High Chemical Index Of Alteration Research Articles

Palaeoclimatic Inferences from Clayey-Iron Palaeosols: A Weathering Event Recorded in the Middle–Upper Jurassic Unconformity (South Iberian Palaeomargin, Western Tethys)

The study of iron crusts containing iron-coated grains from different sections of the Prebetic (SE Iberia) and the overlying marine sedimentary rocks also containing iron-coated grains in the Prebetic and the Iberian Range (NE Iberia) allowed us to determine the palaeoclimatic and palaeoenvironmental conditions under which they originated. The iron crusts are mainly composed of clay minerals (kaolinite and illitic phases) and/or goethite and hematite. The kaolinite texture indicates that it is authigenic, whereas the illitic phases are probably detrital. The mineralogy and texture of the iron crusts allow us to classify them as plinthitic palaeosols. The iron-coated grains consisting of a nucleus and a cortex, both composed of a mixture of kaolinite, goethite, and hematite, originated in situ during the plinthite development. Reworking processes caused the fragmentation and incorporation of the iron-coated grains into the overlying ferruginous oolithic limestones and terrigenous-carbonated breccia. New marine iron-coated grains formed later in the ferruginous oolithic limestones. The high Chemical Index of Alteration and Chemical Index of Weathering values and the geochemical ratios (Ba/Sr, Rb/Sr, Sr/Cu, Ga/Rb) from iron deposits reflect intense weathering under warm and humid conditions in the South Iberian Palaeomargin during the Callovian–Oxfordian, which may also take place in the East Iberian Palaeomargin (Iberian Range).

Geochemistry of Pelites at Chengmen Area: Implications for Sediments Resource and Tectonic Environment.

The pelite samples were collected from drillholes from the Chengmen area of the Fuzhou Basin of Fujian, China to reveal the sediments resource and tectonic environment of these pelites via an analysis of its lithology and geochemistry. The rare earth element distribution pattern of the pelites exhibited a high degree of fractionation between light and heavy rare earth elements, moderate negative Eu anomalies, and positive Ce anomalies, which are consistent with the rare earth element distribution pattern of Minjiang sediments and Fujian soils. Moreover, the variations in trace elements are also generally consistent, indicating a common provenance. The extremely high chemical index of alteration (CIA) and index of chemical variability (ICV) values of the pelites suggest that the source area experienced intense weathering, indicating a subtropical hot and humid climate environment in the source area and the Fuzhou Basin at that time. The source rock attribute discrimination diagrams show that the main source of pelites is felsic igneous rocks. The ratios of REDOX parameters show that during the sedimentary period, the water body in the study area was predominantly in an oxidizing environment. Furthermore, the tectonic background diagrams reveal that the source area underwent geological tectonic evolution processes of active and passive continental margins, marking the transition from the continental margin above the subduction of the ancient Pacific plate to the continental margin extension after subduction cessation.

Lower Permian Gondwana sequence of Rajhara (Daltonganj Coalfield), Damodar Basin, India: floristic and geochemical records and their implications on marine ingressions and depositional environment.

Early Permian floral diversity and palaeodepositional environment of the Lower Permian Rajhara sequence of Damodar Basin have been studied based on mega-, microfossil and geochemical proxies. Even though Gondwana sediments are generally considered as fluvio-lacustrine deposits, recent studies indicate marine inundations with patchy records. Here in the present study, an attempt has been made to address the changeover from fluviatile to shallow marine conditions and also to address the palaeodepositional aspects. Luxuriant vegetation during deposition of the Lower Barakar Formation generated thick coal seams. The macroplant fossil assemblage shows Glossopteridales, Cordaitales and Equisetales comprising one palynoassemblage with the dominance of bisaccate pollen grains having glossopterid affinities. However, Lycopsids are absent in the megafloral record and are represented in megaspore assemblage. The present floral assemblage led to envisage the presence of dense forest with swampy conditions and prevalence of warm and humid climate during the deposition of Barakar sediments. Correlation with the coeval Indian assemblages and those from other Gondwanan continents also supports an Artinskian age and reveals a stronger affinity with flora of Africa than that of South America. Biomarker analysis reveals low pristane/phytane values (0.30-0.84), noticeable absence of hopanoid triterpenoids and long-chain n-alkanes that is attributed to the obliteration of organic compounds and subsequent alteration of composition due to thermal effect. The high chemical index of alteration, A-CN-K plot and PIA also suggest severe denudation under a warm/humid climate. The V/Al2O3 and P2O5/Al2O3 indicated freshwater-near-shore conditions. However, signature of possible marine influence is identified from Th/U and Sr/Ba ratios resulted from the eustatic fluctuations during Permian.

