Geochemistry of the Palaeo–Mesoproterozoic Tadpatri shales,Cuddapah basin, India: implications on provenance,paleoweathering and paleoredox conditions

The Palaeo–Mesoproterozoic Tadapatri formation of the Cuddapah basin is comprised of clastic sedimentary rocks with minor carbonates and mafic–ultramafic sill bodies. Geochemistry of the shale is used to study the provenance, paleoweathering and paleoredox conditions of this Tadpatri formation in order to better understand the development of the Cuddapah basin during Palaeo–Mesoproterozoic time. The higher CIA(average 74.39), PIA(average 85.94) and CIW(average 87.59) values of the Tadpatri shales suggest intensely weathered sources.Higher Al2_ O_3/TiO_2(average 30.78) and LREE/HREE ratio(average 8.80) with negative europium anomaly indicate derivation of the clastic sediments from a felsic source rock. The geochemical parameters like U, U/Th, Cu/Zn,Ni/Co, V/Cr ratios reveal that the Tadpatri shales are mainly deposited in an oxic condition.

Geochemistry of metasedimentary rocks of the Sonakhan and Mahakoshal greenstone belts,Central India:Implications for paleoweathering,paleogeography and mechanisms of greenstone belt development

A comparative study of the Precambrian Sonakhan(SGB)and Mahakoshal(MGB)greenstones belts of Central India has been undertaken to decipher their provenance,paleoweathering,paleogeography,and tectonics.As compared to the Upper Continental Crust(UCC),the MGB samples are enriched while the SGB samples are depleted in mafic elements indicating the presence of mafic rocks in the source of the MGB.This is complemented by the Ni–Cr diagram.The REE concentrations,LREE fractionated patterns and negative Eu anomalies of the MGB and SGB samples indicate derivation of sediments from a highly fractionated granitic source.Since MGB samples also contain the geochemical signature of mafic rocks,it is,therefore proposed that the MGB clastic load were derived from two sources(mafic+felsic)with arc character.This is attested by Cr and Zr relationships,and LILE enrichment,and HFSE depletion.These features suggest that the SGB developed as autochthonous while the MGB developed as an allochthonous belt.The chemical alteration indices such as chemical index of alteration(CIA),plagioclase index of alteration(PIA),and index of compositional variability for MGB samples indicate that they were dominantly derived as the first cycle(with minor recycled)sediments from bimodal sources(dominantly continental arcs)by intense chemical weathering as compared to the SGB samples,which were derived from felsic sources(dominantly cratonic rocks),and partly by recycling through a low chemical weathering.The CIA and PIA values of the samples reveal a change in the climatic conditions from Late Archean to Late Paleoproterozoic.Such change is interpreted in terms of migration of the Indian plate from high latitudes in the Late Archean to lower latitudes during the Late Paleoproterozoic.This is consistent with the paleomagnetic data that placed India in the configuration of 2.45 Ga Ur and 1.78 Ga Columbia supercontinents.

Provenance and Paleoclimate of the Triassic to Middle Jurassic Adigrat Sandstone,Blue Nile Basin,Central Ethiopia

Mineralogical and geochemical studies have been undertaken on the Triassic to Lower Jurassic Adigrat Sandstone of the Blue Nile Basin of central Ethiopia to infer its source rock type,paleoweathering,and paleoclimatic history.The Adigrat Sandstone occurs at the basal section of the Mesozoic sedimentary formation and unconformably overlays the Neoproterozoic–Paleozoic crystalline rocks,or locally,the Karroo sediments in the northern Blue Nile Basin.A mineralogical study reveals that quartz(Q),feldspars(F),and lithic fragments(L)are the framework grains of the sandstone.On the QFL diagram,the plot of the modal composition of the sandstone mainly falls within the feldspathic arenite and quartzose arenite fields.The geochemical data of the lower section of the sandstone mainly falls within the arkose and subarkose fields,whereas the upper section data falls within the quartzose and sublithic arenite fields.Mineralogical and geochemical weathering indices indicate that the provenances of the Adigrat Sandstone were exposed to pronounced weathering intensity,where the lower part of the sandstone was controlled by arid to semi-arid conditions,whereas the upper section was linked to humid to semi-humid(tropical to subtropical)climatic conditions.Mineralogical and geochemical data also indicate that mafic to intermediate basement rocks were the primary source rocks of the sediment.Perhaps the sediment was assumed to have been reworked by multi-cyclic sedimentary processes.The discriminant function diagram,the REE pattern,La/Th vs.La/Yb,and the Th–Hf–Co plot are consistent.A comparison of provenance studies for the Adigrat Sandstone in the Blue Nile Basin and the Mekele outlier of northern Ethiopia indicates that the sediment of the former is highly sorted,experienced higher weathering intensity,and compositionally derived from mafic to intermediate crystalline rocks.On the other hand,the sediment of the latter is essentially a weathering product of felsic rocks.

