ECLOGITES RECORD THE TECTONIC TRANSITION FROM OCEANIC SUBDUCTION TO CONTINENTAL COLLISION

It has been generally assumed that continental subduction was generally preceded by oceanic subduction, with subsequent collision between two continental blocks. While this precept is evident in the Himalayan orogen where the Neotethyan ocean was closed, it is not so in the Dabie-Sulu orogenic belt where the Paleotethyan ocean was closed in east-central China. In order to test this precept, we have performed a combined study of whole-rock and zircon geochemistry as well as zircon U-Pb ages HP and UHP eclogite-facies metamorphic rocks from the Hong’an orogen, a westward extension of the Dabie-Sulu orogenic belt. The results provide insights into the tectonic transition from oceanic subduction to continental collision during the closure of the Paleotethyan ocean between the South and North China Blocks. Based on their whole-rock geochemistry, eclogites in the Hong’an orogen are categorized into continental-type and oceanic-type, respectively. The continental-type eclogites were mostly metamorphosed in the Triassic at HP to UHP conditions and occur in the whole orogen. They show arc-like trace element patterns as well as both positive and negative ε_Nd(t) and ε_Hf(t) values for whole-rock and zircon, suggesting that their protoliths are both juvenile and ancient crustal rocks. Zircon U-Pb dating on relict zircon cores of magmatic origin yields protolith ages of about 750 to 1200 Ma, demonstrating a tectonic affinity to the South China Block. In contrast, the oceanic-type eclogites were only metamorphosed in the Carboniferous under HP conditions and only occur in the northwestern edge of the orogen. They exhibit MORB-like flat REE patterns and arc-like trace element patterns as well as variable ε_Nd(t) and positive ε_Hf(t) values and slightly high (⁸⁷Sr/⁸⁶Sr)_i ratios, suggesting that their protoliths are equivalent to forearc basalts. Zircon U-Pb dating on relict zircon cores of magmatic origin yields protolith ages of ~420 Ma. The occurrence of both oceanic- and continental-type eclogites in the Hong’an orogen records the tectonic processes that the subduction of continental crust would be gravitationally pulled by the subduction of oceanic crust at the same subduction zone and the HP eclogite-facies metamorphic mélanges would be exhumed together with the UHP rocks along the same subduction channel.