柴达木-祁连山地区前寒武系-寒武系碎屑锆石年龄数据

过去柴达木-祁连山地区的前寒武系–寒武系的地层框架、地层对比及物源信息不甚清晰。依托第二次青藏科考，我们对柴达木-祁连山地区的前寒武系-寒武系中学界较为关注的地层单元进行了采样和LA-ICP-MS U-Pb年代学测试，采样地层包括柴达木地区欧龙布鲁克地块的原震旦系全吉群（石英梁组、黑土坡组、红铁沟组、皱节山组、红藻山组），南祁连的早古生代增生杂岩，中祁连东段的原震旦系杏儿沟群、寒武系毛家沟群，中祁连西段的原青白口系哈什哈尔组、原震旦系石板墩组，北祁连的原震旦系白杨沟群。我们利用IsoplotR计算了各样品的最大似然年龄作为地层最大沉积年龄。测试和计算结果表明，原全吉群各组的沉积时代跨度较大，包括中元古代至新元古代，其中存在多个不整合面，并与华北板块的不整合记录有着良好的对应关系。南祁连的早古生代增生杂岩的沉积时代晚于约480 Ma，具有单峰的物源特征，可能反映了洋壳或洋内俯冲记录的单一物源贡献。测试和计算结果对中祁连东段的杏儿沟群和毛家沟群的沉积时代没有很好的约束，表明其沉积晚于中元古代末，物源可能来自于中祁连东段的中元古界湟源群。中祁连西段的哈什哈尔组最大沉积年龄为569 ± 2 Ma，物源主要来自于中祁连西段的托赖群以及可能的板块外的输入；石板墩组的最大沉积年龄为465.8 ± 6.6 Ma，具有与哈什哈尔组一致的物源。白杨沟群的最大沉积年龄为1185 ± 36 Ma，碎屑锆石年龄分布特征类似湟源群。上述测试和计算结果表明，柴达木-祁连山地区的前寒武系–寒武系地层框架可能与原有认识差别较大，需要重新厘定各地层单元的沉积时代进而重新构建区内的前寒武系–寒武系地层框架。物源上，欧龙布鲁克地块原全吉群的碎屑锆石分布特征与华北板块极其相似，可能反映了其与华北板块在前寒武纪的亲缘关系和相似的演化历史；祁连山地区的前寒武系–寒武系物源主要来自本地，可能反映了对应时期与其他板块相对隔离的环境。

The stratigraphic framework, stratigraphic correlation and provenance information of the Precambrian-Cambrian strata in the Qaidam-Qilian Mountains area have long been unclear. Supported by the Second Tibetan Plateau Scientific Expedition and Research (STEP) Program, we conducted sampling and LA-ICP-MS U-Pb geochronological testing on stratigraphic units of particular academic concern within the Precambrian-Cambrian strata in the Qaidam-Qilian Mountains area. The sampled strata include: the Quanji Group (previously assigned to the Sinian/Neoproterozoic) of the Olongbuluk Block in the Qaidam area (including the Quartzliang Formation, Heitupo Formation, Hongtiegou Formation, Zhoujieshan Formation and Hongzaoshan Formation), the Early Paleozoic accretionary complex in the South Qilian Mountains, the previously assigned Neoproterozoic Xing'ergou Group and Cambrian Maojiagou Group in the eastern section of the Central Qilian Mountains, the Qingbaikouan-aged Hashaha'er Formation and previously assigned Neoproterozoic Shibandun Formation in the western section of the Central Qilian Mountains, and the previously assigned Neoproterozoic Baiyanggou Group in the North Qilian Mountains. We used IsoplotR to calculate the maximum likelihood age of each sample as the maximum depositional age of the corresponding strata.

The testing and calculation results show that the depositional age span of each formation within the formerly defined Quanji Group is large, ranging from the Mesoproterozoic to the Neoproterozoic, with multiple unconformities present, which show a good correspondence with the unconformity records of the North China Craton. The Early Paleozoic accretionary complex in the South Qilian Mountains has a depositional age later than ~480 Ma, with a unimodal provenance signature, which may reflect a single provenance contribution recorded by oceanic crust or intra-oceanic subduction. The testing and calculation results poorly constrain the depositional age of the Xing'ergou Group and Maojiagou Group in the eastern Central Qilian Mountains, indicating that their deposition occurred after the end of the Mesoproterozoic, and their provenance likely originated from the Mesoproterozoic Huangyuan Group in the eastern Central Qilian Mountains.

The maximum depositional age of the Hashaha'er Formation in the western Central Qilian Mountains is 569 ± 2 Ma, with provenance mainly derived from the Tuolai Group in the western Central Qilian Mountains and possible extra-terrane input; the maximum depositional age of the Shibandun Formation is 465.8 ± 6.6 Ma, with a provenance consistent with that of the Hashaha'er Formation. The maximum depositional age of the Baiyanggou Group is 1185 ± 36 Ma, with detrital zircon age distribution characteristics similar to those of the Huangyuan Group.

The aforementioned testing and calculation results indicate that the Precambrian-Cambrian stratigraphic framework in the Qaidam-Qilian Mountains area may differ significantly from previous understandings, and it is necessary to redefine the depositional ages of each stratigraphic unit to reconstruct the Precambrian-Cambrian stratigraphic framework in the area. In terms of provenance, the detrital zircon distribution characteristics of the formerly defined Quanji Group in the Olongbuluk Block are extremely similar to those of the North China Craton, which may reflect their Precambrian affinity and similar evolutionary history; the provenance of the Precambrian-Cambrian strata in the Qilian Mountains mainly comes from local sources, which may reflect a relatively isolated environment from other terranes during the corresponding period.