扬子地块东缘新元古代造山后构造转化:瓮安穹隆构造岩石与年代学限定

Neoproterozoic Post-orogenic Tectonic Transformation in Eastern Margin of the Yangtze Block: Petrologic and Geochronologic Constraints of the Weng’an Dome

扬子地块Rodinian造山后向伸展作用转化的过程与时间,一直是扬子地块新元古代构造演化的重要科学问题。扬子地块东南瓮安穹隆保存了新元古界板溪群至南华系的完整地层层序与角度不整合接触关系,是回答和理解新元古代造山后构造转化问题的理想区域。对新元古代浅变质板溪群沉积岩和南华纪南沱组沉积层序进行了调查,并采取系列样品进行碎屑锆石U-Pb定年分析和岩石地球化学分析。野外调查表明,南华纪南沱组角度不整合于板溪群之上;年代学分析结果表明,板溪群的沉积时代应在772 Ma之前,而南华纪南沱组沉积时代晚于691 Ma。瓮安穹隆新元古代板溪群沉积岩Si O2含量中等,Si O2/Al2O3平均值为5.53,K2O/Na2O平均值为7.14,TFe O+MgO平均值为3.47%。板溪群物源可能来自上地壳,原岩以长英质物源为主,为活动大陆边缘构造背景,其物源主要来自扬子地块西缘。南沱组沉积岩样品Si O2含量中等,Si O2/Al2O3平均值为4.69,K2O/Na2O平均值为20.41,TFe O+MgO含量平均值为6.64%;碎屑沉积岩稀土元素球粒陨石标准化曲线与上陆壳相似,以轻稀土富集、显著的铕负异常和重稀土平坦为特征。南沱组沉积岩物源可能来自上地壳,原岩以长英质物源为主,有少量中性岩混入,具有裂谷背景特征。综上所述,认为瓮安穹隆板溪群-南沱组地层层序代表了从造山作用向造山后伸展裂谷转化过程,其中板溪群可能与碰撞造山作用对应,主体约在772 Ma结束,而造山后裂谷的形成在691 Ma之后,因此,从造山作用向造山后伸展转化的时间大约为772~691 Ma。

The timing and processes of the transformation from the Rodinian post-orogenic tectonics to extension in the Yangtze block have been an important question in the Neoproterozoic Yangtze tectonic evolution.The Weng’an Dome in the southeastern Yangtze block has preserved the complete stratigraphic sequence,and the unconformity between the Neoproterozoic Banxi Group and the Nantuo Formation in the Nanhua period,and is thus an ideal target to investigate the Neoproterozoic post-orogenic tectonic transformation.We study the sedimentary sequence of the low-grade metamorphosed sedimentary rocks of the Neoproterozoic Banxi Group and Nantuo Formation(Nanhua period),via detrital zircon U-Pb dating and whole-rock geochemical analyses.Field geology shows that the Nantuo Formation(Nanhua period)lies unconformably on the Banxi Group.Age dating results indicate the maximum depositional ages for the Banxi Group and Nantuo Formation are 772 Ma and 691 Ma,respectively.Neoproterozoic rocks in the Weng’an Dome(Banxi Group)have intermediate Si O2 contents,with the average values of SiO2/Al2 O3=5.53,K2 O/Na2 O=7.14 and(TFe O+MgO)=3.47%.The Neoproterozoic Banxi Group sedimentary rocks may have sourced from the feldspar-quartz-dominated upper crust.The Weng’an Dome may have formed in an active continental margin,and the clastic material may have derived from the western margin of the Neoproterozoic Yangtze block.The Nantuo Formation sedimentary rocks have intermediate SiO2 contents,with average values of SiO2/Al2 O3(4.69),K2 O/Na2 O(20.41)and(TFeO+Mg O)(6.64%).The REE chondrite-normalized patterns of the clastic rocks resemble those of the upper continental crust,as characterized by LREE-enrichment,flat HREE patterns and negative Eu anomalies.The Nantuo Formation sediments may have come from the feldspar-quartz-dominated(with minor intermediate rocks)upper crust.The Weng’an Dome was likely formed in a rifting-related tectonic setting.In summary,we suggest that the stratigraphic transition from the Banxi Group to the Nantuo Formation represents the tectonic transformation from syn-collisional orogeny(till^772 Ma)to post-orogenic rifting(after^691 Ma).Therefore,the synto post-orogenic tectonic transition occurred at about 772-691 Ma.