南祁连党河南山地区新元古代拐杖山岩群碎屑锆石U-Pb年代学及其地质意义

传统认为志留系巴龙贡噶尔组广泛分布于南祁连地区,近年来在其中解体出了新元古代拐杖山岩群,对其开展锆石年龄研究对了解该地层的沉积时代、物源性质以及与邻近板块的亲缘性具有重要意义。本文选取党河南山地区拐杖山组岩群中的黑云斜长石英片岩和石英片岩,进行LA-ICP-MS锆石U-Pb定年并讨论其地质意义。实验结果表明:由下及上碎屑锆石样品的U-Pb测年的最大沉积年龄分别为-794Ma、-746Ma和-713Ma,表明该变质地层至少沉积于-794Ma之后,在-740Ma时期伴有火山活动,并且部分地层可能沉积于-713Ma以后,时代应划归为新元古代。样品碎屑锆石以867-700Ma年龄区间最为集中,表明全吉地块与中祁连地块的基底岩系可能作为源区为拐杖山岩群提供了碎屑物质,同时响应了Rodinia超大陆裂解事件。通过与全吉地块、中祁连地块、柴达木地块、华北板块、澳大利亚板块以及扬子板块的碎屑锆石频谱对比,南祁连地区与其周缘地块以及扬子板块在元古宙时期显示了较好的亲缘关系,但其物源主要来自全吉地块与中祁连地块,而与扬子地块关联较小。

Geochronology of detrital zircons from the Guaizhangshan Group from the southern Danghe Mountains in South Qilian and its geological implications

The Silurian Balonggonggaer Formation is widely distributed in the South Qilian Mountains and in recent years Neoproterozoic Guaizhangshan Group were found within. It is of great significance in deciphering its sedimentary time, provenance and affinity with adjacent plate. In this paper, we conduct LA-ICP-MS zircon U-Pb dating for biotite plagioclase quartz schist and quartz schist from the Guaizhangshan Group in southern Danghe Mountains. Detrital zircon ages show that the weighted average ages of the youngest zircons in the three samples are 794±10 Ma, 746±6.5 Ma and 713±7.5 Ma, indicating that the Neoproterozoic rocks were deposited after 794 Ma while volcanic activity began -740 Ma. The detrital zircons were largely concentrated in the age range of 867-700 Ma and indicated that the basement rocks of the Quanji and the Central Qilian blocks may have provide the detrital material. This group of ages is an indicator of the disintegration of the Rodinia supercontinent.After comparison with the Quanji, the Central and the Qaidam blocks, North China plate, Australian plate and Yangtze plate, the U-Pb age spectra of detrital zircons shows that the South Qilian block has good affinity of its surrounding massif and the Yangtze plate, but its provenance was mainly from Quanji block and Central Qilian block while it was less related to Yangtze plate.