Tectonic Evolution Of The Meso-Neoproterozoic Sedimentary Basins And Its Geological Significance On The Southwestern Margin Of The Yangtze Block

The South China Block has been considered as an important piece of puzzle in the Rodinia configuration and its paleoposition within the supercontinent is of great interest to a broad community.However,the paleoposition of the South China Block in Rodinia is still under debated due to controversial interpretations on the tectonic and geodynamic evolution of the South China Block from the late Mesoproterozoic to the Neoproterozoic.The formation and evolution of sedimentary basins are closely related to regional tectonic signatures,and as such they can be used to invert the basin-mountain coupling process,which in turn provides key constraints on supercontinent assembly and breakup.Sedimentary records from the late Mesoproterozoic to the early Neoproterozoic are widely exposed on the southwestern margin of the Yangtze Block of South China.Deciphering the formation and evolution of these sedimentary basins is an important window for understanding the tectonic evolution of the Yangtze Block during this period,which in turn provides a key constraint for reconstructing the paleoposition of the Yangtze Block and thus the South China within the Rodinia supercontinent.In this thesis,systematic field investigations,petrographical,sedimentological and zircon U-Pb-Hf isotopic analyses as well as whole-rock geochemistry of samples from the Kunyang,Julin and Huili groups are carried out to investigate the depositional age,sediment provenance and tectonic evolution of the sedimentary basins on the southwestern margin of the Yangtze Block.These new data,in combination with information from literature,allow the comparison between the contemporaneous sedimentary basins on the Yangtze Block and those on other blocks,to trace and reconstruct the paleoposition of the Yangtze Block in the Rodinia supercontinent.The main conclusions of this study are as follows.(1)The zircon U-Pb age data define the depositional age of the three units.The Huili Group was deposited from 1176 Ma to 958 Ma.The formation age of the Kunyang Group is constrained at between 1152 Ma and 1000 Ma,whereas the deposition of the Meidang Formation,traditionally considered the uppermost part of the Kunyang Group,continued until 866 Ma.By combining field observations,provenance variations and tectonic setting analysis in this study and in previous works,it is suggested that there may be a depositional hiatus between the Meidang Formation and the underlying Dalongkou Formation.This reveals that the Meidang Formation represents a depositional event different from the main part of the Kunyang Group.The Julin Group was deposited between 1167 Ma and 982 Ma.The Haizishao Formation,traditionally considered the uppermost part of the Julin Group,was formed after 982 Ma.The occurrence of basal conglomerate,variation on detrital components and tectonic setting suggest that the Haizishao Formation is not part of the Julin Group and may represent a individual depositional event.(2)Paleocurrent data,U-Pb-Hf isotopes of detrital zircons and whole rock geochemistry are employed to trace the source rocks and their geochemical nature for the sediments of the Huili,Julin and Kunyang groups.Two types of sources,namely intrinsic and extrinsic sources are identified.The ~1600 Ma detrital materials were largely derived from extrinsic sources,whereas ~2500 Ma,2170-2020 Ma and 1040-960 Ma detritus were mainly supplied by intrinsic sources.The extrinsic sources are mainly from the Gawler Craton in Australia,the Transantarctic Mountains in East Antarctica,and the Ongole domain in the Eastern Dharwar Craton of India.The internal sources are mainly from the Yudongzi(~2500 Ma),Douling(~2500 Ma)and Kongling(2170-2020 Ma)complexes in the northern margin of the Yangtze Block,and the early Neoproterozoic magmatic rocks(1040-960 Ma)in the southwestern margin of the Yangtze Block.In generally,the source rocks of detritus in the three strata are mainly from felsic rocks,while mafic and intermediate rocks may contribute significantly to the sediments of the Tianbaoshan Formation.(3)The lithological compositions,sedimentary structures,detrital zircon age distributions and whole rock elemental compositions,in combination with the geodynamics of associated magmatic activity,are used to reveal the tectonic evolution of the Meso-to Neoproterozoic sedimentary basins in the southwestern margin of the Yangtze Block.It can be concluded that the Huili Basin was initially deposited in a continental rift setting and was subsequently transformed into an active continental margin setting when the Tianbaoshan Formation was deposited.The Julin and Kunyang basins were initially deposited in a continental rift setting,then were gradually transformed into a passive continental margin setting,and then into an active continental margin setting when the Haizishao and Meidang formations were deposited.(4)The Huili Group differs from the Julin and Kunyang groups,in terms of depositional age,lithological assemblage,detrital zircon age distribution and tectonic setting.These differences suggest that the Huili Group may deposit in microcontinent different from that of the Julin and Kunyang groups.Based on the tectonic settings of the three sedimentary basins in different periods and the multi-stage rifting in the southern Yangtze Block at 1.8-1.0 Ga,it is inferred that the southwestern margin of the Yangtze Block may be composed of several microcontinents in the late Mesoproterozoic.These microcontinents were gradually subducted northward at the end of the Mesoproterozoic and converged into a united terrane in the early Neoproterozoic.A combination of the extrinsic source locations of the sedimentary basins on the southwestern margin of the Yangtze Block and the comparison of contemporaneous sedimentary strata from different continents reveals that the Yangtze Block was located in the west of Australia and East Antarctica,close to north India but was far from Laurentia during the late Mesoproterozoic to early Neoproterozoic.At this time,the Yangtze Block was close to or had merged with the Cathaysia Block to form the South China Block.According to the paleoposition of the Yangtze Block,it was inferred that the South China Block was at an external position within the Rodinia supercontinent.