Abstract
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 εHf 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.
Highlights
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
Previous chronology studies shows that the metamorphic age of the retrograded eclogite are still under disputes: 325 Metermorphic age (Ma) of SHRIMP zircon U-Pb age was recommended as the peak metamorphic age by Ni et al [10]; 355 Ma of SHRIMP zircon U-Pb age was proposed as the peak metamorphic age by Kong et al [11]; 1847–1840 Ma of SIMS zircon U-Pb age was recommended as the peak metamorphic age by Liu et al [1]; 1.85–1.80 Ga, 460–420 Ma, and 360–270 Ma of zircon U-Pb age was proposed as the peak metamorphic age, retrograded metamorphic age, and amphibolite fancies metamorphic age, respectively, by Zhang et al [8]
Summary
Two samples (sample 160601 and 160602) of retrograded eclogite from Chicheng were used in this research and were performed by zircon ICP-MS U-Pb dating, zircon Lu-Hf isotopic analysis, and zircon trace elements analysis. Zircon ICP-MS U-Pb Dating, Lu-Hf Isotopic Analysis, and Trace Elements’ Analysis. E zircon U-Pb dating of sample 160601 and 160602 was conducted using an Analyte Excite (Bozeman, Montana, USA) 193 nm Laser ablation system combined with a Nu Plasma II ICP-MS (Wrexham, Wales, UK) at Nanjing FocuMS Technology Co. Ltd. E isotope ages of 206Pb/238U were corrected by standard zircons 91500 (1064 Ma) [14]. E zircon trace elements analysis of sample 160601 and 160602 were performed by ICP-MS at the Nanjing FocuMS Technology Co. Ltd. Ltd. e testing procedures and parameters are similar to zircon U-Pb dating and the analytical error is about ±10% for light rare Earth elements and ±5% for the other rare Earth elements
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