华北吕梁地区汉高山群和小两岭组的地层时代——火山岩锆石U-Pb年龄的约束

Timing of the Hangaoshan Group and Xiaoliangling Formation in the Lüliang area,North China:Constraints from zircon U-Pb ages of volcanic rocks

华北吕梁地区出露的以陆源碎屑沉积为主的汉高山群为中元古界底界界线层型的理想候选剖面,其时代的确定对认识华北前寒武纪地质演化历史及初始盖层的确定及Nuna超大陆聚散的研究具有重要的意义。目前,汉高山群的时代、划分对比及其意义还存在较大争议。本研究使用SHRIMP(二次离子质谱仪)对汉高山群安山岩中的锆石进行了U-Pb同位素分析,获得其^(207)Pb/^(206)Pb年龄加权平均值为1769±9 Ma(2σ)。这是首次直接测得汉高山群锆石U-Pb年龄。另测得同一层位的小两岭组3个不同部位火山岩中的锆石^(207)Pb/^(206)Pb年龄加权平均值分别为1792±18 Ma、1773±19 Ma和1778±20 Ma,与汉高山群的年龄在误差范围内一致,故认为汉高山群和小两岭组的时代分别为1769±9 Ma和1792~1773 Ma。结合前人的研究资料,认为汉高山群和小两岭组的形成时代为1780 Ma左右。吕梁地区汉高山群的剖面地层连续,发育区域上分布较为稳定的安山岩,是华北克拉通中南部极好的区域等时标志层。该时期的裂谷沉积和火山岩在扬子陆块西部、劳伦大陆西北部、澳大利亚北部、南美巴西圣弗兰西斯科(São Francisco)克拉通、非洲刚果克拉通及西伯利亚克拉通均有产出,且可以对比,显示同时期由Nuna超大陆初始裂解引起的陆内裂谷沉积地层的全球分布特征,由此提出国际中元古界底界应该下移,并且汉高山剖面可作为国际上新的中元古界底界的候选层型剖面。

Exposed in the Lüliang area, North China, the Hangaoshan Group is dominated by terrigenous clastic deposits and bear an ideal candidate section for the stratotype of the lower Mesoproterozoic or upper Paleoproterozoic sequence, especially for the boundary between them. The determination of formation age is of great significance for understanding the Precambrian geological evolution of the North China Craton, such as emergence of the initial cover sequence of the craton, assembling and breakup of the Nuna supercontinent. To date, there are still great controversies on the age,subdivision,correlation and geological implication of the Hangaoshan Group. In this study,zircon U-Pb isotopes of the andesites from the Hangaoshan Group have been analyzed with SHRIMP technique,giving a weighted mean ^(207)Pb/^(206)Pb age of 1 769±9 Ma( 2σ) . This is the first direct measurement of the zircon U-Pb age of the Hangaoshan Group. In addition,weighted mean ^(207)Pb/^(206)Pb zircon ages of the volcanic rocks in three different parts of the Xiaoliangling Formation of the same stratohorizon are 1 792±18 Ma,1 773±19 Ma and 1 778±20 Ma,respectively,which are consistent with the age of Hangaoshan Group within the error range. Therefore,the ages of the Hangaoshan Group and Xiaoliangling Formation are determined at 1 769±9 Ma and 1 792 ~ 1 773 Ma,respectively.In combination with previous research results,formation time of the Hangaoshan Group and Xiaoliangling Formation is ca. 1 780 Ma. As regards sedimentology,the section strata of the Hangaoshan Group in Lüliang Mts area are continuous,and a horizon of andesite is stably and widly distributed in the region. So the horizon is an excellent regional isochronous marker in the central and southern part of the North China Craton. Furthermore,rift deposits and volcanic rocks in this period occur in the western Yangtze Block,the northwestern Laurentia,northern Australia,the São Francisco,Congo and Siberian cratons,and all can be well correlated,showing the global distribution of intracontinental rift sediments during initial breakup of Nuna supercontinent in the same period. We propose here that the international Mesoproterozoic bottom boundary should be moved down,from 1.6 Ga to 1.8 Ga,and the Hangaoshan section can be used as a new candidate stratigraphic type section of the international Mesoproterozoic bottom boundary.