Interpreting Tillite Geochemistry In Terms Of Continental Crust Composition And Weathering Evolution

Glacial diamictites are poorly sorted sedimentary aggregates of material derived from the abrasion and erosion of soil and bedrock,followed by transport and deposition of the resulting sediments by melting glaciers and sea ice on land or under seawater,so the chemical weathering in glacial diamictites is limited.In addition,glacial tills are recognized throughout much of the geologic record,going back at least to 2.9 Ga.Pre-Cenozoic glacial deposits occur in four broad geological intervals:Mesoarchean(~2.9Ga),Paleoproterozoic(2.4~2.2Ga),Neoproterozoic(750~550Ma),and Paleozoic(300 and 450Ma).Because of these features,glacial diamictites,with ages ranging from~2900 Ma to 0.01 Ma,occurring broadly across the world,can be used as effective rocks to record the changing composition of the upper continental crust through time.Thus,it's important to carefully evaluate the origin of the chemical weathering signature preserved in glacial diamictites before we use them to trace temporal evolution of the upper continental crust composition and furtherly the information of paleoclimate and paleoenvironment.We have sampled glacial diamictites from all five of these glacial intervals,covering five regions(North America,South America,South China and Greenland).We mainly work on three targets in this study:evaluate the chemical weathering degree preserved in glacial diamictites,interpret the origin of these signatures and finally trace the temporal evolution of the chemical weathering and upper continental crust composition.Li concentrations and isotopic compositions,combined with Pb isotopic compositions,chemical index of alteration(CIA)and relative Sr concentrations are used here to assess the degree of chemical weathering recorded in these glacial deposits and the origin of this signature.The CIA values of most of the diamictites are higher than that of fresh igneous rocks and the?~7Li values of most of the diamictites(-3.9 to+3.5)are lower than those of mantle-derived basalts(+3.7±2,2?),and the low?~7Li values are generally accompanied by high CIA and low Sr/Sr*values.The weathering signature preserved in the ancient glacial diamictites may have derived from pre-depositional,syn-depositional,and/or post-depositional weathering processes.In order to evaluate the origin of the signature and the effectiveness of using glacial diamictites to estimate the upper crust composition,it's important to observe glacial diamictite outcrops,dirll cores and the geochemistry of glacial diamictite profiles.Nearly all of the diamictites studied here were deposited underwater.These submarine deposits are generally conformably overlain by other sedimentary deposits,suggesting that there was little opportunity for subaerial post-depositional weathering to have occurred.Furthermore,if subaerial weathering had occurred and was severe enough,a paleosol would be developed at the upper boundary of the unit.No paleosols were observed in any of the glacial diamictites sampled in outcrop or in drill cores.Profiles through three glacial diamictites with relatively high CIA(a fresh road cut of the Neoproterozoic Nantuo Formation(CIA=62 to 69)and drill cores through the Paleoproterozoic Timeball Hill(CIA=66 to 75)and Duitschland Formations(CIA=84-91))do not show evidence of significant post-depositional weathering.Timeball Hill shows changes with depth that are consistent with post-depositional chemical weathering:CIA increases,and Sr/Sr*decreases towards the surface.However,there is no significant change in?~7Li through this section,suggesting that the Li signature of the Timeball Hill diamictite was mainly inherited from the provenance of the sediments,which was also weathered.Thus,post-depositional weathering appears to have played a minimal role in generating weathering signature in the majority of deposits investigated here.Modern glaciomarine sediments record little weathering(CIA=47,Sr/Sr*=0.7,?~7Li=+1.8)suggesting that the cold temperatures during glaciations limit syn-depositional weathering.High Th/U,reflecting loss of uranium during oxidative weathering,is seen in all Paleozoic and Neoproterozoic diamictites and a few Paleoproterozoic deposits.Pb isotopic systematics suggest that this signature was largely inherited from preexisting crust,although a subset of samples(the Neoproterozoic Konnarock,Paleozoic Dwyka and several of the Paleoproterozoic Duitschland samples)appears to have experienced post-depositional U loss.Thus,the chemical weathering signature observed in ancient glacial diamictites appears to be largely inherited from the upper continental crust(UCC)over which the glaciers traversed.We have studied on the major and trace elements of the glacial diamictites,and found the geochemistry of the samples can be related to the local geology over which the glaciers flowed.The transition metal abundances in Archean and Paleoproterozoic diamictites relative to Neoproterozoic and younger diamictites document a change from upper crust dominated by basalt and komatiite,to one dominated by granitic rocks.The?~7Li averages of upper crust show no obvious changes through time,except that of the Archean crust lower;whereas,Li concentrations of upper crust increase from Archean to the present.The elevated Th/U and pervasive Mo depletion in post-Paleoproterozoic glacial diamictites reflect oxditive weathering signature of the provenance.The intense oxidative weathering signature(Mo depletion and high Th/U)preserved in pre-GOE Duitschland diamictite suggests the presence of measurable atmospheric oxygen prior to the Great Oxidation Event(GOE).Combining our data with Archean shales and other types of post-Paleoproterozoic sedimentary rocks(i.e.,shales,mudstones,etc.),we run student's t test to evaluate the weathering intensity of the old-group(Archean and Paleoproterozoic)rocks and the young-group(post-Paleoproterozoic)rocks.The results show the strength of this weathering signature in old-group rocks is statistically greater than that of young-group rocks.Plotting all the data with time,it appears that post-Paleoproterozoic upper continental crust experienced less intense chemical weathering than Archean and Paleoproterozoic upper continental crust,suggesting a warm,humid and possibly more fluid-acidic climate in Archean.