This paper reports LA-ICP-MS zircon U-Pb ages and REE compositions on detrital zircons （Type 1） and hydrothermal zircons （Type 2） from low-metasedimentary rocks in the Mesoproterozoic Gaoyuzhuang Formation, Pingqu...This paper reports LA-ICP-MS zircon U-Pb ages and REE compositions on detrital zircons （Type 1） and hydrothermal zircons （Type 2） from low-metasedimentary rocks in the Mesoproterozoic Gaoyuzhuang Formation, Pingquan area, Hebei Province that provide important constraints on the tectonic evolution along the northern margin of the North China Block. The detrital zircons are characterized by an oscillatory magmatic core, surrounded by a narrow structureless rim in CL images. They yield 2~7pb/2~tpb ages ranging from 1703 to 2543 Ma with two age peaks at 2473 and 1794 Ma, which is consistent with the Neoarchean and Paleoproterozoic tectono-thermal events recorded in the NCB basement, indicating that the source of the detritus was locally derived. By contrast, the hydrothermal ones are euhedral, sector zoning or internally structureless. They have relatively higher U and Th contents and Th/U ratios （U=139-2918 ppm, Th=35-1327 ppm, Th/U （average） =0.78 vs. U=15-1044 ppm, Th=8-341 ppm, Th/U （average） =0.57 for detrital zircons）. Moreover, they are also enriched in REEs relative to detrital zircons and show a lower positive Ce anomaly （ZREE = 659-2418 ppm vs. 231-611 ppm for detrital zircon; Ce/Ce＊=2-13 vs. 33-174 for detrital zircons）, similar to known hydrothermal zircons derived from many locations. These characteristics, combined with our field petrographic observations, indicate that the hydrothermal zircons possibly formed from a low temperature aqueous fluid. Twelve concordant or near-concordant analyses on hydrothermal zircons yield weighted mean 206pb/238U ages of 325-327 Ma, which provide unambiguous evidence that the northern NCB underwent late Paleozoic low temperature hydrothermal modification. This timing of this hydrothermal event is compatible with that of the late Paleozoic magmatic and metamorphic-deformational events occurred on the northern margin of the NCB, it appears likely that the hydrothermal activity resulted from southward subduction of the Paleo-Asian Oceanic plate underneath the NCB during late Paleozoic.展开更多
Early Paleozoic volcanic rocks in Nuomuhong area occurred as basalt slice and meta-volcanic slice. SHRIMP zircon U-Pb dating of the basalt slice and the meta-volcanic slice show that the age of the basalt slice is 419...Early Paleozoic volcanic rocks in Nuomuhong area occurred as basalt slice and meta-volcanic slice. SHRIMP zircon U-Pb dating of the basalt slice and the meta-volcanic slice show that the age of the basalt slice is 419±5 Ma, and that of the meta-volcanic slice is 401± 6 Ma. These ages directly testify that there existed Early Paleozoic ocean-continent transform in East Kunlun, the basalt slice was formed in an extensional mid-ocean ridge setting and the meta-volcanic rock slice was formed in an extrusion subduction and collision setting. The inherited zircon age of 1734 Ma in volcanic rocks reflects that the base of East Kunlun may be Middle Proterozoic.展开更多
基金supported by National Basic Research Program of China (Grant No. 2011CB808800)SINOPEC Project (Grant No. G0800-06-ZS-319)+1 种基金National Natural Science Foundation of China (Grant No. 40621002)"111 Project" (Grant No. B08030)
文摘This paper reports LA-ICP-MS zircon U-Pb ages and REE compositions on detrital zircons （Type 1） and hydrothermal zircons （Type 2） from low-metasedimentary rocks in the Mesoproterozoic Gaoyuzhuang Formation, Pingquan area, Hebei Province that provide important constraints on the tectonic evolution along the northern margin of the North China Block. The detrital zircons are characterized by an oscillatory magmatic core, surrounded by a narrow structureless rim in CL images. They yield 2~7pb/2~tpb ages ranging from 1703 to 2543 Ma with two age peaks at 2473 and 1794 Ma, which is consistent with the Neoarchean and Paleoproterozoic tectono-thermal events recorded in the NCB basement, indicating that the source of the detritus was locally derived. By contrast, the hydrothermal ones are euhedral, sector zoning or internally structureless. They have relatively higher U and Th contents and Th/U ratios （U=139-2918 ppm, Th=35-1327 ppm, Th/U （average） =0.78 vs. U=15-1044 ppm, Th=8-341 ppm, Th/U （average） =0.57 for detrital zircons）. Moreover, they are also enriched in REEs relative to detrital zircons and show a lower positive Ce anomaly （ZREE = 659-2418 ppm vs. 231-611 ppm for detrital zircon; Ce/Ce＊=2-13 vs. 33-174 for detrital zircons）, similar to known hydrothermal zircons derived from many locations. These characteristics, combined with our field petrographic observations, indicate that the hydrothermal zircons possibly formed from a low temperature aqueous fluid. Twelve concordant or near-concordant analyses on hydrothermal zircons yield weighted mean 206pb/238U ages of 325-327 Ma, which provide unambiguous evidence that the northern NCB underwent late Paleozoic low temperature hydrothermal modification. This timing of this hydrothermal event is compatible with that of the late Paleozoic magmatic and metamorphic-deformational events occurred on the northern margin of the NCB, it appears likely that the hydrothermal activity resulted from southward subduction of the Paleo-Asian Oceanic plate underneath the NCB during late Paleozoic.
文摘Early Paleozoic volcanic rocks in Nuomuhong area occurred as basalt slice and meta-volcanic slice. SHRIMP zircon U-Pb dating of the basalt slice and the meta-volcanic slice show that the age of the basalt slice is 419±5 Ma, and that of the meta-volcanic slice is 401± 6 Ma. These ages directly testify that there existed Early Paleozoic ocean-continent transform in East Kunlun, the basalt slice was formed in an extensional mid-ocean ridge setting and the meta-volcanic rock slice was formed in an extrusion subduction and collision setting. The inherited zircon age of 1734 Ma in volcanic rocks reflects that the base of East Kunlun may be Middle Proterozoic.
