Thermobaric structure of a traverse across western Dabieshan: Implications for collision tectonics between the Sino-Korean and Yangtze cratons

Abstract

As the best preserved high- and ultrahigh-pressure (HP and UHP) metamorphic terrane in the Qinling-Dabieshan-Sulu orogen, western Dabieshan is divided into six lithotectonic units along a traverse across the orogen, i.e. from north to south, the Nanwan, Balifan, Huwan, Xinxian, Hong'an and Mulanshan units. In this terrane five eclogite-bearing zones (I-V) are developed. The garnet and clinopyroxene in eclogites from these zones exhibit chemical zoning, suggesting that the rims record general peak temperature and pressure. Thermobarometric study indicates that the peak P-T conditions of eclogite are 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 symmetrical thermobaric pattern, in conjunction with structural and geochronological data, demonstrates that the Huwan and Hong'an units belong to the same HP slice overlying the UHP slice. This pattern, together with the Mulanshan LT/HP blueschist-greenschist belt in the south, roughly constitutes a 'normal' metamorphic zonation. However, clear metamorphic gaps occur between different slices. It is inferred that the LT/HP, HP and UHP slices were broken up from the downgoing slab during subduction and reached different depths along different geothermal gradients. The successive subduction of underlying slices leads to a nearly concomitant uplift of overlying slices, whereas exhumation of the deepest UHP slice was effected by underthrusting of the lower crust of the Yangtze craton. © 2004 Blackwell Publishing Ltd.