Age and tectonic setting of the Yanbian Group in the southwestern Yangtze Block: Constraints from clastic sedimentary rocks.

The southwestern margin of the Yangtze Block is occupied by the thick volcanic-sedimentary rocks of the Yanbian Group, which is of significance in reconstructing the Neoproterozoic tectonic environment of this region. The clastic sedimentary rocks of the Yanbian Group have been studied on petrology, clastic composition and geochemistry. In addition, two sandstone samples are collected from the Xiaoping Formation and Zhagu Formation, respectively, for zircon U-Pb dating and Hf isotope analysis. In the metasandstone of the Yanbian Group many volcanic lithoclasts and mineral debris of quartz and feldspar have been discerned, which show poor sorting and roundness. In the clastic sedimentary rocks, Al2O3/SiO2 is ranging from 0.12 to 0.4, and K2O/Na2O from 0.14 to 9.45, in which most slates with K2O/Na2O ratio over 1.0, while most sandstones less than 1.0. The LREEs and HREEs in the clastic sedimentary rocks are slightly to moderately differentiated with (La/Yb)N ratios from 1.6 to 9.37 and most with substantial negative Eu anomaly. The eNd(t) gives the range of -1.77 to +5.01. The detrital zircons U-Pb result is mainly concentrated at 900Ma to 910Ma with minor ages of Archean to Paleoproterozoic. The weighted mean ages of the young detrital zircons of the Xiaoping and Zhagu formations are 888±8Ma and 884±14Ma, respectively. The clastic composition, geochemistry and zircon U-Pb ages all indicate that clastic sediments in the Yanbian Group are proximal deposits, and the sedimentary provenances are from intermediate-felsic volcanic rocks and granites in island arc environment. Combining with the results of the Guandaoshan diorite and the basalt in the Huangtian Fm., the age of the Yanbian Group is constrained to 880～830Ma, deposited in the back-arc basin environment. Provenance of the clastic sediments is from the volcanic arc at the western margin of the Yangtze Block when the Cathaysian Block was being subducted beneath the Yangtze Block.