Chemical Index Of Alteration Values Research Articles

Sediment petrography, mineralogy and geochemistry of the Miocene Islam Dağ Section (Eastern Azerbaijan): Implications for the evolution of sediment provenance, palaeo‐environment and (post‐)depositional alteration patterns

AbstractThe reconstruction of regional long‐term patterns recorded in marine sedimentary successions of the Eastern Paratethys is important in understanding the role of Cenozoic climate change and orogenic activity on the depositional environment and sedimentation dynamics in Western Asia. In this study, the environmental conditions in the early to middle Miocene (Islam Dağ section) in eastern Azerbaijan are elucidated using petrographic–mineralogical relations, detrital indicators, weathering indices and δ13C and δ18O signatures of organic‐rich (total organic carbon: ca 3 to 6 wt. %) argillites. Sedimentary facies and chemical proxies (Na/K, K/Al, Si/Al, Ti/Al ratios, chemical index of alteration values) indicate arid conditions, reduced weathering rates in the hinterland and sediment deposition in an euhaline and poorly oxygenated deep‐water basin during the early Miocene, followed by a shift to humid conditions, higher weathering rates and an oxygenated water column in the mid‐early Miocene. Long‐term aridification and deposition of gypsiferous and calcareous argillites under generally more oxygenated, euhaline to polyhaline conditions in a lacustrine or restricted shelf setting until the middle Miocene is evidenced by gradual changes in element ratios and the chemical index of alteration. Discriminant function analysis suggests the Russian Platform, drained by the Palaeo‐Volga and Palaeo‐Don river systems, to be the source area for the siliciclastic input throughout the Miocene, although a minor contribution of volcanogenic detritus and mafic components from the Greater Caucasus is possible. The C–S–Fe associations and increasing Fe/Al ratios towards the middle Miocene support the concept of continuous influx of detrital Fe and total organic carbon. The formation of ferruginous smectite from alteration of volcanic ash layers could have affected the preservation of total organic carbon and therefore the sedimentary C and Fe budget in the Eastern Paratethys basins. Palaeo‐climatic reconstructions based on δ13C (−34·5 to +1·7‰ Vienna Pee Dee Belemnite) and δ18O (−34·7 to −4·8‰ Vienna Pee Dee Belemnite) records of authigenic carbonates should be made with great caution, as the pristine marine signatures may be affected by the oxidation of organic matter and meteoric diagenesis.

Geochemical characteristics of Early Cretaceous marine oil shale from the Changshe Mountain area in the northern Qiangtang Basin, Tibet: Implications for palaeoweathering, provenance, tectonic setting, and organic matter accumulation

The Early Cretaceous Shengli River–Changshe Mountain marine oil shale belt, located in the northern Qiangtang Basin, Tibet, is estimated to be the largest marine oil shale resource in China. However, its palaeoweathering, provenance, and tectonic setting during the depositional history have received little interest. In this paper, 19 outcrop marine oil shale samples from the ChangsheMountain (CSM) area were studied to investigate the provenance, palaeoclimate, palaeoweathering, and tectonic setting during the marine oil shale deposition. The total organic carbon and organic sulphur contents of the CSM oil shales range from 2.2% to 13.44% and 0.19% to 1.08%, respectively. The CSM oil shale samples contain abundant carbonate (e.g., calcite and dolomite), quartz, and clay minerals. The clay minerals of the CSM oil shale samples is dominated by illite and then by smectite. Other minerals, including haematite, apatite, pyrite, baryte, and celestite, were also identified by scanning electron microscope–energy‐dispersive X‐ray spectrometer. The total rare earth element (∑REE) contents of the oil shales range from 9.01 to 105.96 ppm (average 57.56 ppm), close to the average REE contents of U.S. coals. The REE contents in oil shale samples are mainly controlled by terrigenous input. The CSM oil shales show LREE‐enriched, HREE‐depleted, with the LREE/HREE ratios ranging from 5.12 to 10.18, negative Eu anomalies (0.56–0.84) and slightly negative Ce anomalies (0.87–1.05). The chemical index of alteration values of the CSM oil shales range from 67 to 76 (average 73), indicating a moderate chemical weathering condition. Sr/Cu and Rb/Sr ratios suggest the CSM oil shales were mainly deposited under a warm and humid climate regime. Sr/Ba ratios, ranging from 0.54 to 7.84, suggest a palaeoenvironment with fluctuant salinity. Major element, trace element, and REE relationships indicate that the CSM oil shales are mainly from the felsic and intermediate igneous rocks. The REE distribution patterns of the CSM oil shales are similar to the Late Triassic Nadi Kangri felsic volcanic rocks in the Qiangtang Basin, indicating that the CSM oil shale may be derived from the Late Triassic volcanic rocks and the oil shales were mainly formed in rift tectonic setting. Favourable preservation condition (e.g., warmer palaeoclimate, higher palaeosalinity, and dysoxic to anoxic water column environment) could be the main controlling factor for organic matter accumulation.

