Metamorphic Zircon
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Ultramafic rocks as irregular lenses and blocks are commonly identified within gneissic rocks in the Sulu ultrahigh-pressure (UHP) metamorphic terrane,eastern China.A combined study of mineral inclusions,cathodoluminescence (CL) images,and in situ U-Pb LA-ICP-MS dates of zircons provided insight into the protolith and UHP metamorphic ages of these rocks.Zircon grains separated from olivine-bearing pyroxenite (samples W1 and W2) in Weihai area,North Sulu can be subdivided into three types:(1) inherited (magmatic) zircons with clear magmatic CL images and Ol+Cpx+Ap inclusions,which record Paleoproterozoic protolith ages of 1835~1845Ma;(2) some inherited zircons with blurry CL images and Ol+Cpx+Ap inclusions give a younger age group of 250~784Ma related to partial recrystallization and Pb loss during late magmatic and metamorphic events;and (3) metamorphic zircons with homogeneous CL images and Grt+Cpx inclusions that record UHP metamorphism at 230~234Ma,which are consistent with the UHP ages (235~225Ma) recorded by the coesite-bearing zircons from eclogite and its country-rocks in Sulu UHP terrane.Existence of UHP metamorphic zircons in ultramafic rocks indicates that the fluid may play an important role in the UHP metamorphic process of the Sulu terrane.The results from this study not only exactly constrain the protolith and UHP metamorphic ages of ultramafic rocks,but also show important significance for an in-depth insight into of the fluid property and evolution feature,fluid-rock interaction mechanism in the Sulu-Dabie UHP terrane.
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An unusual zircon SHRIMP dating result of a granitic gneiss from the Qinglongshan eclogite-gneiss roadcut section is presented in this paper. The very peculiar and complicated internal structures, as well as the very low Th/U ratios (0.01-0.08) of the zircons indicate that they were formed by metamorphic recrystallization. Strongly in contrast with previously published zircon U-Pb ages of the Dabie-Sulu UHP metamorphic rocks where protolith ages of 600-800 Ma are commonly recorded, only metamorphic age of 218+5 Ma, defined by 18 analytical spots either in rim or in core of zircons, are recorded in this granitic gneiss. This age represents the time of the complete metamorphic recrystallization overprint on primary magmatic zircons. The recrystallization was derived by the UHP metamorphism,and was strengthened by the early stage of retrograde metamorphic fluid activity.
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Laser ablation inductively coupled plasma mass spectrometry analyses of U–Pb isotopes and trace elements in zircon and titanite were carried out on epoxy mounts and thin sections for ultrahigh-pressure (UHP) eclogite in association with paragneiss in the Dabie orogen. The results provide a direct link between metamorphic ages and temperatures during continental subduction-zone metamorphism. Zircon U–Pb dating gives two groups of concordant ages at 242 ± 2 to 239 ± 5 Ma and 226 ± 2 to 224 ± 6 Ma, respectively. The Triassic zircon U–Pb ages are characterized by flat heavy rare earth element (HREE) patterns typical of metamorphic growth. Ti-in-zircon thermometry for the two generations of metamorphic zircon yields temperatures of 697 ± 27 to 721 ± 8 °C and 742 ± 19 to 778 ± 34 °C, respectively. We interpret that the first episode of zircon growth took place during subduction prior to the onset of UHP metamorphism, whereas the second episode in the stage of exhumation from UHP to HP eclogite facies regime. Thus, the continental subduction-zone metamorphism of sedimentary protolith is temporally associated with two episodes of fluid activity, respectively, predating and postdating the UHP metamorphic phase. The significantly high Ti-in-zircon temperatures for the younger zircon at lower pressures indicate the initial ‘hot’ exhumation after the peak UHP metamorphism. There are two types of titanite. One exhibits light rare earth element (LREE) enrichment, steep MREE–HREE patterns and no Eu anomalies, and yields Zr-in-titanite temperatures of 551 to 605 °C at 0.5 GPa, and the other shows LREE depletion and flat MREE–HREE patterns, and gives Zr-in-titanite temperatures of 782–788 °C at 2.0 GPa. The former is amenable for U–Pb dating, yielding a discordia lower intercept age of 252 ± 3 Ma. Thus, the first type of titanite is interpreted to have grown in the absence of garnet and plagioclase and thus in the early stage of subduction. In contrast, the second one occurs as rims surrounding rutile cores and thus grew in the presence of garnet during the ‘hot’ exhumation. Therefore, there is multistage growth of zircon and titanite during the continental subduction-zone metamorphism. The combined studies of chronometry and thermobarometry provide tight constraints on the P–T–t path of eclogites during the continental collision. It appears that the mid-T/UHP eclogite facies zone would not only form by subduction of the continental crust in a P–T path slightly below the wet granite solidus, but also experience decompression heating during the initial exhumation.
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Abstract An unusual zircon SHRIMP dating result of a granitic gneiss from the Qinglongshan eclogite‐gneiss roadcut section is presented in this paper. The very peculiar and complicated internal structures, as well as the very low Th/U ratios (0.01–0.08) of the zircons indicate that they were formed by metamorphic recrystallization. Strongly in contrast with previously published zircon U‐Pb ages of the Dabie‐Sulu UHP metamorphic rocks where protolith ages of 600–800 Ma are commonly recorded, only metamorphic age of 218±5 Ma, defined by 18 analytical spots either in rim or in core of zircons, are recorded in this granitic gneiss. This age represents the time of the complete metamorphic recrystallization overprint on primary magmatic zircons. The recrystallization was derived by the UHP metamorphism, and was strengthened by the early stage of retrograde metamorphic fluid activity.
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Micro-scale in situ REE analyses of zircon cores and rims have been carried out for eclogites from Xindian, Shuanghe, Huangzhen and jadeite quartzite from Shuanghe, Dabie Mountains. Combining with the CL internal structure and U-Pb dating, we studied the zircon REE composition change from the protolith inherited zircon to the metamorphic zircon during the ultra-high pressure metamorphism. The results show that the inherited core and metamorphic rim have quite different REE contents. It depends on how metamorphic zircon forms, overgrowth or recrystallization of protolith zircon. In general, the metamorphic zircon rims have lower REE contents compared with the protolith cores, especially in HREE. The metamorphic rims also have lower Th/U ratio than that of the cores. Detailed comparative study on REE and Th/U ratio of inherited zircon cores and metamorphic zircon rims may provide important information on forming process of metamorphic zircons and physico-chemical environment when it forms.
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