Unveiling growth histories of multi-generational garnet in a single skarn deposit via newly-developed LA-ICP-MS UPb dating of grandite |
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| Institution: | 1. CAS Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China;2. University of Chinese Academy of Sciences, Beijing 100049, China;3. Guangdong Provincial Key Laboratory of Mineral Physics and Materials, Guangzhou 510640, China;4. State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China;5. Key Laboratory of Metallogenic Prediction of Nonferrous Metals and Geological Environment Monitor, Central South University, Changsha 410083, China;1. State Key Laboratory of Geological Processes and Mineral Resources, Faculty of Earth Resources and Collaborative Innovation Center for Exploration of Strategic Mineral Resources, China University of Geosciences, Wuhan 430074, Hubei, China;2. State Key Laboratory for Mineral Deposits Research, Department of Earth Sciences, Nanjing University, Nanjing 210093, Jiangsu, China;1. School of Marine Sciences, Sun Yat-sen University, Guangzhou 510006, China;2. Guangdong Provincial Key Laboratory of Marine Resources and Coastal Engineering, Guangzhou 510006, China;3. School of Earth Science and Engineering, Sun Yat-sen University, Guangzhou 510275, China;1. Department of Earth System Sciences, Yonsei University, 50 Yonsei-ro, Seoul 120-749, Republic of Korea;2. Korea Institute of Geoscience and Mineral Resources, 124 Gwahang-ro, Daejeon 305-350, Republic of Korea;3. Korea Basic Science Institute, Ochang Center, Ochang 363-883, Republic of Korea;1. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China;2. Department of Geological Sciences, Indiana University, Bloomington, IN 47405, USA;3. Institute of Geological Research, China University of Geosciences, Beijing 100083, China;4. Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China |
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| Abstract: | Although garnet U Pb dating method has been reported recently, yet the accurate concordia 206Pb/238U ages and growth histories of multi generation of garnets based on ages were still lacked. LA-ICP-MS UPb dating on multi-generational grandite (grossular-andradite) garnet from the large Tonglvshan Cu-Fe-Au skarn deposit was applied in this study. Based on petrographic observation, in chronological order, three generation garnets have been distinguished, namely homogeneous Grt1-exo (in the exoskarn zone) and Grt1-endo (in the endoskarn zone), oscillatory zoning Grt2 and vein-type Grt3 cutting magnetite ores. LA-ICP-MS UPb dating on four grandite samples from the Grt1-exo, Grt1-edno, Grt2 and Grt3 yields Tera-Wasserburg lower intercept 206Pb/238U ages of 139.1 ± 1.0 Ma (2σ, MSWD = 0.79), 134 ± 11 Ma (2σ, MSWD = 2.5), 143.4 ± 8.3 Ma (2σ, MSWD = 2.3) and 140.3 ± 1.4 Ma (2σ, MSWD = 0.95), respectively. More importantly, two concordia 206Pb/238U ages of 139.2 ± 0.6 Ma (2σ, MSWD = 1.4) and 139.8 ± 1.5 Ma (2σ, MSWD = 0.13) were firstly obtained from the sample of Grt1-exo with highest U concentrations (U]avg > 80 ppm) contents. The precision UPb ages of 139–140 Ma from Grt1-exo and Grt3 can be considered as the timing of Cu-Fe-Au skarn mineralization, and consistent with the majority of published zircon UPb ages of the quartz dioritic stock and 40Ar39Ar plateau ages of phlogopite at Tonglvshan (142–140 Ma). The precision grandite UPb ages also indicate that the entire metasomatic hydrothermal mineralization activity in the Tonglvshan Cu-Fe-Au skarn deposit occurred within a relatively short time span of <1 (or 2.5 considering errors) Myr. In addition, we found that the grandite garnet is more easily to be enriched in U and can obtain the high-precision concordia UPb ages with higher andradite Mol%, euhedral and larger crystals, and relevant oxidized magmatic rocks or skarns. |
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