Age and nature of eclogites in the Huwan shear zone, and the multi-stage evolution of the Qinling-Dabie-Sulu orogen, central China

摘要

In situ LA-ICPMS U-Pb, trace element, and Hf isotope data in zircon demonstrate a Carboniferous age for eclogite-facies metamorphism in Siluro-Devonian protoliths in the Huwan shear zone, Dabie Mountains, Central China. This age contrasts with the more prevailing Triassic age for high- to ultrahigh pressure (HP to UHP) metamorphism in the Qinling-Dabie-Sulu orogen. Metamorphic zircon in two eclogite samples from Sujiahe is characterized by low Th/U ratios, small negative Eu anomalies, flat HREE patterns, and low Lu-176/Hf-177 ratios. These geochemical signatures suggest that the zircon crystallized in the presence of garnet and in the absence of plagioclase feldspar. Furthermore, temperatures of similar to 655 and similar to 638 degrees C, calculated using the Ti content of zircon, are consistent with their formation during eclogite-facies metamorphism. The weighted mean Pb-206/U-238 age of 309 +/- 4 Ma (2 delta) for this zircon improves previous age estimates for eclogite-facies metamorphism in the Huwan shear zone, ranging from 420 to 220 Ma. Metamorphic zircon from one eclogite sample from Hujiawan, most likely formed during prograde metamorphism, yields an equivalent age estimate of 312 +/- 11 Ma. Magmatic zircon cores in the three samples yield ages for the magmatic protoliths of the eclogites ranging from 420 +/- 7 to 406 +/- 5 Ma, and post-dating the middle Paleozoic collision of the North China and the Qinling terrain. The zircon crystals in the three eclogite samples display a large variation of epsilon(Hf) (t) values of -4.9 to 21.3. The metamorphic zircon overgrowths show the same range of epsilon(Hf) (t) values as those of the inherited magmatic crystal interiors. This suggests that the metamorphic zircon overgrowths may have formed by dissolution-reprecipitation of pre-existing magmatic zircon thereby preserving their original Hf isotopic composition. The high epsilon(Hf) (t) values suggest that the protoliths were derived from depleted mantle Sources, most likely Paleotethyan oceanic crust; while the low epsilon(Hf) (t) values are attributed to crustal contamination. Some eclogites in the Huwan shear zone have a distinctive signature of continental crust most probably derived from the Yangtze Craton. The coexistence of Paleozoic oceanic crust and Neoproterozoic continental crust with similar metamorphic ages in the Huwan shear zone implies that Paleozoic Paleotethyan oceanic crust was produced within a marginal basin of the northern Yangtze Craton. The opening of the Paleo-Tethyan ocean was slightly younger than the collision of the North China Craton and the Qinling terrain during the Late Paleozoic in the Qinling-Dabie-Sulu orogen. Subduction of the Paleo-Tethyan oceanic crust and associated continental basement resulted in the 309 +/- 2 Ma (2 sigma) eclogite-facies metamorphism in the Huwan shear zone. The subsequent Triassic continent-continent collision led to the final coalescence of the Yangtze and Sino-Korean cratons. Amalgamation of the Yangtze and North China cratons was, therefore, a multistage process extending over at least 200 Ma.