Zircon U-Pb Dating and Lu-Hf Isotope of the Retrograded Eclogite from Chicheng, Northern Hebei Province, China

We report zircon U-Pb ages and Lu-Hf isotopic data from two sample of the retrograded eclogite in the Chicheng area. Two groups of the metamorphic zircons from the Chicheng retrograded eclogite were identified: group one shows characteristics of depletion in LREE and flat in HREE curves and exhibit no significant Eu anomaly, and this may imply that they may form under eclogite facies metamorphic condition; group two is rich in HREE and shows slight negative Eu anomaly indicated that they may form under amphibolite facies metamorphic condition. Zircon Lu-Hf isotopic of epsilon Hf from the Chicheng eclogite has larger span range from 6.0 to 18.0, which suggests that the magma of the eclogite protolith may be mixed with partial crustal components. The peak eclogite facies metamorphism of Chicheng eclogite may occur at 348.5-344.2 Ma and its retrograde metamorphism of amphibolite fancies may occur at ca. 325.0 Ma. The Hongqiyingzi Complex may experience multistage metamorphic events mainly including Late Archean (2494-2448 Ma), Late Paleoproterozoic (1900-1734 Ma, peak age = 1824.6 Ma), and Phanerozoic (495-234 Ma, peak age = 323.7 Ma). Thus, the metamorphic event (348.5-325 Ma) of the Chicheng eclogite is in accordance with the Phanerozoic metamorphic event of the Hongqiyingzi Complex. The eclogite facies metamorphic age of the eclogite is in accordance with the metamorphism (granulite facies or amphibolite facies) of its surrounding rocks, which implied that the tectonic subduction and exhumation of the retrograded eclogite may cause the regional metamorphism of garnet biotite plagioclase gneiss.

Automated summary

The study reveals that the eclogite in the Chicheng area experienced high temperature and high pressure eclogite metamorphism as well as later retrograde metamorphism, forming metamorphic zircons with different characteristics. Isotopic data indicates that the eclogite protolith magma may have mixed with partial crustal components, and the metamorphic events coincide with those of the surrounding rocks, suggesting tectonic subduction and exhumation may have caused regional metamorphism.