Mid-neoproterozoic Characteristics Of Sedimentary Basin In The Dahongshan Area Of The Northern Yangtze Block,South China,and Precambrian Crustal Evolution

The Dahongshan area is located at Jingshan,Zhongxiang,and Suizhou in Hubei Province.It's tectonic location is the eastern part of the northern Yangtze Block,which is an important link between the Qinling orogenic belt and the interior of the Yangtze Block.The Huashan Group is a key mid-Neoproterozoic stratigraphic unit in the Yangtze Block,South China,which is composed of low-grade metamorphic terrigenous clastic-volcanic rock sequences;it is the initial sedimentary filling of the sedimentary basin in the Yangtze Block above the“Jinning-Sibao orogen”angular unconformity and it formed a product of the Rodinia supercontinent cycle.However,the depositional age,provenance,characteristics of sedimentary basin of the Huashan Group are still debated.Here,based on the field geological survey,we research for systematic sedimentological study,U-Pb geochronology and Lu-Hf isotope analysis of the Huashan Group.In addition,combined with the whole-rock geochemistry,U-Pb dating and Lu-Hf isotope studies from the mid-Paleoproterozoic granites in Zhongxiang Terrane of the northern Yangtze Block and the Mid-Neoproterozoic granites in Jinkouhe area from the western Yangtze Block.We obtained the following results:?1?Zircon U–Pb geochronology yields a ²⁰⁶Pb/²⁰⁸U age of 836±4 Ma?MSWD=0.74,n=29?for the tuff from the second section of the Hongshansi Formation near the bottom of the Huashan Group.The six youngest grains?823–809 Ma?f form a weighted mean ²⁰⁶Pb/²³⁸U age of 815±5 Ma?MSWD=0.65,n=6?from the upper part of the Liufangzui Formation.Combined with previous research results,the Huashan Group was deposited from ca.830 to 800 Ma from bottom to top.?2?According to the sedimentology characteristics,the sedimentary facies of the Huashan Group are interpreted to record a transition from an alluvial fan,to a fan delta,to semideep to deep lacustrine facies?from base to top?.?3?According to depositional time and sedimentary filling sequence of the Huashan Group and correlations of mid-Neoproterozoic strata in the Yangtze Block,the Huashan Group was correlated the Cangshuipu Formation and Madiyi Formation of the Banxi Group,the Jialu Formation and Wuye Formation of the Xiajiang Group,the Baizhu Formation and the lower part of the Hetong Formation of the Danzhou Group,the Gegongzhen Formation in the southeastern Yangzte Block,and the Luliang Formation in the western Yangzte Block,and the Sunjiahe Formation of the Xixiang Group and the lower part of the Tiechuanshan Formation in the northern Yangzte Block;which show the initial volcanic-sedimentary sequence of sedimentary basin above the“Jinning-Sibao orogen”angular unconformity.?4?The detrital zircons of the Huashan Group are mainly from magmatic rocks related to an island arc/orogenic environment of continental crust.And detrital zircons ages yield four major peaks at ca.2670 Ma,ca.2040 Ma,ca.940 Ma and ca.840 Ma and five subordinate peaks at?ca.2850 Ma,ca.2490 Ma,ca.1840 Ma,ca.1600 Ma and ca.1240 Ma,which suggest the provenance of the Huashan Group includes Archean to Paleoproterozoic basement,reworked underlying Dagushi Group sedimentary rocks,early Neoproterozoic magmatic rocks and mid–Neoproterozoic syn–sedimentary magmatic rocks.?5?Based on the characteristics by the differences between the crystallisation and depositional ages of the detrital zircons,sedimentology,and zircon Hf isotope ratios of the tuff from the Huashan Group,the Huashan Group formed in a continental rift basin and consistent with early sedimentation in continental rift basins.And the depositional age and characteristics of sedimentary basin show that the rifting was underway by ca.830 Ma in South China;this time is consistent with the initial breakup time?ca.830 Ma?of Adelaidean Rift in southeastern Australia.In addition,the Huashan Group was deposited during the first and the second episodes of a Neoproterozoic superplume associated with the breakup of the Rodinia supercontinent.?6?Zircon Lu-Hf isotope can provide important information for Precambrian crustal evolution.We report the Paleoproterozoic granite from the Zhongxiang area of the northern Yangtze Block to further research the crustal evolution in Yangtze Block.The research show that zircon U–Pb dating yields crystallization ages of 2039±36Ma and 1950±16 Ma for two monzogranite samples,respectively.Both samples show S-type granite affinities.They have similar zircon Hf isotope ratios with?_Hf?t?values of-17.11 to-9.69 and T_DM2 ages of ca.3.67–3.22 Ga,suggesting that the monzogranites could have been generated by partial melting of Archean crustal materials.And we believe that the Yangtze Block is an important part of Columbia supercontinent.According to the zircon Lu-Hf isotope comprehensive analysis show that although the crust keeps crustal growth occurred in the Archean to Neoproterozoic,a larger proportion occurred in the late-Mesoproterozoic and early to middle Neoproterozoic in the Yangtze Block,South China.And reworking of ancient crust in the Yangtze Block occurred from the Archean to the Neoproterozoic.The crustal evolution of the Kenorland supercontinent and the Columbia supercontinent caused mainly re-melting the old basement rocks and the addition of minor juvenile material to the crust,whereas the Rodinia supercontinent cycle mainly caused mixture of re-melting the old basement rocks and more juvenile material addition.