generation especially in the marine carbonate basin, to identify source units and to determine the activity age of fault within bitumen filling. Six bitumen samples from the Lower Cambrian Changjianggou formation in the northern Longmen Shan thrust belt were selected for Re-Os analysis. Re and Os contents are ~283.3–563.3 ppb and ~4058.2–15347.3 ppt, respectively. These values are significantly elevated from those of the average continental crust and previously reported bitumen samples from both hydrocarbon and metalliferous systems, and also the majority of marine and lacustrine organic-rich sedimentary rocks (REFS), which has also been shown by previous studies. The
187Re/
188Os values are high, ranging from ~ 230.7 to 718.4, and the Os isotopic composition is radiogenic, with
187Os/
188Os ratios ranging from 2.79 to 3.48. Repeat analysis yield similar Re and Os concentrations, and
187Re/
188Os and
187Os/
188Os values.
All the Re-Os data yield an isochron age of 158 ± 77 Ma with an initial 187Os/188Os value of 1.85 ± 0.61 (2σ; MSWD = 76). Calculated initial 187Os/188Os ratios at 158Ma are variable and can be divided to two groups. One yield a Re-Os date of 162 ± 14 Ma with an initial 187Os/188Os value of 1.87 ± 0.12 (2σ; MSWD = 0.95). The remaining samples yield a Re-Os date of 172.7 ± 8.1 Ma with an initial 187Os/188Os value of 1.655 ± 0.056 (2σ; MSWD = 0.000).
Two different initial 187Os/188Os values suggest that all the oil generated from different source horizons with different Os isotope compositions. The Re-Os bitumen geochronology was coeval with intense thrust fault movement of the northern Longmen Shan thrust belt, which also provides a new direct absolute age evidence for the Late Mesozoic deformation of the northern Longmen Shan thrust belt.
