New 40Ar/39Ar age constraints on the Late Palaeozoic tectonic evolution of the western Tianshan (Xinjiang, northwestern China), with emphasis on Permian fluid ingress

Koenraad de Jong; Bo Wang; Michel Faure; Liangshu S. Shu; Dominique Cluzel; Jacques Charvet; Gilles Ruffet; Yan Chen

New 40Ar/39Ar age constraints on the Late Palaeozoic tectonic evolution of the western Tianshan (Xinjiang, northwestern China), with emphasis on Permian fluid ingress

Koenraad de Jong ¹ Bo Wang ^{1, 2} Michel Faure ¹ Liangshu S. Shu ² Dominique Cluzel ^{1, 2} Jacques Charvet ¹ Gilles Ruffet ³ Yan Chen ¹

1 ISTO - Institut des Sciences de la Terre d'Orléans

2 Department of Earth Sciences [Nanjing]

3 GR - Géosciences Rennes

Abstract : Laser-probe dating of mylonite whole-rock samples from the North Tianshan—Main Tianshan fault zone that cross-cuts the North Tianshan domain's southern margin yielded 40Ar/39Ar spectra with 255–285 Ma ages. Biotite from an undeformed, Early Carboniferous granite, which cuts the steep mylonitic foliation in the Proterozoic basement of the Yili arcs's southern margin, gave a 263.4 ± 0.6 Ma plateau age (1σ). Pre-Carboniferous metasediments overlying this basement yielded plateau ages (1σ) of 253.3 ± 0.3 (muscovite) and 252.3 ± 0.3 (biotite) Ma. The Permian ages of mylonites date movement on these ductile, dextral strike-slip shear zones, whereas the mica ages are interpreted by recrystallisation as a result of fluid flow around such transcurrent faults. We propose that the Tianshan's Permian syn-tectonic bimodal magmatism was created in a non-plume-related Yellowstone-like extensional–transtensional tectonic regime. Gold mineralisation, tracing aqueous flow in the crust, peaked in Permian time and continued locally into the Triassic. The picture is emerging that a convective fluid system partly driven by magmatic heat, existed in a strongly fractured and weakened crust with an elevated heat flow, leading to regional-scale isotope resetting. We suggest that surprisingly young isotopic ages in the literature for early orogenic (ultra)high-pressure metamorphism are similarly due to fluid-mediated recrystallisation.

Keywords : Isotope resetting Fluids Central Asia Chinese Tianshan 40Ar/39Ar geochronology

Document type :

Journal articles

Domain :

Sciences of the Universe [physics] / Earth Sciences / Tectonics

Environmental Sciences / Global Changes

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Submitted on : Monday, September 29, 2008 - 11:33:43 AM
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New 40Ar/39Ar age constraints on the Late Palaeozoic tectonic evolution of the western Tianshan (Xinjiang, northwestern China), with emphasis on Permian fluid ingress

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New 40Ar/39Ar age constraints on the Late Palaeozoic tectonic evolution of the western Tianshan (Xinjiang, northwestern China), with emphasis on Permian fluid ingress

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