Major, trace element, and Nd isotopic compositions of banded iron formation and shales from the Sirsi shelf, Dharwar Craton, India: Implications for paleo-seawater chemistry, post-depositional alteration, and provenance

We report geochemical and Nd isotopic data for ca. 2.5 Ga old Banded Iron Formation (BIF) and associated clastic rocks which include red- and purple-coloured shales, mudstone, and sandstone from the Sirsi shelf of the Dharwar Craton. Based on lanthanide and non-lanthanide (yttrium) rare earth element (REE) concentration patterns, the Sirsi BIF samples are sub-divided into two groups. The group-1 BIF samples show low shale-normalized (SN) ratios of (La/Yb)SN (0.37–0.53) and (Pr/Yb)SN (0.29–0.34), and superchondritic Y/Ho (41–52); these samples also show insignificant negative Ce anomaly ((Ce/Ce*)SN = 0.83–1.22) compared to modern seawater and seawater-derived precipitates. In contrast, the group-2 BIF samples show strong light REE (LREE) enrichment with high (La/Yb)SN (1.73–18.48) and (Pr/Yb)SN (1.50–7.58), negative Ce anomaly ((Ce/Ce*)SN = 0.02–0.16), and mostly subchondritic Y/Ho (19–39). The group-1 BIF displays modern-day seawater-like REE patterns and their initial εNd at 2.5 Ga overlaps with contemporaneous global BIF, suggesting their precipitation from Archean seawater. The REE compositions of the group-2 Sirsi BIF cannot be explained by contribution of detritus and are best explained by alteration by a LREE-enriched and Ce-depleted fluid. The Sm–Nd isotopic compositions suggest that this alteration event took place approximately 0.5 Ga ago. The timing of the alteration event coincides with the Pan-African orogeny, which had regionally affected the Greater Dharwar Craton. Among the associated clastic rocks, red and purple shales show group-2-BIF-like REE signatures with negative Ce anomaly, and display high Chemical Index of Alteration (CIA) values (78–98), which suggest high degree of chemical alteration. The enrichment factors (calculated as [(Xelement/XTi)sample/(Xelement/XTi)reference-1] relative to the Archean Upper Continental Crust) of La, Pr, and Ce, in the Sirsi red shales further confirm post-depositional modifications by a LREE-enriched and Ce-depleted fluid, and indicate that negative Ce anomaly in ancient sedimentary rocks could be generated by post-depositional fluid overprint. Geochemical compositions of the Sirsi clastic rocks suggest a felsic provenance, while Nd isotopic compositions of selected shale samples indicate that these sediments were most likely derived from granitoids in the Goa-Dharwar Sector.

Relationships between geochemical properties and microbial nutrient acquisition in tropical forest and cropland soils

Land-use conversion can profoundly modify geochemical and microbial properties that drive organic carbon (C) dynamics in tropical soils. However, it is unclear how microbes adjust nutrient acquisition strategies to changing geochemical properties across deeply weathered soils developed from geochemically contrasting parent material. Here we show that along a geochemical gradient in forest and cropland, a proxy variable, the chemical index of alteration (CIA), is sufficiently sensitive to reflect geochemical controls on microbial nutrient acquisition in tropical soils. We found that the CIA negatively correlates with rock-derived nutrient concentrations and pH, but positively with clay content, iron oxides, as well as total iron, aluminum, and manganese concentrations. Thus, using the CIA, which integrates effects of soil fertility and C stabilization by minerals, reduced the complexity of relating microbial nutrient acquisition to geochemical soil properties. Effects of the CIA on microbial C and phosphorus (P) acquisition were stronger in cropland than in forest soil. Microbial nutrient acquisition strategy shifted with increasing CIA from predominating C demand to P. Changes in soil properties at higher CIA (less rich in rock-derived nutrients) were favorable to fungi, which pursue a conservative nutrient allocation strategy to cope with acidic and nutrient-depleted soil conditions, reducing C loss through respiration. In low CIA soils (more rock-derived nutrients), bacteria-dominated communities increasingly invested in C acquisition at the expense of the community biomass, with subsequent greater C loss through respiration. We conclude that microbial communities adapt nutrient acquisition strategies to changing geochemical soil properties in a way that might affect C input versus C storage and release in tropical soils. Here, high soil fertility may favor plants to build up more C in biomass, thus increasing C input, whereas, it may also favor the establishment of microbial communities whose nutrient acquisition and allocation strategies limit long-term soil C storage.