The geochemistry of glacial deposits in Taylor Valley, Antarctica: Comparison to upper continental crustal abundances

Wet-based glacial deposits have been used traditionally as an analog for upper continental crust (UCC) abundances. To provide more information on the validity of using glacial deposits from wet-based glaciers, samples deposited by the dry-based polar glaciers located in Taylor Valley, Antarctica, were collected. Stream channel sediments, comprised of igneous, metamorphic, and sedimentary rocks initially deposited as glacial tills by polar glaciers, were analyzed by XRF, ICP-MS, and SEM. Based on the Chemical Index of Alteration values and A–CN–K ternary diagram, there are low levels of chemical weathering in these tills. Additionally, major and trace element geochemical data are compared to the average UCC values. The observed discrepancies between the mean UCC and Antarctic samples develop from the existence of mafic components, most likely the McMurdo Volcanic Group and Ferrar Dolerite, being present in the Taylor Valley tills. Even though the mafic material typically comprises 3–7% of the till, the volcanic rocks have a significant influence on the tills’ bulk geochemistry. The existence of this mafic fraction in the dry-based glacial tills results from the reduced rate of weathering, as compared to wet-based glaciers. Geochemical analyses of the dry-based glacial tills in polar deserts, such as those found in Taylor Valley, may provide a better representative composition of the original material than wet-based glaciers and need to be incorporated into upper continental crust calculations.

Carboniferous glaciotectonized sediments in the southernmost Paraná Basin, Brazil: Ice marginal dynamics and paleoclimate indicators

Carboniferous glaciogenic strata (Itararé Gp.) in the southernmost Paraná Basin, Brazil exhibit soft-sediment deformation features previously interpreted as glaciotectonism. These sediments were studied in detail to confirm that they were deformed by ice and to assess the nature of the glaciation, depositional environments, and paleoclimate in this region during the Carboniferous. Five outcrops were described along a railroad transect that contains a conglomerate and diamictite facies with striated and faceted clasts, a medium sandstone facies, a fine grained silt/clay rhythmite and mudstone facies with dropstones and diamictite pellets, a sandy clinoform facies, and a folded sandstone with interbedded mudstone facies. The depositional environment for these sediments is interpreted as an outwash fan and fan delta from an ice-proximal, transitional terrestrial-to-estuarine setting. Rb/K values from the rhythmites reflect a transition from a freshwater to brackish environment and the Chemical Index of Alteration (CIA) of the rhythmites ranges from 65 to 73, reflecting a relative increase in the degree of chemical weathering through time. Deformation features include widespread folding, thrust faults, hydrofractures, décollement surfaces, and piggyback fold-thrust complexes. The deformation is interpreted as evidence of a push-moraine complex formed by at least two complete ice advance/retreat cycles. The occurrence of décollement surfaces, plastically deformed proglacial sediments, and hydrofractures indicate a dynamic, warm-based or polythermal glacier. Abundant outwash sediments, rhythmites with dropstones, and the shift to average CIA values all support a temperate paleoclimate. Deformation structures indicate a NW direction of ice shove that is in agreement with the regional-scale hypothesis that a NNW flowing lobe extended out of Uruguay during the Carboniferous and terminated in the southernmost Paraná Basin. This study demonstrates that late Paleozoic glaciation in this region was more dynamic than previously understood, with high frequency fluctuations in ice marginal positions.

