Triassic retrograded eclogites and Cretaceous gneissic granites in the Tongbai Complex, central China: Implications for the architecture of the HP/UHP Tongbai–Dabie–Sulu collision zone

The Permo-Triassic Qinling–Tongbai–Dabie–Sulu collision zone between the Sino-Korean and Yangtze cratons hosts a number of terranes; some of them carry evidence for ultrahigh-pressure (UHP) metamorphism. Discovery of retrograded high-pressure (HP) eclogites in the Tongbai Complex, in the middle of the orogen, brings fresh information to correlate segments of the orogen and understand dynamics of the orogeny. The 20 km wide Tongbai Complex is underlying two HP eclogite bearing lithotectonic units (the northern and southern eclogite zones) in the Tongbaishan area. It consists of gneissic granites and subordinate metamorphic enclaves. Among them, retrograded eclogites carry garnet with a compositional profile indicating a three-stage metamorphic evolution, from epidote-amphibolite, through eclogite, to amphibolite facies conditions. A thermobarometric study yields estimates of 660–700 °C and 8.0–10.3 kbar for the amphibolite facies stage. U–Pb geochronology and trace element characterization of zircon produced two important results. First, the dominant gneissic granites are Early Cretaceous (ca. 140 Ma) and not Neoproterozoic as formerly reported. Second, metaigneous enclaves, i.e. retrograded eclogites, garnet amphibolites, and dioritic–trondhjemitic gneisses, formed between 933 and 742 Ma and were metamorphosed at 232–220 Ma. Metasedimentary enclaves contain detrital zircons yielding two age populations of 2960 and 1966 Ma. Consequently, the tectonometamorphic evolution of the Tongbai Complex is quite similar to that of the Northern Dabie Complex, but the Tongbai Complex is likely to be positioned at a higher crustal level in the orogenic edifice. Furthermore, the lower metamorphic P-T conditions in Tongbaishan relative to the Dabie–Sulu region are hypothesized to have resulted from a shallower subduction. The younger metamorphic age (232–220 Ma) for the Tongbai Complex with respect to the overlying eclogite zones (metamorphism at ca. 255 Ma and cooling at ca. 238 Ma) reflects diachronous subduction and exhumation for these two HP slices. A similar process occurred in the Dabie and Sulu terranes.

Research Highlights
► The Tongbai Complex comprises Cretaceous granites and Triassic metamorphic enclaves.
► Eclogites from the Tongbaishan area formed in a relatively low P-T environment.
► The subduction of the Yangtze craton shallowed towards the west.
► Diachronous subduction and exhumation occurred in the Tongbai-Dabie-Sulu orogen.