Geochemical characteristics of the metapelites from the Xingxingxia group in the Eastern Segment of the Central Tianshan:Implications for the provenance and paleoweathering

Metapelites from the Mesoproterozoic Xingxingxia group in the Eastern Segment of the Central Tianshan,Northwestern China,were analyzed for major and trace elements,includ-ing rare earth elements.Compared with post-Archean shales,the metapelites are enriched in Sr,Hf and Zr.The other elements are similar to the compositions of the Phanerozoic North American Shale Composite and the Post-Archean Australian average Shale(PAAS).The characteristics of Al_(2)O_(3)/TiO_(2),Cr/Zr,Cr/Th,Th/Sc and high Zr concentration suggest that these pelites could be derived from a significant proportion of felsic and a few proportion of participation of mafic mate-rials in nature.These rocks display highly fractionated REE patterns,and average(La/Yb)_(n)=18.6.Several samples have slightly negative Eu anomalies(Eu/Eu^(*)=0.7―0.84)in comparison with PAAS(Eu/Eu^(*)=0.65),indicating contribution from young immature materials.The low K_(2)O/Al_(2)O_(3) values of these metapelites suggest that their source materials could contain minimal alkali feldspar.In the A-CN-K triangular diagram,the sample data array intersects the feldspar join,indicating that the source had a plagioclase:K-feldspar ratio of approximately 5:1,repre-senting weathered products from components that could approximately correspond to granodio-rites and tonalites.The low Chemical Index of Alteration(CIA)and high Index of Compositional Variability(ICV)values suggest low degrees of weathering of the source and low compositional maturity of the sediments,respectively.The geochemical characteristics of the metapelites from the Xingxingxia group also demonstrate that these sediments were deposited in a back-arc background within a continental-arc system.

Geochemistry and sedimentology of sediments in a short fluvial system,NW China:implications to the provenance and tectonic setting

Six outcrop sections in Fenghe River,Northwestern China,were sampled and analyzed.This study aims to determine provenance,tectonic setting,and source-area paleoweathering of the sediments of Fenghe River in combination of lithofacies analysis and bulk-rock geochemical data.The lithofacies in the studied area were classified as gravel,sand,and fine-grained clastic lithofacies,reflecting generally channel fill deposits,channel bar deposits,and over-bank deposits,respectively.The Chemical Index of Alteration(CIA),Plagioclase Index of Alteration(PIA),and Chemical Index of Weathering(CIW)values ranged 50.10–62.29,50.13–66.35,56.52–71.12,respectively,together with element ratios such as Rb/Sr,K/Na,Rb/K,Th/K,Rb/Ti,and Cs/Ti indicate that the source area was under a low to moderate chemical weathering condition probably in cold and semi-arid climates.Moreover,plot of SiO_(2)vs.(Al_(2)O_(3)+K_(2)O+Na_(2)O)suggests that the sediments were deposited in a semi-arid climate.Plots of Cr/Th vs.Th/Sc,TiO_(2)vs.Zr,La/Yb vs.rare earth element(REE),and La-Th-Sc ternary diagrams,along with the lithology,indicate that the sediments in Fenghe River were mainly originated from felsic igneous rocks.Major elements-based discrimination diagrams and Th-Sc-Zr/10 and La-Th-Sc ternary diagram of the samples indicates that the source rocks of Fenghe River developed in a composite active continental margin and continental island arc field.

陆生蜗牛壳体日分辨率氧同位记录定量重建"千年一遇"超级暴雨

A"once-in-a-millennium"super rainstorm battered Zhengzhou,central China,from 07/17/2021 to 07/22/2021(named"7.20"Zhengzhou rainstorm).It killed 398 people and caused billions of dollars in damage.A pressing question is whether rainstorms of this intensity can be effectively documented by geological archives to understand better their historical variabilities beyond the range of meteorological data.Here,four land snail shells were collected from Zhengzhou,and weekly to daily resolved snail shellδ^(18)O records from June to September of 2021 were obtained by gas-source mass spectrometry and secondary ion mass spectrometry.The daily resolved records show a dramatic negative shift between 06/18/2021 and 09/18/2021,which has been attributed to the"7.20"Zhengzhou rainstorm.Moreover,the measured amplitude of this shift is consistent with the theoretical value estimated from the flux balance model and instrumental data for the"7.20"Zhengzhou rainstorm.Our results suggest that the ultra-high resolutionδ^(18)O of land snail shells have the potential to reconstruct local synoptic scale rainstorms quantitatively,and thus fossil snail shells in sedimentary strata can be valuable material for investigating the historical variability of local rainstorms under different climate backgrounds.