Controls of the paleoenvironment on differential organic matter enrichment of lacustrine fine-grained rocks: A case study of the Jurassic Da’anzhai Member, central Sichuan Basin, SW China

• The majority of siliceous shales in the Da’anzhai member have high TOC contents. • The Da’anzhai member was deposited under oxic-suboxic conditions. • OM accumulation was controlled by clastic input and sedimentation rate . • The differential OM enrichment is explained by high and low clastic input models. Organic matter (OM) is not only a source of oil and gas but also the dominant host for hydrocarbon storage in unconventional reservoirs; thus, understanding the controls on OM accumulation are of great significance for unconventional hydrocarbon exploration. Lacustrine fine-grained rocks of the Jurassic Da’anzhai Member have variable total organic carbon (TOC) contents ranging from 0.11 to 3.11 wt%. However, it is still debatable regarding mechanisms responsible for such variable enrichment. Here, we present petrological, organic, and elemental geochemical data of three wells in the Jurassic Da’anzhai Member in the central Sichuan Basin, South China. The low enrichment of redox-sensitive trace elements (Mo EF , U EF , and V EF ) in lacustrine fine-grained rocks indicates oxic to suboxic conditions during deposition of the Da’anzhai Member, which are not conducive to OM preservation. The chemical index of alteration (CIA) values vary between 75 and 100, suggesting a warm/hot and humid paleoclimate. Relatively high productivity coincides with high CIA, suggesting that weathering fluxes may have played important role in the regulating primary productivity. Furthermore, the relationship between TOC and (La/Yb) N implies the influence of specific sedimentation rates on OM enrichment. Our results suggest that the OM accumulation in the Jurassic Da’anzhai Member is mainly controlled by clastic input and sedimentation rate. We propose that under conditions of variable clastic input, the appropriate sedimentation rate may have played a key role in the accumulation of OM. Finally, our study suggests that the siliceous shale may be a potential lithology for unconventional hydrocarbon production in the Jurassic Da’anzhai Member.

Geochemistry of the Kali River terrace sediments in Central Himalaya, India: Implication on nature of provenance in the known tectonic environment

In the Central Himalaya, the Kali River flow through the rocks of Tethys sedimentary (TS) sequences and Higher Himalayan crystallines (HHC) towards the upstream and through the Lesser Himalayan metasedimentary (LHS) the Siwaliks rocks in the downstream. The terrace sediments of the Kali River are geochemically investigated to infer nature of source region i.e. provenance and degree of chemical changes in the known tectonic setting during their journey. The low to high chemical index of alteration (CIA) of sediments (CIA = 22.24–87) indicate a dominant role of chemical and subordinate physical processes during sediment-forming mechanism. The longitudinal course emanating from TS in upper reaches to the Siwaliks in the lower reaches, the sediments along the main Kali River bear higher content Na2O, K2O, CaO and silica fractionation. The high and low ratios of La/Sc, Th/Sc and Co/Th advocate the dominance of felsic component in the source rocks. The content of Al2O3, TiO2, Fe2O3, La, Th, Sc, and Co/Th, La/Sc and Sc/Th ratios indicate the derivation of sediments dominantly from the leucogranite, granite to granodiorite and its gneissic sources, which commonly prevail in the rocks of Higher Himalayan crystallines sequences (HHCS). However, recycling and contribution of some aluminous Tethyan metasediments in the Kali River cannot be ruled out. Geochemical plots involving mobile and immobile elements show that erosion (sediment sorting) and weathering played a dominant role in the gradual change of sediment chemistry from source regions to catchments bed-load in the basin.

Provenance of the siliciclastic rocks in the Lower Yangtze region: Implications for Paleozoic tectonic switching of the northeastern South China Block

• Permian zircon U-Pb age spectra are different from those of Silurian and Devonian . • The Silurian-Devonian detritus was from the proximal igneous rocks. • Increased ca. 430 Ma zircons indicates the onset of foreland stage. • The Permian detritus was from the basement of the Cathaysia Block. • Foreland basin was replaced by intraplate basin in Late Paleozoic. Integrated analysis of zircon U-Pb dating, in-situ Lu-Hf and whole-rock geochemical data was performed on Silurian-Permian siliciclastic rocks from the Lower Yangtze region (LYZ), northeastern South China Block, to understand the tectono-sedimentary evolution during the Paleozoic. These rocks have a high chemical index of alteration (CIA = 71.3–91.8) and variable index of compositional variability (ICV = 0.2–3.5), indicating intensive weathering. The high Zr/Sc ratios (12.1–75.3) indicate that their sources contain recycled old sediments. The Silurian and Devonian samples contained abundant detrital zircons with ages of 1.0–0.8 Ga and 460–420 Ma. However, the Permian samples show bimodal age spectra with peaks at ∼ 1800 Ma and 280–260 Ma. Integrated with published geochronological data, the proportion of the 460–420 Ma zircons drastically increased from the Early to Mid-Silurian and decreased in the Early Carboniferous, indicating two tectonic transformations, respectively. Based on these variations and relative paleogeographic data, we suggest three evolution stages with different provenances for the LYZ: (1) from Cambrian to Ordovician, the LYZ region inherited the Cryogenian tectonic framework of South China Block, receiving detritus from East Gondwana. (2) from Silurian to Devonian widespread uplift in South China Block led to the development of an isolated foreland basin with independent sedimentary sources and (3) from Carboniferous to Permian, subduction of the Paleo-Pacific plate exposed the Paleoproterozoic basement of the Cathaysia Block, providing detritus for the LYZ intraplate basin.

