Mid-late Neoproterozoic Volcanic And Sedimentary Records In Southeastern South China: Implication For The Supercontinent Reconstruction

The tectonic affinity of mid-late Neoproterozoic magmatism in the South China Block provides a first order constraint on its role in the break-up of Rodinia and the subsequent amalgamation of Gondwana.Strata within the basin range from mid to late Neoproterozoic,recording a complete sedimentary sequence from late Tonian to Ediacaran units,which is an ideal object of tectonic evolution study of the Nanhua Basin.In this thesis,we studied the sedimentary rocks and few volcanic rocks overlying the Cathaysia Block and the Jiangnan Orogen.Based on the study of detrital zircon provenance analyses,origin of the volcanic rock source and intensity of rock chemical weathering of sedimentary successions,we evaluate the provenance source areas,paleogeography,paleosurface climate shifts and tectonic setting of the Nanhua Basin.Mainly achieved the following understanding:1.Several sedimentary successions related to Neoproterozoic glaciations were found in Changting and Sizhoushan areas of the Cathaysia Block,providing new evidence for stratigraphic framework.In combination with our detrital zircon U-Pb dating results and corrected chemical weathering index data(CIAcorr)of time-equivalent Cryogenian and Ediacaran sedimentary successions from South China(including Changting and Sizhoushan areas)and Oman,multiple negative excursions of the corrected values of these chemical index of alteration(CIAcorr)are showed in this study.These excursions reflect climate cooling and correlate with the Sturtian?,?,Marinoan and Gaskiers crychrons.2.Detrital zircon age spectra of the most Neoproterozoic samples overlying the Cathaysia Block display age patterns distinct from the samples that accumulated on the Jiangnan Orogen.Detrital zircons from Cryogenian strata overlying the Jiangnan Orogen(Chang'an and Datangpo formations)and western margin of the Cathaysia Block(Sizhoushan Formation)are largely in the range of 900-700 Ma with scattered Archean and Paleoproterozoic grains.These ages are consistent with derivation from local sources within the South China.In contrast,late Tonian to early Ediacaran sedimentary rocks from western margin and Wuyi-Nanling areas of the Cathaysia Block(Louziba,Nanyan,Huanglian,Xixi,Xiafang,Laohutang,Dajiangbian,Tianzidi,Aiqiling formations and upper part of the Yingyangguan Group)contain abundant late Mesoproterozoic to early Neoproterozoic(1150-870 Ma)and some midNeoproterozoic(800-720)detrital zircons.The timing and character of the ages of detrital zircons,Hf isotope,and MDS plots match the records of North India(Lesser Himalaya,Greater Himalaya)and Indo-Antarctica,indicating that the Cathaysia Block received detritus from North Gondwana.3.Detrital zircon from the Zhengyuanling diamictite provides evidence of bidirectional sources in the western margin of Cathaysia Block at ca.635 Ma,suggesting the closure of paleo-South China Ocean during late-Neoproterozoic.4.Volcanic events with age of ca.770 Ma and 750 Ma have been recognized in the Cathaysia Block.Rarer evidence about Mid-Neoproterozoic magmatic rocks from the Cathaysia Block than that from northwestern part which traditionally been considered plume-associated.U-Pb zircon age data indicate the Louqian Formation rhyolitic tuffs deposited at ca.770 Ma and 750 Ma magmatism of the Yingyangguan Group in the western margin of Cathaysia Block.The zircons within the tuffs display largely positive ?Hf(t)indicate involvement of Neoproterozoic to late Paleoproterozoic crustal materials in their generation.Geochemically,the tuffs are peraluminous and characterized by both A-type and arc affinities which are comparable to contemporaneous(800-750 Ma)felsic rocks on the northwestern side of Nanhua Basin,suggesting the Nanhua Basin lie inboard of a convergent plate margin arc at770 Ma.The geochemical characteristics of mid-Neoproterozoic mafic and intermediate rocks in the Yingyangguan Group suggest crustal assimilation and fractional crystallization play an important role in magma evolution,and they were derived from a source dominated by modified lithospheric mantle sources previously metasomatized by slab-derived fluids.Analyzed samples,as well as contemporaneous mafic rocks in neighboring areas plot in the within-plate basalt field at ca.750 Ma.This evolving tectonic record of magmatic activity suggests a transformation from a convergent plate margin setting on the periphery of Rodinia to one involving within plate extension at ca.750 Ma.5.The paleoposition of the South China during the transitional period from the Rodinia supercontinent to the Gondwana,and the tectonic evolution process from ca.750 – 541 Ma are defined in this thesis.The detrital records of the early Tonian strata in the Cathaysia Block(Dajiangbian Formation and the lower part of Yingyangguan Group)present abundant late Mesoproterozoic to early Neoproterozoic(1200-950 Ma)ages.These ages match the record of East Gondwana,and imply a peripheral setting for South China with respect to the supercontinent and most likely adjacent to India from at least 750 Ma.Thus,we suggest that the Yangtze Block accreted to Cathaysia on the northern margin East Gondwana in the early Neoproterozoic,and the active plate boundaries located along the northern and western margins of South China continue to ca.750 Ma.After this time,the tectonic setting within the South China Block transformed into a stable within-plate siliciclastic depositional environment that continued to receive detritus from East Gondwana until the early Devonian.