Provenance and tectonic setting of the Lower Cambrian Niutitang formation shales in the Yangtze platform, South China: Implications for depositional setting of shales

Geochemical compositions of the Lower Cambrian Niutitang Formation shales in the southeastern Yangtze Platform margin were investigated for provenance, tectonic setting, and depositional environment. The shale samples are characterized by higher abundances of large ion lithophile elements (Cs, Ba, and Pb), lower abundances of high field strength elements (Cr, Sc, and Co) and transition elements (Th, Zr, Hf, Nb, and Ta) relative to average shale. North American shale composition (NASC) -normalized rare earth element (REE) patterns are observed, with negative Ce anomalies, negative Eu anomalies, and positive Y anomalies. The chemical index of alteration (CIA) varies from 68.67–74.93. Alkali and alkaline element contents and CIA values suggest that the source rocks have undergone moderate weathering. The index of compositional variability (ICV), Zr/Sc and Th/Sc ratios vary from 0.53 to 1.07, 5.31 to 8.18 and 0.52–1.02, respectively. ICV values and relationships between Zr/Sc and Th/Sc ratios indicate negligible sedimentary recycling. The Al2O3/TiO2 (14–26) and TiO2/Zr (56–77) ratios imply that the source rocks of the investigated shales had intermediate igneous compositions. However, Cr/V ratios and a La/Th–Hf discrimination diagram suggest that the intermediate compositional signal of the source rocks was derived from a mixture of 75% mafic and 25% felsic igneous rocks rather than intermediate igneous rocks. The major source was the Jiangnan continental island arc with bimodal igneous rocks, lying to the south of the study area, together with a contribution from granites and gneisses uplifted and eroded in the Yangtze Block. Discrimination of tectonic setting using major and trace elements indicates that the source rocks originated in a transitional setting from active continental to passive margin, consistent with the failed intracontinental rift model for the evolution of the South China plate. The Niutitang Formation shales were deposited in a rift basin setting under conditions of anoxic bottom water in a redox-stratified water column, with organic-rich shales prospective for shale-gas production being found in deep-water downslope and basin environments rather than the shallow-water shelf.

Changes in the profile characteristics of cultivated soils obtained from reconstructed farming plots undergoing agricultural intensification in a hilly mountainous region in southwest China with regard to anthropogenic pedogenesis

Farming plot reconstruction engineering is important in mitigating cultivated land fragmentation and promoting intensified production in hilly mountainous regions. As an anthropogenic disturbance, farming plot reconstruction and subsequent agricultural intensification have considerable effects on cultivated land, ecological environments, and pedogenesis. Although researchers have given considerable attention to changes in the physical and nutrient properties of soil in reconstructed farming plots, the connection between soil engineering and anthropogenic pedogenesis is often neglected. In this paper, we compared samples obtained during farming plot reconstruction engineering and agricultural intensification in terms of different soil properties. The results indicated that reconstruction changed the soil profile patterns from A–C to A–B–C, thereby significantly increasing the soil thickness, bulk density, clay content, pH, soil organic carbon, total nitrogen, C/N ratio, and cation exchange capacity. The geochemical composition notably varied in topsoil; the chemical index of alteration (CIA) and the chemical index of weathering (CIW) decreased markedly, and the Na2O/K2O ratio (Na/K) increased slightly. After a period of agricultural intensification, the physical and chemical properties of soil improved gradually with a high coefficient of variation. However, the soil exhibited Mg and Ca leaching and K and Na accumulation, and the intensity of chemical weathering was not significantly stronger with higher CIA values, higher CIW values, and lower Na/K ratios. These changes indicated that the anthropogenic disturbances from soil engineering accelerated the physical weathering and uniformity of purple soil but depressed soil chemical weathering to some extent. The anthropogenic disturbances of agricultural intensification disrupted the homogeneity of soils but did not significantly promoted the physical and chemical weathering of purple soil. This study provides guidelines for rational soil usage in reconstructed farming plots and may aid in soil classification.

Effects of grain size on the chemical weathering index: A case study of Neogene fluvial sediments in southwest Japan

