南祁连党河南山地区巴龙贡噶尔组的解体与岩石地层单位厘定——来自岩石学与年代学的证据

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

南祁连党河南山地区分布一套变形较强的区域低温动力变质岩系,前人将其划归为巴龙贡噶尔组。通过对该套地层开展岩石学、同位素年代学及地层对比研究,发现其明显区别于典型的巴龙贡噶尔组,将该地层重新厘定为拐杖山组。该组岩石由片岩段与火山岩段组成,构造变形强烈,片岩段原岩为成熟度较低的杂砂岩与岩屑砂岩,火山岩段则以中基性火山岩与火山碎屑岩为主,总体表现为火山-沉积建造特征。在火山岩段获得玄武安山岩LA-ICP-MS锆石U-Pb年龄为734±8.5Ma,确定其形成时代为新元古代。地层对比结果显示,研究区拐杖山组在南祁连地区出露较好,分布广泛。在沉积环境上,拐杖山组的物源与沉积建造均指示其具有大陆裂谷盆地特征,是对Rodinia超大陆裂解事件的沉积响应。因此,拐杖山组的发现与确立为正确建立祁连陆块地层格架、研究祁连山构造演化提供了重要依据。

There exist a series of regional low- temperature dynamometamorphiic rocks in Danghenanshan area of South QilianMountain, which were previously classified as Balonggongge’er Formation. Studies of petrology, isotope chronology and stratigraphiccorrelation show that these rocks are obviously different from typical Balonggongge’er Formation and hence the authors redefined itas 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, suggestingthat it was formed in Neoproterozoic. The results of stratigraphic correlation suggest that the Guaizhangshan Formation had a widedistribution. 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 stratatigraphicframework in Qilian block and studying tectonic evolution of the Qilian Mountain.