Retrograded Eclogite From Chicheng, North China Craton: New Insights Into the Fidelity of U-Pb-Hf-O Isotopes in Zircon During High-grade Metamorphism

Zircon is the most abundantly used mineral for dating igneous and metamorphic events and for tracing source characteristics. Understanding the geochemical behavior of the U-Pb-Hf-O isotope systems during high-grade metamorphism is therefore important for accurate interpretation of the isotopic information. We report zircon U-Pb-Hf-O isotopes and trace elements of retrograded eclogites and host gneisses from Chicheng, North China Craton, with the aim to obtain new insights into the delity of U-Pb-Hf-O isotopes in zircon as recorders of high-grade metamorphism. U-Pb dating suggested that the Chicheng mélange experienced eclogite facies metamorphism at ~1.84 Ga, and then exhumed to amphibolite facies at 320– 300 Ma. Zircons with Paleoproterozoic ages formed in metamorphic melts-derived from the gneiss during the eclogite facies metamorphism. Zircons with ages of 300–320 Ma formed by recrystallization of peak metamorphic zircons during uid-assisted amphibolite-facies retrograde metamorphism. This process led to the near-complete resetting not only of U-Pb ages but also of Hf-O isotopic compositions of the peak metamorphic zircons, while preserve REE patterns. These results contrast with the sluggish Hf diffusion rate predicted from experimental studies, and support ndings that isotopic data from metamorphic zircons in retrograded high-grade metamorphic rocks need not be faithful recorders of their sources. intense uid-assisted amphibolite-facies retrograde metamorphism. We present an integrated study of U-Pb-Hf-O isotopes and trace elements in zircons from the retrograded eclogites and surrounding gneisses from the Chicheng region within the Trans-North China Orogen (TNCO) of the North China Craton (NCC). This provides a new baseline for an informed interpretation of U-Pb ages and Hf-O isotopic information obtained from metamorphic zircon grains in high-grade rocks.


Introduction
Zircon is one of the most commonly used minerals for isotopic age determination, because of its occurrence in a wide variety of rocks, and its high U yet low common Pb contents, together with its chemically resistant property 1 . Hf-O isotopes in zircon have been widely used to trace the sources and evolution of rocks [2][3][4] . However, in high-grade metamorphic rocks with prolonged and complex histories, the U-Pb-Hf-O isotopic compositions determined from zircon grains are less easily interpreted, because zircon shows complex mineralogical and geochemical behavior during high-grade metamorphism [5][6][7][8] . Zircon can form either by metamorphic recrystallization from pre-existing magmatic or metamorphic crystals or by new growth in metamorphic uids/melts during high-grade metamorphism 6-12 . Metamorphic recrystallization of pre-existing zircon can have via processes involving solid-state diffusion-reaction or dissolution-reprecipitation [5][6][7][8][9] . Both processes can cause resetting of U-Pb ages and O isotope, and weak disturbance of Hf isotope systems, and the different isotopic systems exhibit disparate behavior during metamorphism [13][14][15] . The Pb, Hf and O diffusion rates in crystalline zircon were shown to be sluggish under anhydrous conditions 16-20 , whereas O diffuses faster under hydrous conditions 16, 21 . In addition, the resetting of U-Pb ages is also controlled by the degree of radiation damage of the zircon lattice 10,22 , which variably resets U-Pb ages when zircon recrystallization without uids 5,13,14 . In contrast, during dissolution-reprecipitation and recrystallization in the presence of metamorphic uids, both U-Pb ages and O isotope system in zircon would be partially or completely reset 13,14 . Several studies have demonstrated that Hf isotopes, once incorporated into zircon, would not be fractionated or only weakly disturbed during later metamorphic processes ranging from solid-state recrystallization to dissolution-reprecipitation [13][14][15][23][24][25] . However, in this study, we found that not only U-Pb ages and O isotopes but also Hf isotopes in recrystallized metamorphic zircon were totally reset by intense uid-assisted amphibolite-facies retrograde metamorphism. We present an integrated study of U-Pb-Hf-O isotopes and trace elements in zircons from the retrograded eclogites and surrounding gneisses from the Chicheng region within the Trans-North China Orogen (TNCO) of the North China Craton (NCC).
This provides a new baseline for an informed interpretation of U-Pb ages and Hf-O isotopic information obtained from metamorphic zircon grains in high-grade rocks.

Discussion
The symplectites consisting of ne-grained plagioclase and amphibole formed around or completely replace the garnet grains in the Chicheng retrograded eclogites (Fig. 1e). These observations suggest that they experienced amphibolite facies retrograde metamorphism following early-stage eclogite facies metamorphism, as previously reported 32 . The Paleoproterozoic ages of ~ 1.84 Ga from the upper intercept ages of wall-rock gneiss (Fig. 2a) and one retrograded eclogite (Fig. 2b) are consistent with previous results which suggest that the eclogite facies metamorphism for the Chicheng mélange possibly occurred at 1.80-1.85 Ga 30,31,38 . This event is also consistent with the formation timing of TNCO 26-28 .