Grain size variation of sediments and sedimentary rocks generally complicates the assessment of the degree of chemical weathering as measured by the Chemical Index of Alteration (CIA) value. Mineralogical and geochemical research was carried out on Miocene and Pliocene fluvial sediments in southwest Japan in order to evaluate the effects of grain size on the degree of chemical weathering. A strong positive linear trend is usually exhibited between Al2O3/SiO2 ratios and quartz intensities obtained by X-ray powder diffraction, suggesting that the Al2O3/SiO2 ratio may be suitable as a grain size index for semi-consolidated and unconsolidated sediments. Most of the fine-grained sediments, for which Al2O3/SiO2 ratios are over 0.4, have higher CIA values of over 90. In contrast, coarse-grained sediments, for which Al2O3/SiO2 ratios are lower, especially below 0.2, display a wide range of CIA values. The CIA values for intermediate grain sized samples, for which Al2O3/SiO2 ratios range from 0.2 to 0.4, reflect distinctive differences in the degree of chemical weathering between Miocene and Pliocene sediments. Within this range of Al2O3/SiO2 ratios, the Miocene sediments have higher CIA values (86–98) than Pliocene sediments (71–92). To compare the degree of chemical weathering among some sediments or sedimentary rocks with variable grain sizes, it is desirable to evaluate with the Al2O3/SiO2 ratios limited to the intermediate grain size range.

Chemical alteration of fine-grained sedimentary rocks at Gale crater

From Sol 750 to 1550, the Curiosity rover documented>100 m thick stack of fine-grained sedimentary rocks making up part of the Murray formation, at the base of Mt Sharp, Gale crater. Here, we use data collected by the ChemCam instrument to estimate the level of chemical weathering in these sedimentary rocks. Both the Chemical Index of Alteration (CIA) and the Weathering Index Scale (WIS) indicate a progressive increase in alteration up section, reaching values of CIA of 63 and WIS of 25%. The increase in CIA and WIS values is coupled with a decrease in calcium abundance, suggesting partial dissolution of Ca-bearing minerals (clinopyroxene and plagioclase). Mineralogy from the CheMin X-ray diffraction instrument indicates a decrease in mafic minerals compared with previously analyzed strata and a significant proportion of phyllosilicates consistent with this interpretation. These observations suggest that the sediments were predominantly altered in an open system, before or during their emplacement, contrasting with the rock-dominated conditions inferred in sedimentary deposits analyzed at Yellowknife Bay.

Weathering Controlled Landslide in Deccan Traps: Insight from Mahabaleshwar, Maharashtra

Landslide is one of the devastating natural phenomenon that threatens human life and property. Every year a number of persons lost their lives due to the landslides. Therefore, a better understanding and characterization of landslide is very essential for adopting mitigation strategies to contain the adversities of this natural hazard. Information on landslides from different climatic setup are very essential for better understanding of the influence of weathering, rainfall, or topography on landslide generation. Weathering is one of the important causative factor for landslide generation in the moderate topography or inactive mountainous terrain. The Western Ghats including the Deccan Traps, an inactive mountain range, receives torrential rainfall. Intense rainfall in these areas enhances the weathering processes and fabricates thick soil covers. Mahabaleshwar area, Maharashtra was chosen as a case study, where high elevated part is covered by lateritic layer and each lava flow unit is separated by a thin weathered bed of red bole. The area experiences series of landslides during the summer monsoon months. Mainly two types of landslides have been identified in the area confined with the red bole bed and powdery lateritic soil. The first type of landslides occur at higher elevations (≥1200m) where horizontal beds of permeable laterites underlined by impermeable thick basalt beds. The rain water infiltrates down and spread laterally within the permeable lateritic beds. It finally spouts at lower plateau elevations and triggers mainly debris flows. The other category of landslides occurs where the weathered red bole bed separates two successive lava flows. The percolating water from the secondary porosities (joints and inter connected vugs) comes out from the contact zones of basalt and red bole bed in the form of seepages. It erodes the red bole bed and as a result the overlying masses hang and consequently lead to rock fall. The Chemical Index of Alteration (CIA) of the representative samples from landslide locations indicates significant weathering. The CIA values for the fine lateritic soil are up to 98% whereas for the red bole bed it varies from 77 to 85%. This suggests a high chemical weathering and higher erodibility. The association of active landslide locations with the red bole bed and fine lateritic soil suggests a close relation between weathering and landslide occurrences in the area.

