International Geologiical Congress - Oslo 2008

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HPP-07 Late Neoproterozoic orogenic belts and assembly of Gondwana

 

Geochronology of the neoproterozoic mamianshan group in the cathaysia block-China and its tectonic significance

 

Da Zhang, China University of Geosciences (China)
Ganguo Wu, China University of Geosciences (China)
Yongjun Di, China University of Geosciences (China)
Xinqi Yu, China University of Geosciences (China)
Xiangxin Zhang, Shijiazhuang University of Economics (China)
Qunfeng Wang, China University of Geosciences (China)
Huojian Huang, China University of Geosciences (China)
 

 

In the southeastern China, Several large outcrops of Precambrian metamorphic rocks are present in the northwestern Fujian, eastern Jiangxi, and southwestern Zhejiang areas of southeastern China, an area oringinally called Cathaysia by Grabao. Although Cathaysia has been controversial regarding both its existence and tectonic settings, the geologic consensus is that two metamorphic sequences constitute its basement. The upper metamorphic sequence, a silicic metavolcanic formation, is widely discussed about its sequence, age, tectonic setting et al.. In northwestern Fujian province, the upper metamorphic sequence is called the Mamianshan group.
The Mamianshan group is divided into three formations from bottom to top, the Longbeixi, Dongyan and Daling Formations. Although the Cathaysian block has been studied for a long time, arguments about its origin, evolution and tectonic setting during the Neoproterozoic remain. we report new SHRIMP zircon U¨CPb ages and geochemical data for Neoproterozoic volcanic rocks from the three formations of the Mamianshan group in the Precambrian metamorphic basement of the Cathaysia Block. On the basis of ages of the Daling and Longbeixi Formations we define the sequence of the three Mamianshan formations, determine the age range of Neoproterozoic magmatic activities, and speculate on the possible tectonic significance of the Dongyan Formation, which resulted from the strongest magmatic activity in the Mamianshan Group.
By means of SHRIMP zircon U-Pb dating method, we obtain several SHRIMP zircon U-Pb ages for volcanic rock layers from these formations, which are 825.5 ± 9.8 Ma from Longbeixi Formation, 796.5 ± 9.3 Ma and 851.9 ± 9.2 Ma from Dongyan Formation and 756.2 ± 7.2 Ma from Daling Formation, which indicate that, in Cathaysia, during Neoproterozoic, magmatic activities lasted a long period from 851.9 ± 9.2 Ma to 756.2 ± 7.2 Ma. Characters of Nd isotopic data, trace and REE elements from the plagioamphibolites, greenschists indicate the mafic rocks are drived from depleted mantle source and related to arc-island subduction. Changes of geochemical results among different kinds of rocks suggest that greenschist and leptites may have experienced contamination of crustal materials.
According to the isotopic ages, combined with the geochemical conclusion, we can setup temporal sequence of tectonic evolution and model in Cathaysia. Since the early Neoproterozoic, the Huanan oceanic plate, combined with the northwestern Fujian terrane, had been subducted beneath the Yangtze block, forming Jiangnan island arc, magmatism of 851.9 ± 9.2 Ma, weak volcanism of the Longbeixi Formation, strong arc-related volcanism of the Dongyan Formation and weak volcanism of the Daling Formation. This work was supported by NFSC (grants 40372050) and China Geological Survey (grant 1212010533105).

 

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