Moreover, zircons with Paleoproterozoic ages show signi cant lower HREE contents than the protolith zircon of gneiss from the Sulu orogen (Fig. 5a) 24 . Flat HREE patterns of some zircons (Fig. 5a) suggest that they formed in the presence of garnet 24,39,40 . Some zircons have shallow MREE-HREE patterns, suggesting insigni cant effect of garnet 24 , but they still have lower HREE contents than the protolith zircons (Fig. 5a). Therefore, zircon grains with age of ~ 1.84 Ga formed during the eclogite facies metamorphism of the Chicheng gneiss and eclogite, which is also consistent with the higher Ti-in-zircon temperatures of 755-876°C (Table S2).
Conversely, zircons in the wall-rock gneiss and four retrograded eclogite samples give concordia ages of 301, 317, 305, 314 and 315 Ma (Fig. 2), respectively, which is in accordance with the previous results that show an age range of 300-326 Ma 31,32,34,35 and the 40 Ar/ 39 Ar age (~ 331 Ma) of amphibole from the Chicheng retrograded eclogite 35 . Therefore, we suggest that zircons with apparent ages of 300-320 Ma record amphibolite facies retrograde metamorphism. However, some zircon cores from sample CC14-10 yield ages from 320 to 1494 Ma (Fig. 2c). These cores were possibly in uenced by amphibolite facies retrograde metamorphism, resulting in partial resetting of the U-Pb ages to different degrees. The irregular relict cores show spongy texture (Fig. 2C) as those observed in zircon that underwent dissolution recrystallization, while the rims exhibit no zoning 13,40 . These pairs of core-rim structure may form by dissolution recrystallization, in which the cores and rims represent the inclusion-rich and pure domains, respectively 24 . Thus, the U-Pb ages of zircon cores from the sample CC14-10 do not have geological meaning, and deviate from the previously suggested protolith ages 32,34 .
In high-grade metamorphic rocks, zircon can form either by recrystallization from pre-existing magmatic or metamorphic crystals or by new growth in metamorphic uids/melts during metamorphism 6-12 . The gneisses and retrograded eclogites in this study experienced a polyphase evolution, with peak metamorphism in the Paleoproterozoic and amphibolite facies retrograde metamorphism upon exhumation at ~ 320-300 Ma. In the following, combined petrographic and geochemical information are used to distinguish between these various origins for zircons from the Chicheng gneiss and retrograded eclogites: (1) Zircon structure, (2) Th, U contents and Th/U ratios obtained from U-Pb geochronology, (3) Hf-O isotopic composition and (4) trace element compositions.
The unzoned or patchy zoned structures (Figs. 2A-E) of zircons from Chicheng gneiss and retrograded eclogites re ect their metamorphic origin. The grains with peak metamorphic age might have crystallized from metamorphic melts with higher Th and U contents and Th/U ratios (Fig. 3a), as suggested in previous studies 11,12 . The similar lower ε Hf (t) (Fig. 3b) and higher δ 18 O (Fig. 3c) in zircons with peak metamorphic age from the gneiss and retrograded eclogite suggest that the metamorphic melts could be from the wall-rock gneiss during the eclogite facies metamorphism. The interpretation is compatible with the wall-rock gneiss representing the uppermost meta-sedimentary unit within the ophiolitic mélange on the continental margin 29,41 , which could have elevated δ 18 O (up to 19.2) 42 .
In contrast to zircons with Paleoproterozoic ages, zircon grains with ages of 300-320 Ma have low Th contents and Th/U (< 0.1) (Fig. 3a). One explanation is that U is more uid-mobile than Th under conditions that are oxidizing enough for some water-soluble U 6+ to be present 43 . This is taken to indicate the formation of these zircons in the presence of uids 5 . In addition, the observations that garnets are replaced by symplectite of amphibole and plagioclase in Chicheng retrograded eclogites (Fig. 1e), and the occurrence of uid inclusions in zircon cores from sample CC14-10 (Fig. 2C), suggest extensive uids activities during the amphibolite facies retrograde metamorphism 23,44 . Metamorphic uids can be liberated from eclogite via decompression exsolution of structural hydroxyl and molecular water during exhumation [45][46][47] . The higher ε Hf (t) in zircons with ages of 300-320 Ma (Figs. 3b and 4) imply that the metamorphic uids were characterized by high 176 Hf/ 177 Hf ratios, which could result from garnet breakdown during retrograde metamorphism 13,23 . During zircon growth by breakdown of garnet, the 176 Hf/ 177 Hf ratio for the newly grown zircon would be elevated 48 . The δ 18 O in zircons with ages of 300-320 Ma fall in the eld for the altered oceanic crust (Fig. 3c), which could be the protolith of Chicheng retrograded eclogite. The geochemical composition of the Chicheng retrograded eclogite indicates that their igneous precursors were oceanic basalts 32  However, the large ranges of Hf-O isotopic compositions in zircons with ages of 300-320 Ma suggest that they are not simple new growth grains in the metamorphic uids, but could be recrystallization of pre-existing zircons. Alternatively, or in addition, Hf-O isotopes were disturbed during interaction with the metamorphic uid. Fluids would increase the O diffusion rates in zircon 16 , and act as a catalyzer for the healing of radiation-damages zircon 5,49,50 . The ε Hf (t) show obvious negative relationship with δ 18 O in zircons with U-Pb ages of 300-320 Ma from the retrograded eclogites (Fig. 4). This might imply that the degree of modi cation of Hf-O isotopic compositions in pre-existing zircons increases with increased accessibility to eclogite-derived uids with radiogenic Hf and light O. Zircons formed by dissolution-reprecipitation recrystallization of pre-existing zircon in a closed system, the isotopic compositions would be the weighted mean values of the pre-existing zircons and uids derived from the matrix, e.g., garnet 13,15,23 . In this scenario, the initial Hf isotope ratios of the peak metamorphic zircons from the gneiss and retrograded eclogite are calculated at t = 301 and t = 317 Ma, respectively. The result indicates that the Hf-O isotopic compositions of zircons with ages of 300-320 Ma could be mixing between peak metamorphic zircons and metamorphic uids-derived from the eclogite (Fig. 4). Therefore, dissolutionreprecipitation recrystallization of peak metamorphic zircons during amphibolite facies retrograde metamorphism would have resulted in the complete resetting of U-Pb ages and Hf-O isotopes in peak metamorphic zircons. Zircons with ages of 300-320 Ma from the wall-rock gneiss have lower and more variable ε Hf (t) from − 36.5 to -0.3 (Fig. 4), and zircons with peak metamorphic ages show large variations of δ 18 O but almost constant ε Hf (t) (Fig. 4). These observations imply that zircons from the gneiss were weakly affected by the metamorphic uids during the retrograde metamorphism, which only lead to the resetting of U-Pb ages and δ 18 O to different degrees but did not or minimally disturb the Hf isotopic compositions.
Zircon with ages of 300-320 Ma and Paleoproterozoic have similar trace element compositions (Fig. 5a), and most of zircon with ages of 300-320 Ma show at HREE patterns (Figs. 5b-d). This also implies that the 300-320 Ma zircons could have been formed by recrystallization of peak metamorphic zircons during amphibolite-facies retrograde metamorphism. Zircon recrystallization is commonly characterized by the preservation of the REE signature of the precursor due to their lower diffusion rates 16,18 . The decoupled variation between Pb and REE in zircons during the amphibolite-facies retrograde metamorphism has been observed in other studies of Dabie-Sulu orogen belt 14,24,39,40 .
Hafnium isotopic compositions are considered as the most robust system relative to U-Pb and O during high-grade metamorphism because of the slower diffusion rate of Hf under dry conditions 16, 19 . In addition, previous studies have suggested that the Hf isotopes in zircon are not or only minimally disturbed during the solid and dissolution recrystallization process [13][14][15][23][24][25] . However, our study of zircons from the Chicheng retrograded eclogite indicates that not only the U-Pb ages and O isotopic compositions of peak metamorphic zircon could be completely reset, but also the Hf isotopic compositions can be strongly altered during the retrograde metamorphism assisted by extensive uids. Thus, we speculate that the Hf diffusion rate is not so sluggish under hydrous conditions, as previously considered 19 . Therefore, the U-Pb ages and Hf-O isotopic information in zircons obtained from the highgrade metamorphic rocks need to be interpreted with caution.

Methods
Zircon O-U-Pb isotope analyses were performed using the same Cameca IMS1280 at Institute of Geology and Geophysics in Chinese Academy of Sciences (IGGCAS) following the methods described by 51 . The Cs + primary ion beam was accelerated at 10 kV, with an intensity of ~ 2 nA and rastered over a 10 µm area, and with spot about 20 µm in diameter for oxygen isotopic measurement. The O 2− beam was accelerated at 13 kV, with an intensity of ∼8-10 nA and ellipsoidal spot of ca. 20×30 µm in size for U-Pb geochronology. The instrumental mass fractionation factor (IMF) was corrected using zircon 91500 as a reference. Two working zircon reference materials "Qinghu" and "Penglai" were used to monitor the machine stability. The reproducibility of the two reference samples was better than 0.26‰, and the internal precision of a single analysis was generally better than 0.2‰. Values of δ 18 O were standardized to VSMOW.
Lu-Hf isotopes and trace element compositions were conducted using the Geolas 193 nm laser coupled to a Neptune Plasma II multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) and an Agilent 7500 inductively coupled plasma-mass spectrometry (ICP-MS), respectively, at the State Key Laboratory of Continental Dynamics, Northwest University, Xi'an, China. The laser was operated at a nominal ablation diameter of 40 µm, repetition rate of 8 Hz and laser beam energy density of 10 J/cm 2 .
Details of the technique are described in 52,53 . If space allowed, all analyses were placed in the same zircon domain, as shown in CL images.  The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.    εHf(t) versus δ18O in zircon grains from Chicheng gneiss and retrograded eclogite. The initial Hf isotope ratios of zircon with age of ~1.84 Ga from gneiss and retrograded eclogite are calculated at t = 301 Ma and t = 317 Ma, respectively. εHf(t = 310 Ma) and δ18O values of uid from altered oceanic crust (i.e., protolith of eclogite) are from37 and36, respectively.