New Lu-Hf and Sm-Nd geochronology constrains the subduction of oceanic crust during the Carboniferous-Permian in the Dabie orogen

摘要

Eclogites from the Huwan shear zone in the western Dabie were investigated in terms of their P-T evolution, geochemistry, and combined Lu-Hf and Sm-Nd geochronology. Trace element and isotope data suggest a normal mid-ocean ridge rather than an intraplate or ocean island setting for the protoliths of the eclogites. Electron microprobe analyses of representative garnets show typical prograde zoning profiles. Estimated peak metamorphic temperatures of 540-590 ℃ most likely did not exceed the closure temperature of the Lu-Hf and Sm-Nd systems. The consistent Lu-Hf and Sm-Nd ages, therefore, most likely reflect garnet growth and are interpreted to reflect high-pressure eclogite-facies metamorphism due to the occurrence of omphacite inclusions from core to rim in garnets and the spherical geometry effect despite the well-preserved prograde zoning in the garnets. The high-pressure mineral assemblage of the eclogite yielded a statistically robust Lu-Hf age of 260.0 ± 1.0 Ma (2σ, 10 points, MSWD - 1.0) and a Sm-Nd age of 260.4 ± 2.0 Ma (2σ, 9 points, MSWD = 1.4), which are younger than the Carboniferous zircon U-Pb ages of ca. 310 Ma. The new Lu-Hf and Sm-Nd data, in combination with published geochro-nological data, define two distinct Carboniferous and Permian population ages for the oceanic-type eclogites from the Huwan shear zone, which may require that these rocks experienced two episodes of high-pressure metamorphism within less than 50 Myr.