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 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.,e peak eclogite facies metamorphism of Chicheng eclogite may occur at 348.5–344.2Ma and its retrograde metamorphism of amphibolite fancies may occur at ca. 325.0Ma. ,e Hongqiyingzi Complex may experience multistage metamorphic events mainly including Late Archean (2494–2448Ma), Late Paleoproterozoic (1900–1734Ma, peak age� 1824.6Ma), and Phanerozoic (495–234Ma, peak age� 323.7Ma). ,us, the metamorphic event (348.5–325Ma) of the Chicheng eclogite is in accordance with the Phanerozoic metamorphic event of the Hongqiyingzi Complex. ,e 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.


Introduction
e tectonic evolution of the central part of the northern margin of the North China Craton has become a hot academic research focus in recent years. As a significant component of the dismembered ophiolite mélange of the Hongqiyingzi Complex [1], the retrograded eclogite which recorded the evolution history of the Paleoasian Ocean has been attracting more and more attention [2][3][4][5][6][7][8][9]. e tectonic evolution of the Chicheng retrograded eclogite can be classified into four stages [10]: (1) the protolith formation stage, (2) the peak eclogite facies stage, (3) the granulite facies stage, and (4) the amphibolite facies stage. Previous chronology studies shows that the metamorphic age of the retrograded eclogite are still under disputes: 325 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].
Because zircon geochemistry and chronology are very effective methods applied to metamorphism chronology research of the eclogite, this paper attempts to provide more zircon U-Pb and Lu-Hf isotopic composition of the eclogite and may be beneficial to solve these disputes.

Geological Background and
Petrological Characteristics e Chicheng area, including Zhenningpu, Luhepu, and Qilidun, is located on the north side of the Chongli-Chicheng Fault, northern margin of the North China Craton (NCC) (Figures 1(a)-1(c)). Retrograded eclogite, which occur as separate tectonic lenses within the Hongqiyingzi Complex, range in size from 1-5 cm to 30-50 m and their elongation is consistent with regional schistosity or gneissosity (Figure 2(a)).
Based on the differences of the mineral assemblages and the degree of retrograde metamorphism, the retrograded eclogite from Chicheng can be divided into the weakly amphibolitized eclogite and the intensely amphibolitized eclogite [11]. e intensely amphibolitized eclogite are characterized by the existence of amphibole and plagioclase, very little or no omphacite and vermicular symplectite of Na-poor clinopyroxene and plagioclase; garnets are replaced completely by a granular symplectite of amphibole and plagioclase.
e weakly amphibolitized eclogite are characterized by the existence of remnant omphacite and vermicular symplectite of Na-poor clinopyroxene and plagioclase.

Sample Selection and Analytical Methods
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 Selection and Cathodoluminescence Study.
Zircons from sample 160601 and 160602 were sorted in the laboratory of the Hebei Regional Geological Investigation Brigade in Langfang city. First, the rock samples were broken into 80-100 mesh grains; then, the zircons were sorted by conventional magnetic separation and then purified by manual picking under a binocular microscope. Before dating, the purified zircons were mounted in an epoxy resin target and polished to half of their thickness.

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 details of the analytical procedures and methods are similar to the dissertations elaborated by Griffin and Yuan [12,13]. e diameter of the ion beam was approximately 32 μm at 8 Hz repetition rate, the depth of the ablation was 20-35 μm, and the duration of the ablation was 40 seconds. In the process of testing, 1 point of the standard zircons was measured after every 5 points of the purified zircons to control the stability and the accuracy of the ion counts. e uncertainty for each measuring point is ±1σ, and uncertainty of the final concordia ages is ±1σ.
Calculation of Hf TDM1 ages is based on a depletedmantle source with present-day 176 Hf/ 177 Hf � 0.28325, using the 176 Lu decay constant 1.867 × 10 −11 year −1 [20]. e zircon trace elements analysis of sample 160601 and 160602 were also performed by ICP-MS at the Nanjing FocuMS Technology Co. 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.

Zircon Trace Elements' Analysis.
e chondrite-normalized REE (rare-earth element) patterns of zircon from sample 160601 (Table 2) are characterized by flat HREE curves and no significant Eu anomaly, indicating that the zircon may form under eclogite facies metamorphic conditions ( Figure 3 Figure 4), and thus, they may grow under amphibolite facies metamorphic condition [22].

