Coherence of the Dabie Shan UHPM terrane investigated by Lu-Hf and ⁴⁰Ar/³⁹Ar dating of eclogites.

F.M. Brouwer (1), M. Groen (1), O. Nebel (1,2), J.R. Wijbrans (1), H.N. Qiu (3,4), Q.J. Yang (3), L.H. Zhao (3), Y.B. Wu (5)

1) Department of Petrology, Faculty of Earth & Life Sciences, VU University Amsterdam, Amsterdam, The Netherlands.
2) Present address: Research School of Earth Sciences, ANU, Canberra, ACT, Australia.
3) State Key Laboratory of Isotope Geochronology and Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, China.
4) State Key Laboratory of Tectonics and Petroleum Resources, China University of Geosciences, Wuhan, China.
5) State Key Laboratory of Geological Processes and Mineral Resources, Faculty of Earth Sciences, China University of Geosciences, Wuhan, China.

In Press: UHPM: 25 year after discovery of coesite and diamond. Edited by L. Dobrzhinetskaya, S. Cuthbert, W.Faryad and S. Wallis. Elsevier.

Abstract

Dabie Shan is a central part of the largest known ultrahigh-pressure (UHP) metamorphic belt. Peak UHP metamorphism in Central and Eastern Dabie Shan is thought to be Triassic, while further west ages associated with UHP metamorphism are older: late Carboniferous in Western Dabie Shan and Ordovician in Qinling and Altyn Tagh. Contrasting reports of decidedly older UHP metamorphism in both Eastern and Western Dabie Shan question a simple East-West gradient. Here, pressure-temperature-time (PTt) data for localities throughout Dabie Shan are presented to determine how far west the Triassic UHP event is documented, and how far east the Carboniferous and Ordovician events can be traced. We present Lu-Hf Grt-Cpx isochrons that yield consistent Triassic ages for the main volume of Dabie Shan, and a Permian age for the Huwan unit at its northwestern end. The data suggest that the Lu-Hf system in the main Dabie unit was reset by an early Triassic UHP event, whereas younger (~220 Ma) Lu-Hf ages are interpreted to result from fluid-assisted retrograde recrystallization. The 40Ar/39Ar ages of ~215 Ma reflect cooling after early Triassic UHP metamorphism, and post-Triassic ages are related to Cretaceous granitic magmatism. These results support the view that a tectonic break exists at the northwestern end of the Dabie Shan terrane, while the rest of Dabie Shan was essentially tectonically coherent since the earliest Triassic. Pre-Triassic metamorphism extends as far east as the Huwan unit, but can not be traced by Lu-Hf Grt-Cpx geochronology in the main tract of the Dabie Shan Terrane, where early Triassic (U)HP metamorphism prevailed.

Fieldwork was partly supported by the KNAW-CAS Collaboration Program (06CDB002).