Neoproterozoic Glacial Deposits And Mafic Magmatism In The The Yangtze Block

South China is composed of the Yangtze Block to the northwest and the Cathaysia Block to the southeast.It has been widely accepted that these two blocks collided in the Neoproterozoic,resulting in the formation of the Jiangnan Orogen.However,some key issues have been subject to much debate,i.e.,the exact timing of collision,the position of the suture zone,Paleogeographic reconstructions of South China,one or two Neoproterozoic glacial epochs,Neoproteorozoic geodynamics,tectonic evolution of South China and the nature of collision.The Huangling anticline and eastern Jiangnan Orogen are located in the northern part and southeastern margin of the Yangtze Block,respectively.This study focuses on Neoproterozoic glacial deposits and mafic dykes in both areas and the Shexian complex which is in close geographic relation with the Fuchuan ophiolite.Detrital zircon U-Pb ages of three sedimentary samples from the Liantuo Formation,two sedimentary samples from the Nantuo Formation and one sample from the Silurian Shamao Formation in the Huangling anticline reveal three major age groups of 740-890 Ma,1.9-2.1 Ga and 2.4-2.55 Ga.Although Lu-Hf isotope analysis suggests both juvenile crustal growth and reworking of old crust for all the age populations,juvenile input mainly occurred in the Archean and Neoproterozoic time,whereas Paleoproterozoic is a period dominated by recycling of pre-existed crust.The predominant Neoproterozoic detritus is mainly derived from coeval igneous rocks distributed along the western and northwestern margin of the Yangtze Block.The pre-Neoproterozoic sediments were likely eroded from the Kongling terrane and unexposed Yangtze basement.The similar age patterns of the sedimentary rocks in this study imply only a slight variation in the source regions from the Neoproterozoic to Silurian.The Lantian section in southern Anhui Province at the eastern end of the Jiangnan Orogen comprises the Xikou Group below an angular unconformity,and the Xiuning Formation and overlying glacial Leigongwu Formation above the unconformity.The Leigongwu Formation is again overlain by cap carbonates from the lower Lantian Formation.Detrital zircons from eight samples in all three units are dominated by 2.6-2.4 Ga,2.1-1.9 Ga and 960-740 Ma age populations suggesting erosion from similar source regions,and an increasing proportion of older grains up through the sequence reflects a change in depositional environment from syn-collisional to extensional.Pre-Neoproterozoic zircon grains were derived from the Yangtze Block basement,much of which is now concealed by younger rocks,while the Neoproterozoic population was eroded from local igneous rocks in the Jiangnan Orogen.Zircon Hf isotope compositions indicate that 2.6-2.4 Ga source rocks were a mix of juvenile and reworked crust,while 2.1-1.9 Ga source rocks were dominated by reworked crust.Neoproterozoic sources show a switch from 960-860 Ma juvenile crust to 860-740 Ma juvenile and reworked crust,reflecting a transition in the Jiangnan Orogen from subduction to collision and extension.The youngest detrital grains in the upper Xikou Group and overlying Xiuning Formation indicate deposition after ca.810 Ma and ca.732 Ma,respectively,correlating closely with comparable sequences elsewhere in the Yangtze Block.This demonstrates that the unconformity in South Anhui is part of a regional erosion surface that formed more or less synchronously throughout the orogen at 830-800 Ma,consistent with it dating the end of collision between the Yangtze and Cathaysia blocks.The Leigongwu diamictite in the Lantian section is a single Marinoan-age glacial unit deposited after 649±13 Ma.It follows that a carbonate layer in the middle of the Leigongwu diamictite in the Lantian section most likely reflects later faulting,rather than two separate glacial sequences,although we cannot rule out the presence of both Sturtian and Marinoan diamictite elsewhere in South Anhui.Mafic dykes in the northern Huangling anticline were emplaced at ca.800 Ma,indicating large percentages of mafic dykes intruding the Kongling TTG rocks are Neoproterozoic in age.Zircon Hf isotope data of the mafic dykes vary with different locations but all show negative ?Hf(t)values.Two groups of mafic dykes can be identified due to their distinct major and trace element characteristics,both of which yield highly fractionated REE patterns,indicative of derivation from partial melting of a heterogeneous lithospheric mantle source beneath a thickened crust.They were produced at relatively low temperatures by fluid-present partial melting,thus precluding their direct relationship with a mantle plume,as well as the model in which South China is located in the center of Rodinia supercontinent.The tonalite located in the southern Huangling anticline was formed at ca.815 Ma.The tonalite shows variable?Hf(t)values of-19.8 to+6.9,suggesting input of juvenile material during the magma emplacement.The intrusion of Neoproterozoic mafic dykes and tonalite was likely caused by subduction event along the western Yangtze Block which may have provided juvenile material as well as slab-derived fluids.The Guanyinshan maf c dykes from South Anhui in eastern Jiangnan Orogen were emplaced at ca.776 Ma.The dykes display mainly tholeiitic feature and are characterized by negative Nb,Ta and Ti anomalies,positive Pb,Hf and LILE anomalies and superchondritic Nb/Ta ratios(19.6-24.5).The arc signatures were inherited from mantle metasomatism by subduction-related fluid/melt.Modelling of non-modal batch melting suggests that these NE trending mafic dykes were formed by 1%to 9%partial melting of a rutile(or high-Mg amphibole)bearing spinel lherzolite source,pointing to a relatively thin crust in the eastern Jiangnan Orogen at that time.These unmetamorphosed mafic dykes were formed in a within-plate tectonic setting during the second rift-related episode in South China(780-750 Ma),possibly in a post-collisional setting.Combined U-Pb-Hf isotopic and REE analysis of zircon from gabbroic and dioritic samples of the Shexian complex,located 10 km southwest of the main Fuchuan ophiolite body,indicate that the complex crystallized at ca.870-860 Ma with a large variation of zircon ?(t)values from-480 to+13.30.Whole-rock geochemistry reveals that the magma mainly experienced fractionation of olivine,clinopyroxene and plagioclase and was partly affected by crustal contamination,which resulted in elevated Th/Nb,Th/La and La/Sm ratios,as well as the scattered ?Hf(t)values.The most mafic and least contaminated sample shows MORB affinity and was probably formed by partial melting of a depleted subduction-metasomatized mantle wedge.Other samples exhibit arc-like signatures and were probably modified by both melt-and fluid-related subduction metasomatism.The emplacement of the Shexian complex corresponds to the time that subduction switched from a ca.1000-880 Ma intra-oceanic island arc to a 870-830 Ma continental arc along the southeastern Yangtze Block.The sequence of igneous rocks associated with this continental arc resemble those preserved in forearc Tethyan ophiolites,with magma evolving from ca.870-860 Ma MORB to ca.860-850 Ma arc tholeiite and ca.830 Ma boninite.Arc magmatism concluded with the final assembly of the Yangtze and Cathaysia blocks at 830-800 Ma.Non-thickened crust was formed during the final collision,which is a kind of soft collision.