New SHRIMP U-Pb zircon ages of Yanbian Group: Evidence for neoproterozoic retro-arc rifting in the western margin of Yangtze continent
|Location||International Geological Congress,oslo 2008|
|Author||Zhang, Chuanheng۱; Gao, Linzhi۲; Shi, Xiaoying۱; Wu, Zhenjie۱; Sun, Yujian۱|
|Holding Date||08 October 2008|
The Yanbian Group is important for understanding the tectonic evolution and for defining the position of Yangtze continent within Rodinia supercontinent, but its geological age and tectonic settings are still highly debated. The Yanbian Group, outcropping in the Yumen region north of Panzhihua city, West Sichuan, angularly overlain by the Nantuo Formation, is a succession of epimetamorphic intermediate and basic volcanic rocks, terrigenous clastic rocks and a few of beds of chert and carbonate. The volcanic rocks, mainly exposed in the southern part of Yumen region, belong to Huangtian Formation, which was traditionally considered as the oldest in the Yanbian Group. Their tectono-geochemical characteristics support a tectonic interpretation of retro-arc rifting. The trachyandesite from the middle of Huangtian Formation yields a weighed-mean U-Pb age of 825±12Ma, suggesting the retro-arc rifting developed in early Neoproterozoic. The terrigenous clastic rocks are divided into three formations: Yumen, in ascending order, Xiaoping and Zhagu formations. A few layers of andesitic tuff are present in lower Zhagu Formation, and the lowest layer yields a weighed-mean U-Pb age of 866±8Ma.
This SHRIMP age shows that volcanic activity older than 850Ma occurs in the western margin of Yangtze continent. The triangular plots of the clastic composition of the 27 sandstone samples from Xiaoping Formation indicate that these terrigenous sediments derived from a volcanic arc, suggesting that Yanbian Group developed in an active continental margin. The above data lead to the conclusion that a convergence plate margin developed along the western margin of Yangtze continent during Rodinia dispersal, which took place from ca. 850Ma at earliest.