Geochemistry of semi-arid Cryosols on volcanic and sedimentary materials from James Ross Island, Antarctica

In most parts of Antarctica, specifically in arid and semi-arid condition, soils are the results of weak weathering and little pedogenic processes. There, the use of geochemical data is considered an important tool to interpret possible pedological processes through the changing molecular ratios of elements with depth. The semi-arid soils of Antarctica remains little explored. In this study, the geochemistry of soils developed on different parent materials under a typical semi-arid climate was investigated, based on the quantification of the elements by various methods. Based on the major and trace elements geochemistry, the soil types are clearly differentiated by their geochemical composition, and highly affected by their parent materials. Based on the major elements abundances, chemical weathering is very limited. Using geochemical parameters to identify lithologic discontinuities, five profiles showed this characteristic. The apparent high chemical index of alteration and mineralogical composition, with kaolinite in the clay fraction, soils developed on the marine sedimentary rocks showed a pre-weathered nature, related to the ancient inheritance of Cretaceous paleoclimates, during which warmer climate led to intense weathering under subtropical conditions. Therefore, pre-weathering has a role in the mineralogical composition of Antarctic soils on sedimentary rocks.

Geochemistry of the mudrocks and sandstones from the Bredasdorp Basin, offshore South Africa: Implications for tectonic provenance and paleoweathering

Abstract An inorganic geochemical investigation of mudrocks and sandstone from the southern Bredasdorp Basin, off the south coast of South Africa was carried out to unravel the provenance, paleoweathering, and tectonic setting of the basin. Seventy-seven representative samples from exploration wells E-AH1, E-AJ1, E-BA1, E-BB1, and E-D3 underwent geochemical analysis involving major and trace elements. The major oxide compositions show that the sandstones could be classified as sub-arkose and sub-lithic arenite. The provenance discrimination diagrams based on major oxide geochemistry revealed that the sandstones are mainly of quartzose sedimentary provenance, while the mudrocks are of quartzose sedimentary and intermediate igneous provenances. The discrimination diagrams indicate that the Bredasdorp sediments were mostly derived from a cratonic interior or recycled orogen. The bivariate plots of TiO2 versus Ni, TiO2 against Zr, and La/Th versus Hf as well as the ternary diagrams of V–Ni–Th∗10 suggest that the mudrocks and sandstones were derived from felsic igneous rocks. The tectonic setting discrimination diagrams support passive-active continental margin setting of the provenance. Also, the closely similar compositions of the analysed samples and recent sedimentary rocks of the East African Rift System perhaps suggest a rifted basin tectonic setting for the Bredasdorp Basin. Chemical index of alteration (CIA) indices observed in the sandstones suggest that their source area underwent low to moderate degree of chemical weathering. However, the mudrocks have high CIA indices suggesting that the source area underwent more intense chemical weathering, possibly due to climatic and/or tectonic variations.

Mineralogical and geochemical imprints to determine the provenance, depositional environment, and tectonic setting of the Early Silurian source rock of the Qusaiba shale, Saudi Arabia

