Architecture of the western Dabieshan HP/UHP terrane, China:
constraints from thermobaric pattern and isotopic dating
Xiaochun Liu (Institute of Geomechanics, CAGS; Department of Geosciences,
National Taiwan University); Bor-ming Jahn (Department of Geosciences, National
Taiwan University); Shuwen Dong (Institute of Geomechanics, CAGS); Sanzhong Li
(College of Marine Geosciences, Ocean University of China)
As the best preserved high- and ultrahigh-pressure (HP and UHP) metamorphic
terrane in the Qinling-Dabie-Sulu orogen, western Dabieshan could be divided into
six lithotectonic units, i.e., from north to south, the Nanwan flysch, Balifan tectonic
mélange, Huwan HP eclogite, Xinxian UHP eclogite, Hong’an HP eclogite and
Mulanshan blueschist-greenschist units. In these units five eclogite-bearing zones (I-V)
are developed. Based on various thermobarometries, the peak P-T conditions of
eclogite are estimated to be 550-570C and 21 kbar for Zone I, 470-500C and 14-17
kbar for Zone II, 620-670C and 26-29 kbar for Zone III, 530-560C and 20-22 kbar
for Zone IV, and 490-510C and 19-20 kbar for Zone V. The zircon U-Pb data of
eclogites from the Huwan and Hong’an units suggest that their protoliths were formed
during the Neoproterozoic, from 733  10 Ma to 684  15 Ma, whereas the
metamorphic ages are all the Triassic, about 230 Ma. The symmetrical thermobaric
pattern and identical age data demonstrates that the Huwan and Hong’an units belong
to the same HP slice overlying the UHP slice. Zircons from a metagabbro in the
Balifan unit also yield a Neoproterozoic emplacement age, but slightly younger at 582
 11 Ma. Taking into account the Yangtze signature for the Nanwan unit, we infer that
the Balifan tectonic mélange and Nanwan flysch units might represent the basement
and cover accretionary complexes, respectively, sliced from the Yangtze craton during
its northerly subduction. Accordingly, the northern boundary fault of the Nanwan unit
should be the Triassic suture between the Sino-Korean and Yangtze cratons. Clear
metamorphic gaps between different slices and their diachronous exhumation (from
ca. 240 Ma to ca. 210 Ma) could be explained by a successive subduction of
underlying slices with a nearly concomitant exhumation of overlying slices.