Zircon U-Pb geochronology of the volcanic rocks from Fanchang-Ningwu volcanic basins in the Lower Yangtze region and its geological implications |
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Authors: | Jun Yan HaiQuan Liu ChuanZhong Song XiSheng Xu YaJun An Jia Liu and LiQun Dai |
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Institution: | (1) School of Resources and Environmental Engineering, Hefei University of Technology, Hefei, 230009, China;(2) State Key Laboratory for Mineral Deposits Research, Department of Earth Sciences, Nanjing University, Nanjing, 210093, China |
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Abstract: | The latest eruptions in two important Mesozoic volcanic basins of Fanchang and Ningwu located in the middle-lower reaches
of the Yangtze River formed the bimodal volcanic rocks of the Kedoushan Formation and ultrapotassic volcanic rocks of the
Niangniangshan Formation, respectively. The representative volcanic rocks of the two Formations were selected for LA-ICPMS
zircon U-Pb dating. The results indicate that there exist a large amount of magmatic zircons as indicated by high Th/U ratios
in these volcanic rocks. The weighted mean age of 21 analyses is 130.7±1.1 Ma for the Kedoushan Formation, and that of 20
analyses is 130.6±1.1 Ma for the Niangniangshan Formation. These U-Pb ages are interpreted to represent the formation times
of the volcanic rocks. In combination with other known geochronological data for Mesozoic volcanic rocks from the Lower Yangtze
region, it is proposed that the latest volcanic activations in the Jinniu, Luzong, Fanchang and Ningwu volcanic basins probably
came to end prior to ca. 128 Ma. There is no significant time interval between the early and later volcanic activities in
the Luzong and Ningwu basins, suggesting a short duration of volcanic activities and thus implying the onset of an extensional
tectonic setting at about 130 Ma in the Lower Yangtze region. Integrated studies reveal that the Early Cretaceous magmatic
activities and their geochronological framework in the Lower Yangtze region are a response to progressively dynamic deep processes
that started with the transformation of tectonic setting from compression to extension, followed by delaminating of the lower
part of the thickened lithosphere, lithospheric thinning, asthenosphere upwelling, and crust-mantle interaction.
Supported by the National Basic Research Program of China (Grant No. 2006CB403508), Key Project of Ministry of Education of
China (Grant No. 108511) and National Natural Science Foundation of China (Grant No. 40730313) |
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Keywords: | Zircon U-Pb dating volcanic rocks Early Cretaceous extensional setting Lower Yangtze region |
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