The Early Silurian source rock of the Qusaiba shale from Saudi Arabia was analyzed for its mineralogical and geochemical compositions to investigate its provenance, depositional environment, and tectonic setting, which still has not been studied. The shale is approximately 85 m thick gray to dark gray to black shale intercalated with siltstone and sandstone. The non-clay minerals are quartz and dolomite, while the clay minerals are kaolinite, illite, and smectite in some samples. An intermediate igneous source is suggested for the Qusaiba shale based on the Al2O3/TiO2 ratio, calculated SiO2, and Zr–TiO2, Al2O3–TiO2, SiO2–Al2O3/TiO2 and Cen–La/Ybn binary plots. The Zr/Sc versus Th/Sc cross plot indicate that the Qusaiba shale was not subjected to sedimentary recycling. The abundance of kaolinite, C–values [Σ(Mn + Fe + Ni + Cr + Co + V)/Σ(Mg + Ca + Sr + Ba + Na + K)], Sr/Cu ratios and high chemical index of alteration (CIA) values indicate humid and warm conditions for the period of deposition. Geochemical proxies such as V–Al2O3 and P2O5–Al2O3 binary plots suggest a shallow marine depositional environment for the Qusaiba shale was prevailing during the Paleozoic in northeastern Gondwana. The Qusaiba shale represents a transgressive phase in the Early Silurian followed the Late Ordovician regressive event marked by the glacial non-marine sandstone of the Sarah Formation. The Qusaiba Formation shows another regressive event characterized by storm deposits at its top. The tectonic setting interpreted from major oxides, ratios of immobile–trace elements and rare earth elements (REE) parameters such as (La/Y)n, (La/Y)n, (Gd/Yb)n and Eu/Eu* depicts a passive margin matching with the regional setting for the northeastern Gondwana. Illite contents (13 %–33%) and IC values (0.52°–0.75° Δ2θ) suggest a late diagenesis stage for the Qusaiba shale.

The relationship between the geochemical and mineralogical characteristics of Calcocambisol, colluvium and recent marine lake sediment of the narrow seashore intertidal zone: a case study from the Veliko Jezero (Mljet Island, Croatia)

This study investigates the mineral composition, particle size distribution and geochemical characteristics of Calcocambisol, colluvium and recent marine lake sediment in a narrow intertidal seashore zone of the Veliko Jezero on the Island of Mljet (Croatia). The obtained results show that the fractions of Calcocambisol/colluvium less than 2 mm and 2 µm display similar particle size distribution (PSD) curves compared to marine lake sediments containing larger particles in these fractions. The smallest fractions of the investigated materials that are less than 1 µm show identical PSD curves. The bulk and clay mineral composition of the marine lake sediment show that the non-carbonate fraction is derived from weathering of the surrounding soils and colluvium containing quartz, feldspars and phyllosilicates (illitic material, kaolinites, chlorite, and a mixed-layer clay mineral, MLCM), as well as the authigenic formation of early-diagenetic pyrite, while one part is related to the yield of material by aeolian deposition (amphibole). The observed difference between the phyllosilicate mineral phases in the clay fraction of the Calcocambisol/colluvium and the carbonate-free clay fraction of the marine lake sediment is related to 1) the presence of chlorite in the marine lake sediment and 2) the higher content of MLCM in the Calcocambisol/colluvium. The chlorite in the marine lake sediment was inherited from the Calcocambisol/colluvium as a result of soil erosion prior to its complete destabilization in the soil. High Chemical Index of Alteration (CIA) values in the Calcocambisol and colluvium clearly indicate their intense weathering. Based on the Sm/Nd and Ti/Al ratios, it can be concluded that the aluminosilicates in the Calcocambisol, colluvium and marine lake sediment are of the same provenance. The distribution of each analysed element among the sequential fractions is very similar in both the Calcocambisol and colluvium. The highest concentrations for most of trace elements in the Calcocambisol, colluvium and marine lake sediment was determined in their residual fraction. Mn, Co and Pb show a different partitioning between the Calcocambisol/colluvium and marine lake sediment, respectively.

Mineralogy, Geochemistry, and Genesis of Kaolinitic Claystone Deposits in the Datong Coalfield, Northern China

AbstractGray-black kaolinitic claystones of industrial value are abundant in Upper Carboniferous–Lower Permian coal-bearing strata of the Datong Coalfield of northern China. The main types are tonsteins and cryptocrystalline kaolinitic claystones, distinguished by the thinness and greater crystallinity of kaolinite in the former and by the presence of detrital illite and authigenic pyrite in the latter. In order to determine the formation history of these two types of kaolinitic claystone, the petrological, mineralogical, and geochemical characteristics of borehole samples from the Upper Carboniferous Taiyuan Formation which comprises siliciclastics and coal seams deposited in a coastal environment, were analyzed. In addition to kaolinite, the claystones contain subordinate illite, quartz, pyrite, anatase, feldspar, siderite, and calcite. The tonsteins and cryptocrystalline kaolinitic claystones have different sources, as shown by petrographic data, elemental ratios, and chondrite-normalized rare earth element (REE) patterns. The volcanic origin of the tonsteins is revealed by an abundance of volcanic quartz and vitric fragments as well as Al2O3/TiO2, Zr/Hf, and Nb/Ta ratios consistent with a felsic igneous source. Their REE fraction was derived from feldspars or micas of the parent rocks, and the fraction decreased with alteration of these minerals to kaolinite. The sedimentary origin of the cryptocrystalline kaolinitic claystones is revealed by an abundance of detrital quartz and illite grains derived from either granite or sedimentary upper crust, and by the total REE contents (ΣREE) and (La/Yb)N values which are consistent with granitic material. Their depositional environment was in a transitional (coastal) setting (as shown by intermediate Sr/Ba ratios) hosting an open acidic hydrologic system (as shown by high chemical index of alteration (CIA) values indicative of intensive chemical weathering) that was suboxic to anoxic (as shown by high U/Th ratios and trace-metal enrichment factors). The present chemistry of these claystones was thus controlled by a combination of parent rock type and diagenetic alteration.

