| The Tongbai-Dabie Orogen as a collisional belt between the North China plate and the Yangtze plate has been recognized,but its tectonic and evolution process have not been solved: first,as one same orogenic belt,whether the tectonic pattern of Qinling could be extended to the Tongbai-Dabie Orogen,especially whether the Paleozoic Mesozoic MIANLUE belt of South Qinling exists in the Tongbai-Dabie Orogen.The“Huashan Group” in Dahongshan area on the south margin of Tongbai and the "Hong’an Group" on the south margin of Dabie have been porposed as the eastern extension of MIANLUE belt.However,geological evidences inconsistent with the above views have appeared.Second,the Neoproterozoic magmatic and sedimentary records have been found,So what kind of tectonic meaning do they show,whether they represent a important orogeny of Neoproterozoic in the south margin of the Tongbai-Dabie Orogen,and what is the participation of the Yangtze block in the Rodinia supercontinent event.Based on the magmatic and sedimentary records of the Huashan Group and the Hong’an Group,through geological,trace elements and isotope geochemical analysis,this thesis discriminates the tectonics of these two strata,restores the original rocks and determines the geological attributes;Based on systematic isotopic dating,the chronological framework in the south margin of the Tongbai-Dabie orogen is preliminarily established,and the Neoproterozoic orogenic process is discussed.The main progress as follows:1.The whole sedimentary section of the Huashan Group shows an orderly process,with clastic sources mixed with sand debris from the Yangtze block and volcanic debris from island-arc,indicating that the volcanic rocks and sedimentary rocks are associated products in an ordered stratum rather than a mélange.The basalts of the Huashan Group has geochemical characteristics between MORB and IAT,which is actually BABB.The formation age of basalts are 823.9±5.5 Ma and 829.9±6.9 Ma,with 1637 Ma and 2670 Ma captured zircon ages,indicating that there was a continental basement in the bottom of the volcanic channel,but not a mid ocean ridge.Therefore,it is considered that the Huashan Group are volcanic-sedimentary rock series formed in a back arc basin,and the“Huashan ophiolite” is not supportedThe Sanligang intrusive rocks include granodiorite and gabbro which form in the island arc.The zircon U-Pb dating are 878.7±3.8 Ma and 899.9±2.4 Ma.Therefore,the Sanligang intrusive rocks and the Huashan Group form a Neoproterozoic island arc and back arc basin system in the Dahongshan area.In the clastic rocks of the Liantuo Formation which represents the continental basin deposits,overlying the Huashan Group,the lower limits are 771.7±6.6 Ma and774.1±9.4 Ma,as well as the age of 724 Ma is obtained from the sericite quartzite of the Wudang Group,which represents stable continental margin,indicating that the Neoproterozoic orogeny of the Dahongshan area has ended at less than 770 Ma,and has changed to a stable tectonic at 725 Ma.Thus,the evolution sequence of subduction orogeny,opening-closing of the back arc basin and the end of orogeny in the Dahongshan area is from 899 Ma to 770 Ma.2.The felsic gneiss dominated in the Hongan Group of Western Dabie are actually parametamorphic rocks,combined with the occurrence of strata bound minerals,it is considered that the Hong’an Group is an orderly strata,which denies the previous view of “disordered stratum” or the protolithis granite.Through the lower limited age of689.8±4.8 Ma obtained from the paragneiss,combined with the fact that it covers the Sinian system,it is speculated that the main part of the Hong’an Group formed in later Sinian,and the volcanic rocks in the upper layer may formed in early Paleozoic.The phosphorus clastic rocks at the bottom of the Hong’an Group are near-source deposits.Combined with the zircons come from Neoproterozoic magmatic rocks and ancient continental clastic source areas,it is considered that the sedimentary rocks form in an active continental margin transforming to passive continental margin.The basic rocks in the Hong’an Group are mainly tholeiitic basalts.The magma comes from the homologous depleted mantle.The volcanic rocks from bottom to top show an increased degree of partial melting which generally formed in intracontinental cracking.Therefore,the Hong’an Group is an orderly strata between Sinian convergence and early Paleozoic rifting activities in the northern margin of the Yangtze block,and the “Paleozoic ophiolite” in the Hong’an Group has not been confirmed by this study.3.The Neoproterozoic active continental margin with a mélange belt composed of Neoproterozoic intrusive rocks,volcanic rocks,sedimentary rocks and Neoarchean-Paleoproterozoic granites,basic rocks and sedimentary rocks is identified in the Hong’an Group in Qingshuihe area of Eastern Dabie.The Neoproterozoic intrusive rocks formed in the island arc or active continental margin,with 830~805 Ma zircon U-Pb ages.They have high negative age εNd(t)values which coincide with the Neoarchean granites in the εNd(t)-Age diagram.The two-stage model ages and 2549 Ma of captured zircon ages are also consistent with the formation ages of the Neoarchean samples,indicating that the Neoproterozoic rocks comes from the remelting of the Neoarchean continental crust in an active continental margin.Together with the Dahongshan island arc(899 Ma ~ 878 Ma)and back arc basin(829 Ma ~ 823 Ma)determined earlier,it indicates the orogeny in the late Neoproterozoic on the southern margin of the Tongbai Dabie Orogen,which lags behind the evolution of the Rodinia supercontinent.4.The~2.5 Ga and~2.0 Ga high-K granites were found in the Dabie Orogen for the first time.The ages determined by zircon U-Pb method are 2443~2463 Ma and1995 Ma~2048 Ma.The~2.5 Ga granite has low negative εNd(t)value and the 2.87 Ga model age slightly earlier than the formation age,while the 2.0 Ga granites have negative medium εNd(t)value and the 3.03~3.24 Ga model ages which are much older than the formation ages.Compared with the basic rocks in the same period,it is considered that the granites formed by partial melting of the pre-existing crust caused by mantle upwelling.Therefore,there was a relatively mature continental crust in the northern margin of the Yangtze block in the Neoarchean Paleoproterozoic,which is basically equivalent to the core of this block,indicating that the Yangtze block in this period has a considerable scale,and its genetic mechanism is mainly vertical proliferation model of crust mantle differentiation and remelting in the crust.At this time,it may not have a typical plate structure. |