Zircon U-Pb
Dating. e zircons from sample 160601 are colorless with subhedral to oval crystals, lengths ranging from 40 to 120 μm with length/width ratios of 1 : 1 to 2 : 1 ( Figure 4(a)). Cathodoluminescence photos show cloudy zoned or unzoned features for metamorphic zircon. e 17 analytical points were made on this sample, which have similar low /U values (0.002-0.029) and yield mean ages of 348.5 ± 3.7 Ma (Figure 3(a)). e zircons from sample 160602 are also colorless with subhedral to oval crystals, lengths ranging from 50 to 180 μm with length/width ratios of 1 :1 to 3 :1 (Figure 4(b)). Cathodoluminescence photos show metamorphic features of nebulous zoned or unzoned. e 9 analytical points of group one zircons have low /U values (0.005-0.091) and yield mean ages of 344.2 ± 7.3 Ma (Figure 3(b)). e 2 analytical points of group two zircons have low /U values (0.017-0.04) and yield mean ages of 325.0 ± 4.1 Ma (Figure 3(c)).

Zircon Lu-Hf Dating.
Fifteen Lu-Hf spots on the zircon cores were obtained on zircon grains from sample 160601. On the basis of a metamorphic age of approximate 348.5 Ma, initial 176 Hf/ 177 Hf ratios and εHf (t) values are calculated by assuming t � 348.5 Ma (Table 3). e metamorphic zircons have a 176 Lu/ 177 Hf ratio of 0.000003-0.000013 and 176 Hf/ 177 Hf (348.5 Ma) of 0.282726-0.283025, corresponding to εHf (348.5 Ma) ranging from 6.0 to 16.6 ( Figure 5).
Nine Lu-Hf spots on the zircon cores were obtained on zircon grains from sample 160602. On the basis of a metamorphic age of approximate 344.2 Ma, initial 176 Hf/ 177 Hf ratios and εHf (t) values are calculated by assuming t � 344.2 Ma (Table 3).

e Protolith and Metamorphic Condition of the Eclogite.
Previous studies shows that the protolith of the Chicheng retrograded eclogite is tholeiitic oceanic crust with geochemical characteristics of midocean ridge basalt (MORB) or island arc tholeiite (IAT) [10] and experiences the peak eclogite facies metamorphism caused by the subduction of Paleo-Asian Ocean crust beneath the NCC and the granulite facies or amphibolite facies metamorphism resulted from its exhumation into the Hongqiyingzi Complex. e peak eclogite facies metamorphism is marked by the existence of a small amount of granular omphacite within the garnets (P >1.40-1.50 GPa and T � 680-730°C) [10]. ese features indicate that the Chicheng eclogite has the oceanic or mantle affinity, and thus, the protolith of the eclogite is of "foreign" origin [23,24]. Zircon Lu-Hf isotopic of εHf from the Chicheng eclogite has larger spans range from 6.0 to 18.0, which suggests that the magma of the eclogite protolith may mix with partial crustal components.

6.2.
e Metamorphic Ages of the Eclogite. As presented above, the mineral assemblages from the retrograded eclogite are omphacite, garnet, amphibole, plagioclase, and quartz. Considering the low /U ratios and flat HREE curves with no significant Eu anomaly, the zircon age of 348.5 ± 3.7 Ma from sample 160601 and 344.2 ± 7.3 Ma from group one of sample 160602 are interpreted as the time of the eclogite facies metamorphism. Due to low /U ratios and steep HREE curves with slight negative Eu anomaly, the zircon age of 325.0 ± 4.1 Ma from group two of sample 160602 is interpreted as the time of the retrograde metamorphism of amphibolite facies. e eclogite facies metamorphic age of 348.5-344.2 Ma is older than the previous study of 325 Ma [10] and a little younger than the previous study of 355 Ma (Kong et al. [11]).

e Relationship of the Metamorphism between the Eclogite and Its Country Rock.
According to previous studies, the Hongqiyingzi Complex may experience multistage metamorphic events (Figure 6)   e metamorphism (granulite facies or amphibolite facies) of the eclogite country rock (garnet biotite plagioclase gneiss) occurred from 351 Ma to 343 Ma (our unpublished data). Combined with the fact that the retrograded eclogite occurs as separate tectonic lenses or lumps in the garnet biotite plagioclase gneiss and its major axes direction is in line with regional gneissosity, we come to the conclusion that the tectonic subduction and exhumation of the eclogite may lead to the regional Mesozoic metamorphism of Hongqiyingzi Complex.

Conclusions
(1) 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 and indicate that they may form under amphibolite facies metamorphic condition. (2) 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 mix with partial crustal components.  Shock and Vibration and implies that the tectonic subduction and exhumation of the eclogite may lead to the regional metamorphism of garnet biotite plagioclase gneiss.

Data Availability
e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare that they have no conflicts of interest.