Carboniferous and triassic eclogites in the western Dabie Mountains, east-central China: evidence for protracted convergence of the North and South China Blocks

摘要

SHRIMP U-Pb dating and laser ablation ICP-MS trace element analyses of zircon from four eclogite samples from the north-western Dabie Mountains, central China, provide evidence for two eclogite facies metamorphic events. Three samples from the Huwan shear zone yield indistinguishable late Carboniferous metamorphic ages of 312+/-5, 307+/-4 and 311+/-17 Ma, with a mean age of 309+/-3 Ma. One sample from the Hong an Group, 1 km south of the shear zone yields a late Triassic age of 232+/-10 Ma, similar to the age of ultra-high pressure (UHP) metamorphism in the east Qinling-Dabie orogenic belt. REE and other trace element compositions of the zircon from two of the Huwan samples indicate metamorphic zircon growth in the presence of garnet but not plagioclase, namely in the eclogite facies, an interpretation supported by the presence of garnet, omphacite and phengite inclusions. Zircon also grew during later retrogression. Zircon cores from the Huwan shear zone have Ordovician to Devonian (440-350 Ma) ages, at to steep heavy-REE patterns, negative Eu anomalies, and in some cases plagioclase inclusions, indicative of derivation from North China Block igneous and low pressure metamorphic source rocks. Cores from Hong an Group zircon are Neoproterozoic (780-610 Ma), consistent with derivation from the South China Block. In the western Dabie Mountains, the first stage of the collision between the North and South China Blocks took place in the Carboniferous along a suture north of the Huwan shear zone. The major Triassic continent continent collision occurred along a suture at the southern boundary of the shear zone. The first collision produced local eclogite facies metamorphism in the Huwan shear zone. The second produced widespread eclogite facies metamorphism throughout the Dabie Mountains-Sulu terrane and a lower grade overprint in the shear zone. [References: 43]