Trace elements in zircon and coexisting minerals from low-T/UHP metagranite in the Dabie orogen: Implications for action of supercritical fluid during continental subduction-zone metamorphism |
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| Authors: | Qiong-Xia Xia Yong-Fei Zheng Zhaochu Hu |
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| Institution: | 1. Geological Survey of Western Australia, Department of Mines and Petroleum, Mineral House, 100 Plain Street, East Perth, WA 6004, Australia;2. ARC Centre of Excellence for Core to Crust Fluid Systems, Earth and Planetary Sciences, Macquarie University, NSW 2109, Australia;3. Western Australian School of Mines, Department of Applied Geology, Bentley Campus, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia;1. Geochemical Research Center, Graduate School of Science, The University of Tokyo, Tokyo 113-0033, Japan;2. Department of Earth Science, University of Ottawa, Ottawa, ON K1N 6N5, Canada;3. ISTerre, Université Grenoble Alpes, CNRS, 1381 rue de la Piscine, 38400 Grenoble Cedex 09, France;4. Geological Survey of Canada, 601 Booth Street, Ottawa, ON K1A 0E8, Canada |
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| Abstract: | Simultaneously in-situ analyses of U–Pb isotopes and trace elements were carried out for zircons, in combination with the in-situ analyses of trace elements in coexisting minerals, from low-T/UHP metagranite in the Dabie orogen. The results provide geochemical evidence for the existence of supercritical fluid during continental subduction-zone metamorphism. The zircons are categorized into three types based on their patterns of REE distribution. Type I zircons show increasing enrichment from La to Lu, with prominent positive Ce anomalies and negative Eu anomalies, which are typical of magmatic zircon. Some of them display regular or blurred oscillatory-zoned texture and apparent 206Pb/238U ages of 341 to 780 Ma, suggesting metamorphic modification by solid-state recrystallization with no significant involvement of metamorphic fluid. Type II zircons share similar Th, U and HFSE contents and REE patterns to Type I zircons. However, they exhibit blurred oscillatory-zoned texture or are unzoned, have apparent 206Pb/238U ages of 348 to 709 Ma, and are LREE-enriched relative to Type I zircons. This suggests that they underwent metamorphic reworking by replacement recrystallization in the presence of metamorphic fluid. The LREE enrichment is due to the presence of microscale LREE-bearing mineral inclusions (such as apatite, monazite or epidote) in the zircons. Type III zircons, representing the majority of the present analyses, are characterized by spongy texture and consistent enrichment of LREE, HREE, Th, U and HFSE relative to Type I zircons. They yield nearly concordant U–Pb ages close to the discordia lower-intercept. The consistent enrichment of trace elements relative to the magmatic zircon indicates involvement of a special UHP metamorphic fluid that has a strong capacity to extract significant amounts of LREE, HREE, Th, U and HFSE from such accessory minerals as allanite, garnet, rutile and zircon. Because these minerals are stable in the field of hydrous melt in granite–water systems, they are not able to be decomposed during the exhumation of deeply subducted continental crust. Thus, a supercritical fluid is suggested to transport the LREE, HREE, Th, U and HFSE in the accessory minerals to recrystallized zircons. The mechanism of dissolution recrystallization is responsible for the spongy texture and the very high concentration of trace elements in this type of metamorphic zircons. Therefore, the action of supercritical fluid is evident under the low-T/UHP metamorphic conditions. |
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