Geochemistry and detrital zircon geochronology of the Lower Jurassic clastic rocks of the northern Qiangtang Basin, northern Tibet: Implications for palaeoenvironment, provenance, and tectonic setting

The Lower Jurassic clastic rocks in the Qiangtang Basin, northern Tibet, are generally regarded as the early sediments of the new Mesozoic Qiangtang Basin (Late Triassic to Early Cretaceous). However, little attention has been paid to the palaeoenvironment, provenance, and tectonic setting of this unit. The QZ‐16 well, located in the east of the northern Qiangtang Basin, firstly recovered the Lower Jurassic strata (Quemo Co Formation) in the Qiangtang Basin. In this paper, the whole‐rock major‐trace elemental characteristics of 34 mudstone samples and detrital zircon geochronology of one sandstone, collected from the Quemo Co Formation of the QZ‐16 well, are reported in order to infer the palaeoenvironment, provenance, and tectonic setting. Quemo Co mudstones are dominated by SiO2 and Al2O3, while Fe2O3, CaO, K2O, and MgO are the second most abundant oxides. They are generally characterized by high REE concentrations, enrichment in light rare earth elements (LREEs), marked negative Eu and Ce anomalies. The REE contents of the mudstones are mainly controlled by terrigenous detrital minerals input. The Lower Jurassic mudstones have relatively high chemical index of alteration (79.1–83.6) values, indicating moderate to high degree of chemical weathering conditions of the source area. Sr/Ba and Rb/K ratios reflect that the marine transgression impact was limited and the mudstones were generally deposited in a brackish water environment. Major, trace, and REE contents indicate that the mudstones were mainly derived from intermediate igneous rocks with subordinate felsic igneous rocks. Prominent age peaks at ca. 200 ~ 400 Ma, 700 ~ 970 Ma, 1,700 ~ 1,900 Ma, and 2,100 ~ 2,700 Ma of the detrital zircon grains suggest that the Lower Jurassic clastic rocks were derived from the Central Qiangtang and Songpan–Ganzi complex. Moreover, tectonic discrimination diagrams based on major and trace elements show that the source of the Lower Jurassic sediments were most likely derived from a collision tectonic setting.

Harsh or balmy weathering conditions onto the first continent surface?

The weathering conditions prevailing during Proterozoic times differ likely from Present-day ones mainly because of the climate conditions driven by high CO2 atmospheric levels. The question of weathering intensity onto one of the first continent surface, so-called “Columbia”, is here addressed using the combination of a thermochemical and climatic model. Based on the dissolution of a mix of silicates representative of the presumed fresh rocks and the precipitation of secondary phases, the plausible reaction pathways of weathering seem to indicate that approximately 10% of initial silicates minerals should be dissolved to explain the measured high Chemical Index of Alteration (CIA), classically measured in the rocks belonging to this geological period. In addition, global climatic simulations were performed to evaluate the respective contributions of pCO2, pluviometry, temperature and topographic parameters upon the weathering intensity. This finding could suggest the weathering intensity have been, at the very most, two orders of magnitude greater than tropical present-day one over the Paleoproterozoic era: the runoff seemed to be similar whereas the dissolution reaction extent was enhanced. These calculations are confronted with the geological record of weathering conditions, namely well-preserved sandstone deposits belonging to the 1.9 Ga Roraima Supergroup in the Guiana Shield, Brazil. The field data indicating very high Chemical Index of Alteration are in line with the model calculations.

