A growing body of evidence shows that volcanism near the Permian-Triassic boundary（PTB） may be crucial in triggering the Permian–Triassic（P–Tr） mass extinction. Thus, the ash beds near the PTB in South China may...A growing body of evidence shows that volcanism near the Permian-Triassic boundary（PTB） may be crucial in triggering the Permian–Triassic（P–Tr） mass extinction. Thus, the ash beds near the PTB in South China may carry information on this event. Three volcanic ash layers, altered to clay, outcropped in the PTB beds in Zunyi Section, Guizhou Province, Southwest China. The U-Pb ages, trace elements, and Hf-isotope compositions of zircon grains from these three ash beds were analyzed using LA-ICPMS and LA-MC-ICPMS. The zircons are mainly magmatic in origin（241-279 Ma） except for two inherited/xenocrystic zircons（939 and 2 325 Ma）. The ages of these magmatic zircons indicate three episodes of magmatism which occurred around the MiddleLate Permian boundary（-261.5 Ma, MLPB）, the Wuchiapingian-Changhsingian boundary（-254.5 Ma, WCB）, and the PTB（-250.5 Ma）, respectively. The first two episodes of magmatism near the MLPB and WCB may be attributed to magmatic inheritance or re-deposition. All magmatic zircons share similar trace-element and Hf-isotope compositions. They have Y, Hf, Th and U contents and Nb/Ta ratios are typical of zircons from silicic calc-alkaline magmas. These zircons also exhibit enriched Hf-isotope compositions with _（εHf）（t） values of-11.4 to ＋0.2, which suggests that the three magmatic episodes involved melting of the continental crust. The more enriched Hf-isotope composition （_（εHf）（t）=-11.4--4.8） of Bed ZY13（-250.5 Ma） implies more input of ancient crustal material in the magma. Integration of the Hf-isotope and trace-element compositions of magmatic zircons suggest that these three episodes of magmatism may take place along the convergent continent margin in or near southwestern South China as a result of the closure of the Palaeo-Tethys Ocean.展开更多
The Tongshankou Cu-Mo deposit, located in southeast Hubei province, is a typical skarn–porphyry type ore deposit closely related to the Tongshankou granodiorite porphyry, characterized by a high Sr/Y ratio.Detailed i...The Tongshankou Cu-Mo deposit, located in southeast Hubei province, is a typical skarn–porphyry type ore deposit closely related to the Tongshankou granodiorite porphyry, characterized by a high Sr/Y ratio.Detailed in situ analyses of the trace elements and U–Pb and Lu–Hf isotopes in zircons from the Tongshankou granodiorite porphyry were performed.Scarcely any inherited zircons were observed, and the analyzed zircons yielded highly concordant results with a weighted mean 206Pb/238 U age of 143.5 ± 0.45 Ma（n=20, mean square weighted deviation was 0.75）, which was interpreted to represent the crystallization age of the Tongshankou granodiorite porphyry.The chondrite-normalized rare-earth element pattern was characterized by a slope that steeply rises from the light-group rare-earth elements（LREE） to the heavy-group rare-earth elements（HREE） with a positive Ce-anomaly and inconspicuous Eu-anomaly, which was coincident with the pattern of the zircons from the Chuquicamata West porphyry, Chile.The analyzed zircons also had relatively low 176Hf/177 Hf ratios of 0.282526–0.282604.Assuming t=143 Ma, the corresponding calculated initial Hf isotope compositions（εHf（t）） ranged from-5.6 to-2.9.The results of the in situ analysis of trace elements and U–Pb and Lu–Hf isotopes in zircons from the Tongshankou granodiorite porphyry suggest that a deep-seated process involving a thickened-crust/enriched-mantle interaction may play an important role in the generation of high Sr/Y-ratio magma and potentially in the generation of porphyry Cu-Mo systems.展开更多
基金supported by an aid grant from Chengdu Center, China Geological Survey (No. 12120113049100-1)the National Natural Science Foundations (Nos. 40572068, 40839903 and 41272044)+1 种基金the "111" Program (No. B08030)an aid grant (No. GBL11206) from the State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences (Wuhan), China
文摘A growing body of evidence shows that volcanism near the Permian-Triassic boundary（PTB） may be crucial in triggering the Permian–Triassic（P–Tr） mass extinction. Thus, the ash beds near the PTB in South China may carry information on this event. Three volcanic ash layers, altered to clay, outcropped in the PTB beds in Zunyi Section, Guizhou Province, Southwest China. The U-Pb ages, trace elements, and Hf-isotope compositions of zircon grains from these three ash beds were analyzed using LA-ICPMS and LA-MC-ICPMS. The zircons are mainly magmatic in origin（241-279 Ma） except for two inherited/xenocrystic zircons（939 and 2 325 Ma）. The ages of these magmatic zircons indicate three episodes of magmatism which occurred around the MiddleLate Permian boundary（-261.5 Ma, MLPB）, the Wuchiapingian-Changhsingian boundary（-254.5 Ma, WCB）, and the PTB（-250.5 Ma）, respectively. The first two episodes of magmatism near the MLPB and WCB may be attributed to magmatic inheritance or re-deposition. All magmatic zircons share similar trace-element and Hf-isotope compositions. They have Y, Hf, Th and U contents and Nb/Ta ratios are typical of zircons from silicic calc-alkaline magmas. These zircons also exhibit enriched Hf-isotope compositions with _（εHf）（t） values of-11.4 to ＋0.2, which suggests that the three magmatic episodes involved melting of the continental crust. The more enriched Hf-isotope composition （_（εHf）（t）=-11.4--4.8） of Bed ZY13（-250.5 Ma） implies more input of ancient crustal material in the magma. Integration of the Hf-isotope and trace-element compositions of magmatic zircons suggest that these three episodes of magmatism may take place along the convergent continent margin in or near southwestern South China as a result of the closure of the Palaeo-Tethys Ocean.
基金supported by geological survey projects of the China Geological Survey (1212011120863, 12120114039401, 12120114005901 and 12120115029401)
文摘The Tongshankou Cu-Mo deposit, located in southeast Hubei province, is a typical skarn–porphyry type ore deposit closely related to the Tongshankou granodiorite porphyry, characterized by a high Sr/Y ratio.Detailed in situ analyses of the trace elements and U–Pb and Lu–Hf isotopes in zircons from the Tongshankou granodiorite porphyry were performed.Scarcely any inherited zircons were observed, and the analyzed zircons yielded highly concordant results with a weighted mean 206Pb/238 U age of 143.5 ± 0.45 Ma（n=20, mean square weighted deviation was 0.75）, which was interpreted to represent the crystallization age of the Tongshankou granodiorite porphyry.The chondrite-normalized rare-earth element pattern was characterized by a slope that steeply rises from the light-group rare-earth elements（LREE） to the heavy-group rare-earth elements（HREE） with a positive Ce-anomaly and inconspicuous Eu-anomaly, which was coincident with the pattern of the zircons from the Chuquicamata West porphyry, Chile.The analyzed zircons also had relatively low 176Hf/177 Hf ratios of 0.282526–0.282604.Assuming t=143 Ma, the corresponding calculated initial Hf isotope compositions（εHf（t）） ranged from-5.6 to-2.9.The results of the in situ analysis of trace elements and U–Pb and Lu–Hf isotopes in zircons from the Tongshankou granodiorite porphyry suggest that a deep-seated process involving a thickened-crust/enriched-mantle interaction may play an important role in the generation of high Sr/Y-ratio magma and potentially in the generation of porphyry Cu-Mo systems.