Detrital zircon record of Mesozoic volcanic arcs in the Lower Cretaceous Mural Limestone, northwestern Mexico

The northwestern part of Mexico, southern Arizona, and southern California witnessed extensive arc magmatism during Late Triassic to Early Cretaceous. Here, we present results from petrography, whole‐rock geochemistry (major and trace including rare‐earth elements), and detrital zircon geochronological studies that were carried out on sandstones and shales of the Mural Limestone exposed in the Rancho Bufalo area. Based on the petrography, sandstones are classified as sublitharenite, litharenite, feldspathic litharenite, and lithic arkose. The sandstones contain higher proportion of monocrystalline quartz than polycrystalline quartz. Sandstones and shales from the various members of the Mural Limestone show large variations in major and trace elements concentrations. The average concentrations of ΣREE (total ΣREE) are lower in sandstones than in shales. The chondrite‐normalized REE patterns of the sandstones and shales are moderately fractionated with light REE (LREE) enrichment relative to heavy REE (HREE) and with flat to slightly depleted HREE patterns. The chemical index of alteration values and A–CN–K relationships of sandstones indicate low to moderate intensity of weathering in the source region; however, shale samples suggesting the source rocks were subjected to moderate intensity of chemical weathering. The chondrite‐normalized REE patterns, Al2O3/TiO2ratio, elemental ratios like Cr/V, Y/Ni, and Eu‐anomaly (Eu/Eu*), and bivariate plots suggest that the sandstones and shales were derived mainly from felsic source rocks. Detrital zircon U–Pb geochronology of sandstones from Fronteras, Rancho Bufalo, and Cerro La Ceja members of the Mural Limestone show five main groups of detrital zircon age populations: (a) Proterozoic, (b) Palaeozoic, (c) Triassic, (d) Jurassic, and (e) Early Cretaceous. Our study suggests that continental volcanic arcs in northwestern Mexico, southern Arizona, and southern California served as the provenance for the Late Triassic‐Jurassic detritus, whereas the Early Cretaceous zircon grains were derived mainly from the active Alisitos volcanic arc along the northwestern coast of Mexico.

Mineralogy, petrography and geochemistry of an early Eocene weathering profile on basement granodiorite of Qaidam basin, northern Tibet: Tectonic and paleoclimatic implications

Weathering, as an important process in the earth surface system, can be significantly influenced by tectonics and climates over long time scales. Here, we use mineralogical, petrographic and geochemical data of a paleoweathering profile developed on basement granodioritic rocks of northern Qaidam basin, northern Tibet, to reconstruct early Eocene weathering conditions and to discuss how paleoclimates and tectonics dominated the weathering process. The results indicate that neoformed mineral phases in weathering products are dominated by smectite, and the profile has overwhelmingly low chemical index of alteration values (ca. 51–59) and significantly decreasing micropetrographic index values (from 25.0 to 0.2) from bottom to top. These findings suggest that the basement rocks experienced mild chemical weathering but relatively intensive physical weathering. We favor that non-steady-state weathering, in which mechanical erosion rates compare favorably with rates of chemical weathering, prevailed in northern Tibet during the early Eocene. The weathering conditions were likely an integrated response to active tectonism and dry climates at that time. Furthermore, chemical element mobility evaluation demonstrates that most of large ion lithophile elements and light rare earth elements (LREEs) of granodioritic rocks are quite active during weathering and can be easily leached even under mild chemical weathering conditions. Significant mass loss of Al and LREEs in upper weathered samples probably reflects acidic weathering conditions, which were likely due to extremely high atmospheric CO2 level during the early Eocene. This study, from the unique perspective of weathering process, suggests that intensive deformation and rapid tectonic erosion occurred in northern Tibet during the early Eocene, as a far-field response to the India-Eurasia collision. It also agrees with warm and relatively dry climates, which were likely attributed to the global greenhouse climates and the Paleogene planetary-wind-dominant climate system in Asia, respectively.

Role of weathering of pillow basalt, pyroclastic input and geomorphic processes on the genesis of the Monte Cerviero upland soils (Calabria, Italy)