Chemical weathering of crystalline rocks in contrasting climatic conditions using geochemical proxies: An overview

Weathering profiles of granitoid rocks from the Sila Massif (southern Italy) have been studied to characterize weathering processes that affect this area. Chemical weathering indices of the Sila Massif weathered granitoids were compared to those of different areas of the world characterized by different climates (Malaysia, Puerto Rico, Georgia, China, Nigeria, Colorado, Switzerland, and Sweden) to decipher their weathering characteristics. The geochemical indices show values of CIA (Chemical Index of Alteration) ranging from 55 to 97, CIW (Chemical Index of Weathering) ranging from 60 to ~100, PIA (Plagioclase Index of Alteration) ranging from 55 to ~100, WIP (Weathering Index of Parker) ranging from 7 to 64 and RR (Ruxton Ratio) ranging from 2.14 to 3.96. All these geochemical proxies closely resemble each other and show variations mainly related to the different climatic and tectonic settings. Malaysia and Puerto Rico granitoids showing high CIA values (respectively 97 and 95 on average) underwent very intensive chemical weathering alteration; Georgia, China and Nigeria showing medium-high CIA values (respectively 86, 90 and 92 on average) underwent moderate-intensive chemical weathering alteration; southern Italy and Colorado granitoids showing medium-low CIA values (68 on average) underwent moderate-weak chemical weathering alteration, whereas Switzerland and Sweden granitoids showing low CIA values (respectively 58 and 55 on average) underwent weak chemical weathering alteration. Those values are mainly related to the climatic conditions in term of temperature and precipitation of each area. CIA vs. climate histograms display a shift towards higher CIA values with increasing of warm-humidity (‘tropical’) conditions, whereas granitoids weathered under conditions inferred to be cold-arid-polar climate show lower CIA values.

Evaluating weathering of palaeosols in Cameroon (Central Africa) as a tool for paleoenvironmental reconstruction

Palaeosols formed from different igneous rocks have been identified and characterized in the humid savannas (1400 mm mean annual rainfall, 22 °C mean annual temperature) of the Adamaoua region in north-central Cameroon by means of macro- and micromorphology, geochemical analyses and bulk mineralogy. The studied sequence at Mangoli is about 11 m deep and is divided into three sections, in terms of weathering products: (1) Precambrian granite, located in the base; (2) vesicular porphyritic basalt, in the middle section; (3) aphanitic basalt in the uppermost layer. The products of weathering from the Precambrian granite consist of the following horizons, from bottom to top: a saprolite with granite structure, a yellowish to whitish fine saprolite, and Btg and Bt horizons. The saprolite horizon is dominated by quartz, feldspars (plagioclases and microcline), and muscovite. Primary minerals, except quartz, show weathering, while clay fills the pores and fractures and kaolinite dominates the clay mineralogy. The granite weathering products are slightly acidic (pH ranging from 5.9 to 6.3) and display low values of Fe extracted with dithionite-soluble Fe (Fed) and high Chemical Index of Alteration (CIA) and Mineral Index of Alteration (MIA) values. The middle section of the sequence consists of different areas of the basalt’s spheroidal weathering zone, from the more advanced to the less weathered phase. The clay mineralogy is dominated by kaolinite. The zones with the most alteration are the fractures, where thick, reddish clay coatings are observable. In these areas, the minerals are well crystallized. The kaolinite crystals share areas with Fe oxides. The upper section of the sequence constitutes the modern soil from the aphanitic basalt and consists of a spheroidal weathering zone divided into three horizons. The lowest is a C horizon, overlaid by Bt horizons. The C horizon shows a high degree of weathering in some areas, but it is still possible to identify the primary minerals. Their products of weathering are slightly acidic (pH = 6) and have high Fed, CIA and MIA values, which are indicative of intense weathering. The results suggest warm and humid climates during the formation of all of the palaeosols, similar to modern environmental conditions. We conclude that Palaeosol 1, formed from the granite, represents a soil formed during the Mid-Miocene Climatic Optimum, when the temperatures are assumed to have been 3–6 °C warmer than today. Features discovered during field investigations reveal that the former valley at the surface of the landscape has been filled by lava flows and today occupies the top of the plateau. Meanwhile, the former top of the plateau is located on the slopes and lower altitudes; the burial of soils by volcanic materials at Mangoli increased the altitudes of the former plateau and resulted in landscape inversion (slopes). These palaeosols and their top layers that induced the slope inversion have not yet been described and appear as a unique example worldwide.

