The disintegration of Balonggongge'er Formation and the definition of lithostratigraphic unit in Danghenanshan area of South Qilian Mountain: Evidence from petrology and chronology

There exist a series of regional low-temperature dynamometamorphiic rocks in Danghenanshan area of South Qilian Mountain, which were previously classified as Balonggongge'er Formation. Studies of petrology, isotope chronology and stratigraphic correlation show that these rocks are obviously different from typical Balonggongge'er Formation and hence the authors redefined it as Guaizhangshan Formation. The Guaizhangshan Formation is composed of schists and volcanic rocks with strong structural deforma-tion. The original rocks of the schists are low mature greywacke and lithic sandstone, and the volcanic rocks are made of intermedi-ate-basic volcanic rock and pyroclastic rock. Therefore, the rock association indicates the characteristics of volcanic-sedimentary for-mation. The LA-ICP-MS U-Pb high precision age of the basaltic andesite in Guaizhangshan Formation is 734±8.5Ma, suggesting that it was formed in Neoproterozoic. The results of stratigraphic correlation suggest that the Guaizhangshan Formation had a wide distribution. The provenance and sedimentary characteristics of Guaizhangshan Formation indicate that its sedimentary environment was a continental rift basin and its formation resulted from the sedimentary response to the Rodinia supercontinent breakup event. Therefore, the discovery and establishment of the Guaizhangshan Formation provide important evidence for building stratatigraphic framework in Qilian block and studying tectonic evolution of the Qilian Mountain.