扬子陆块北缘构造演化新认识:来自原花山群年代学和地球化学的制约

New understanding of the tectonic evolution of the northern margin of Yangtze Block:Constraints from the geochronology and geochemistry of the Huashan Group

原花山群分布于紧邻南秦岭的扬子陆块北缘大洪山地区,出露于重要的构造部位,是研究其形成时期扬子陆块构造演化及其与南秦岭关系的重要载体,其物质组成、形成时代和构造属性长期存在争论。本文将原花山群解体为花山构造混杂岩和正常的火山—沉积地层(本文所指花山群)两部分来讨论。笔者重新厘定了花山群的沉积时限,有针对性地对有构造背景争议的花山群进行玄武岩地球化学研究,对有时代争议的混杂岩进行锆石U-Pb年代学研究。年代学、地球化学和沉积学综合研究表明,花山群的沉积时限为ca.830 Ma至ca.800 Ma,形成于与Rodinia超大陆裂解有关的陆内裂谷盆地。花山构造混杂岩带可能不只是晋宁期的缝合带,而是具有多期物质组成、经历了多期构造叠加的复合型缝合带。结合他人成果,我们提出了扬子陆块与南秦岭从新元古代到早古生代的构造演化新模式。

The Huashan Group,distributed in the Dahongshan area on the northern margin of the Yangtze Block that adjoining the South Qinling,is an important carrier to study the tectonic evolution of the Yangtze Block and South Qinling.Long-standing debates persist on the material composition,depositional ages and tectonic nature of the Huashan Group.In this study,the Huashan Group were dismembered into the Huashan tectonic mélange and the Huashan Group that with normal volcanic-sedimentary stratigraphy,and their tectonic natures were discussed separately.Depositional age of the Huashan Group has been redefined here,geochemical features of the basalt from the Huashan Group with the controversy of the tectonic background and zircon U-Pb dating of the tectonic mélange with the controversy of the age are studied.A synthesis of geochronological,geochemical,and sedimentological study results indicate that the Huashan Group was deposited during ca.830–800 Ma and in an intracontinental rift basin which resulted from the breakup of the Rodinia supercontinent.The Huashan tectonic mélange belt may not a product constrained just by the Jinning orogeny,but a complex suture zone with multistage sources and tectonic superposition.Combining the existing study achievements,we propose a new understanding on the tectonic evolution of the Yangtze Block and the South Qinling during the Neoproterozoic to the Early Paleozoic.