Magnetic characteristics of Guangshan loess from northern piedmont of Dabie Mountains, east-central China

SUMMARY The loess from the northern piedmont of the Dabie Mountains is in a transition area between loess from the Chinese Loess Plateau, the Quaternary red soils of southern China and the Xiashu loess. Despite its significant location, the study has been inadequate. In this study, the Guangshan section in the northern piedmont of the Dabie Mountains was selected for investigation. Environmental magnetism, geochemistry, colour reflectance and optical diffuse reflection spectroscopy analyses were applied to detect the magnetic variations in the loess. The results showed that (1) the magnetic minerals consisted mainly of magnetite, maghemite, hematite and goethite, which are the same as those in the Quaternary loess from the Chinese Loess Plateau, the Xiashu loess and the Quaternary red soils of southern China. The average magnetic particles were in the pseudo-single domain, like those of the Chinese Loess Plateau loess. (2) Unit III of the Guangshan section (2.4–4 m), with high chemical index of alteration and low Ba-index, was demonstrated as the most strongly developed palaeosol in the whole section, in agreement with field observations (more Fe-Mn films and weakly vermiculated development). However, it exhibited minimal susceptibility values and the lowest concentration of fine ferrimagnetic minerals. Simultaneously, the unit had low hematite to goethite ratio (Hm/Gt), suggesting that the pedogenic environment was humid; and it also had high values of b* and Gt%, implying that there was more goethite. Therefore, we can conclude that excessive soil moisture and intensive pedogenesis dissolved the fine ferrimagnetic minerals originally produced by pedogenesis and transformed them into goethite. These results could help to trace the palaeoclimatic evolution of the study area and clarify the magnetic variations of loess in different climates throughout China.

Triassic deposits of Chukotka, Wrangel Island and Mendeleev Rise, Arctic Sea: sedimentology and geodynamic implications

ABSTRACT Upper Triassic sandstones from Mendeleev Rise were analyzed and compared to turbiditic sandstones of the same age from Wrangel Island and Chukotka, Arctic Sea region. Medium-grained lithic arenites dominate all of these. The quartz content is higher in the sandstones from Mendeleev Rise than in those from Wrangel Island and Chukotka. Abundant quartz grains with microfractures and kaolinized feldspar are characteristic of the samples from Mendeleev Rise, whereas such features are lacking in the other areas. Quartz with microfractures is less abundant in Triassic turbidite sandstones of Chukotka and Wrangel and kaolinized feldspar is not present. Upper Triassic sandstones of Mendeleev Rise are characterized by a high Chemical Index of Alteration indicating intensive weathering in the source province. All sandstones have Th/Sc ratios ≤1. The chemical composition of sandstones suggests a quartzose sedimentary province for sandstones of Mendeleev Rise, an intermediate igneous province for Wrangel Island sandstones, and a mafic igneous provenance for Chukotka sandstones. Detrital zircon populations are similar in all Upper Triassic sandstones samples with a dominance of Paleozoic zircon. Facies studies of Triassic deposits in the Eastern Arctic of Russia indicate that sandstones from Mendeleev Rise were located near a continent and formed in a shallow or coastal-marine setting. Triassic deposits of Wrangel Island and Chukotka were deposited in a deeper marine environment.

GEOCHEMICAL APPLICATION IN UNRAVELING PALEOWEATHERING, PROVENANCE AND ENVIRONMENTAL SETTING OF THE SHALE FROM CHIA GARA FORMATION, KURDISTAN REGION, IRAQ

The geochemical characteristics of the shale of the Chia Gara Formation (Middle Tithonian-Berriasian) from the Imbricated Zone (Barsarin section) and High Folded Zone (Banik section) Kurdistan Region, Iraq, was carried out to constrain their paleoweathering, provenance, and depositional environment. There are no clear differences in the major and trace elements of the Chia Gara Formation between the two studied sections. The chemical index of alteration (CIA) is significantly higher in the Barsarin than the Banik shales, suggesting more intense weathering of the Barsarin than the Banik shales. The samples of the Banik and some of Barsarin are clustered near the A-K line in A-CN-K plot suggests intense chemical weathering (high CIA) without any clear-cut evidence of K-metasomatism. The other samples of Barsarin have a weathering trend parallel to the A-CN line, indicating relatively steady state weathering conditions. The geochemical parameters of the shale (Al2O3/TiO2, Th/Sc, La/Th, La/Sc, La/Co, Th/Co, Cr/Th, (La/Lu)cn and Eu/Eu*cn), and the diagrams (Th/Sc-Zr/Sc and La/Th-Hf) indicate that they were derived from felsic (from the Rutba Uplift and/or Mosul High) and intermediate (from volcanic material during the spreading of Southern Neo-Tethys Ocean) components. The chondrite-normalized REE patterns are similar to those of Post Archean Australian Shale (PAAS), with the light rare earth element (REE) enrichment, a negative Eu anomaly, and almost flat heavy REE pattern. The geochemical parameters such as authigenic uranium, U/Th, V/Cr, Ni/Co, and V/Sc ratios, and Al2O3-V and Al2O3-P2O5 diagrams indicate that these shales were deposited under deep marine suboxic to anoxic environment and shale of Banik section was deposited slightly under deeper environment than shale of Barsarin section.