Petrological And Geochemical Constrains On The Neoproterozoic Diamictite And Cap Carbonate(DCC) In The Helan Mountains,North China

Neoproterozoic era was a tumultuous period of Earth history,which witnessed the formation and breakup of the supercontinent Rodinia and the subsequent amalgamation of Gondwana.Accompanied by the supercontinent dispersal and amalgamation,many aspects of Earth system were changed,such as the severe climate change,the rapid increase of oxygen,and the emergence of complex life.Recently,researches on the Neoproterozoic “snowball earth”,Rodinia supercontinent and cap carbonates have made the Neoproterozoic glacier event a new focus around the international geology realm.In China,the Neoproterozoic successions are dominated by carbonates and mostly distribute at the Tarim,North China and South China blocks.Previous works mainly concentrated in the Quruqtagh area of the Tarim Block and the Three Gorges area of the South China Block.Because of the limited outcropping of the Neoproterozoic strata in the North China Block(where only a suit of mountain glaciers and ocean glacier deposits on the southern margin of Sino-Korean Block),researches on the Neoproterozoic glacier event on the North China Block are limited.Although Neoproterozoic glacial deposits were previously described in the Helan Mountains,the overlying cap carbonate in this region remained poorly known.To better understand the Neoproterozoic glacial events in North China,we investigated five sections along a north-south transect across the Helan Mountains capturing a transition from shelf margin to intrashelf basin.This effort revealed an excellent exposure of Neoproterozoic diamictite and associated carbonate succession(Zhengmuguan Formation)on the western shelf margin of North China.We propose that the Zhengmuguan Formation along the western margin of the NCC,as well as the likely contemporaneous Luoquan Formation along the southern margin of the NCC,are glacial sequences partially influenced by reworking into sediment gravity flows.The comparatively low CIA values of the Zhengmuguan diamictite,as for other Neoproterozoic examples,may reflect a cold and arid depositional environment.In this paper,a ~1.6-m-thick carbonate succession is distinguished from the diamictite sequence on the basis of lithologic characteristics,including the carbonate's very low siliciclastic/conglomerate content and the bedding properties.The carbonate succession is patchy in its distribution and limited to the southern areas of the central Helan Mountains.Attributes of our carbonate succession that are found widely in Neoproterozoic cap carbonates around the world include(1)dramatically contrasting lithologic changes,such as that from the underlying Zhengmuguan diamictite to the carbonate sequence;(2)minor siliciclastic contents,including the lack of conglomerate;(3)the stromatactis-like cavities;(4)thinly laminated carbonate with rhythmic laminae;and(5)the comparable negative ?13C values.The presence of the late Ediacaran fossils Helanoichnus and Shaanxilithes suggest that the Tuerkeng Formation is equivalent to the Dengying Formation of South China and the Zhoujieshan Formation in Northwest China.If correct,the depositional age of the underlying Zhengmuguan Formation can be constrained to the mid-late Ediacaran based mostly on the fossils and may be correlative to the Gaskiers event.Additional work is needed to confirm this age relationship.The cap carbonate consists of a lower thinly laminated dolomite and an upper thickbedded dolomite.Based on our petrographic and geochemical measurements,the lower carbonate contains detrital minerals and exhibits(1)moderately positive correlation between Zr and REE;(2)elevated concentrations of continental inputs(as recorded in Al,Ti,and Zr);(3)sympathetic variations between redox sensitive elements(U,V,and Cr),REE,and Zr;(4)relatively large variations in ?13Corg for low levels of TOC,which could reflect detrital controls;and(5)riverine-or lacustrine-style REE+Y patterns(Fig.14A).These observations collectively suggest that deposition of the lower cap carbonate was strongly influenced by terrestrial inputs derived from enhanced continental weathering of the NCC with only limited mixing with seawater.In contrast,the upper cap carbonate consists of over 95% microcrystalline dolomite and displays(1)paired enrichments in Fe and Mn;(2)enrichments in P,Cu,Pb,Zn,Ni,and V;(3)a systematic decrease in negative ?13Ccarb and increase in TOC;(4)positive Eu anomalies;(5)positive shifts in Y/Ho ratios;and(6)deep marine REE+Y patterns(Fig.14B).Viewed together,these data suggest that the upper cap carbonate was deposited in waters dominated by upwelling of deep seawater strongly influenced by hydrothermal fluids.Our petrographic and geochemical results suggest the Zhengmuguan cap carbonate was initially deposited via glacial meltwater with significant inputs from continental weathering subsequently overprinted by upwelling anoxic deep seawaters along a continental margin.In summary,our data from a poorly known mid-late Ediacaran cap carbonate capture a high-resolution record of shifting climatic and oceanographic conditions of at least regional importance.The uncertainties of these interpretations are high,but the potential importance of these findings justifies the need to publish the data and our conclusions—along with the unavoidable caveats that come with any poorly dated frontier section.The Zhengmuguan Formation in North China may provide evidence for a mid-late Ediacaran glaciation of unconstrained but potentially appreciable magnitude.