New Lu-Hf geochronology constrains the onset of continental subduction in the Dabie orogen

The Tongbai-Dabie-Sulu orogen contains several eclogite-bearing terranes that were largely metamorphosed in Triassic time. Although petrologic and geochronologic data from these terranes have provided essential information on the geodynamic history of the orogenic cycle, the timing of onset of continental collision between Sino-Korean craton and the Yangtze craton is still unknown. To better our understanding of the transitional time of oceanic to continental subduction in this orogen, we have dated an eclogite with continental affinity in the Tongbai Mountains, located in the very west part of the Tongbai-Dabie-Sulu orogen, central China, using Lu-Hf geochronology. These eclogites formed at pressure temperature conditions of 1.8-2.1 GPa and 490-540 degrees C. Mineral-whole rock ages of multi-garnet fractions from three localities yield Lu-Hf ages of 256.4 +/- 2.6 Ma, 252.3 +/- 3.4 Ma and 246.9 +/- 3.2 Ma. Garnet porphyroblasts from the three samples display evidence of prograde zoning but the major portion of the garnet grains grew exclusively at eclogite-facies conditions due to the occurrence of omphacite inclusions from core to rim. Taking the spherical geometry effect into account, these Lu-Hf ages are, therefore, interpreted to reflect high-pressure eclogite-facies metamorphism instead of the early phase of garnet growth despite the well-preserved prograde major- and trace-element zoning in garnets. Our results allow for a more detailed and complete picture of the tectonic evolution of the Tongbai-Dabie-Sulu orogen during the transitional time of oceanic to continental subduction. These new Lu-Hf ages mark the timing of onset of continental collision between Sino-Korean craton and the Yangtze craton, suggesting that the entry of the leading edge of the Yangtze craton continent into the Sino-Korean craton trench occurred no later than ca. 256 Ma. (C) 2010 Elsevier B.V. All rights reserved.