In this paper we present new data on weathering and soil formation processes affecting alkaline pillow basalts on the summit of Mt. Cerviero (Calabria, southern Italy). We investigated two representative soil profiles using an integrated approach, including pedological, petrographic, mineralogical and geochemical investigations. We distinguished the main features inherited from the hydrothermal alteration of the pillow basalts in a submarine environment from the chemical weathering processes under meteoric conditions. Irregular geochemical patterns and chemical index of alteration values across the soil profiles indicate a lithological discontinuity between the bedrock and the upper soil horizons, as a response to soil rejuvenation, in turn controlled by erosive processes and an allochthonous pyroclastic input. The soil profiles display poor horizonation and an incipient to intermediate degree of weathering, in line with the clay mineralogy (chlorite or hydroxy-interlayered vermiculite, illite, kaolinite and varying mixed-layers). The degeneration microtextures of clay and iron-manganese coatings in one soil profile suggest their relict genesis, with an emplacement under warm-humid conditions during the last interglacial. A Late Pleistocene to Holocene age of soil development is supported by the trachytic composition of volcanic micropumices, correlated to explosive eruptions from the Campania Province or the Aeolian Islands, in both soil profiles. Geochemical indices obtained from selective extractions of pedogenetic Al, Fe and Si pools point to poor andic properties with humus-Al-complexes prevailing over short-range order minerals. Nonetheless, the latter are consistent with the presence of volcanic glass, an optically isotropic pedogenic matrix and the Andosol-like field features of the soil profiles, even if they developed in a non-volcanic area. This should lead to a partial reassessment of the volcanic versus non-volcanic origin of certain Andosols worldwide and claims a good field work as a basis for choosing the best-suited laboratory methods to fill the gap between ordinary lab and field results.

Intensified chemical weathering during the Permian-Triassic transition recorded in terrestrial and marine successions

Global warming is inferred to have been one of the main causes of the Permian-Triassic (P-Tr) boundary mass extinction. Although a strong temperature rise in tropical sea-surface temperatures during the latest Permian has been documented, coeval climate changes in terrestrial sections are less well-known. Here, we analyzed multiple weathering indexes (including CIA, CIW, and PIA) for two terrestrial sections in North China (Shichuanhe and Yima), all of which show a major excursion toward higher values (greater weathering intensity) around the end-Permian plant extinction (EPPE). At Shichuanhe, the CIA (chemical index of alteration) increases from 70 to 75 over the interval of 11–36 m, which straddles the EPPE at 33 m, and then decreases from 75 to 71 in the overlying strata. At Yima, CIA increases from 63 to 79 over the interval of 57–68 m, extending up to the EPPE at 68 m, and then gradually decreases from 78 to 72 in the overlying strata. These results imply a strong warming event coupled with intense chemical weathering in soil environments leading up to the end-Permian plant extinction. The decline in CIA values following the EPPE may reflect loss of weathered soils through physical erosion rather than climatic cooling. Estimation of atmospheric temperatures from the CIA data indicate rapid rises at Shichuanhe (from ~11.6 °C to 16.5 °C) and Yima (from 9.3 °C to 18.4 °C) during the end-Permian crisis. The findings in this study are consistent with the hypothesis that a sharp temperature rise caused the extinction of terrestrial organisms during the end-Permian crisis.

Mineralogy and geochemistry of modern Yellow River sediments: Implications for weathering and provenance

This study illustrates the textural, mineralogical and geochemical signatures of sediments from the upper Yellow River. Sediments of major tributaries have also been examined to study the influence of provenance, weathering intensity, and hydraulic sorting on the spatial compositional variability of sediments. Similarly to floodplain sediments, riverbed deposits are strongly depleted in MgO and Na2O, and enriched in CaO and Si2O relative to the upper-continental-crust standard. The pattern of rare earth elements (REE) indicates LREE enrichment and negative Eu anomaly (Eu/Eu* = 0.54–0.68). Low CIA (Chemical Index of Alteration) values indicate that the sediments from the trunk-river channel, floodplain, and major tributaries have not experienced intense chemical weathering in arid to semi-arid climatic conditions. The distribution of chemical elements shows a significant increase of Zr and Hf with coarsening of grain-size, whereas Rb, Ni, and V are associated with finer sediment fractions supplied by adjacent dune fields and tributaries. The Al/Si ratio (a proxy for grain size) correlates positively with CIA, Fe2O3, MgO, and K2O, but not with Na2O, reflecting grain-size control. Clay mineral assemblages in the upper Yellow River and in tributaries draining the Loess Plateau (e.g., Qingshui and Zuli Rivers) are dominated by illite and chlorite, whereas the abundance of smectite characterizes the middle reaches of the Yellow River and the Ten tributaries draining the Ordos Plateau.

Detrital zircon geochronology and geochemistry of Jurassic sandstones in the Xiongcun district, southern Lhasa subterrane, Tibet, China: implications for provenance and tectonic setting

AbstractJurassic sandstones in the Xiongcun porphyry copper–gold district, southern Lhasa subterrane, Tibet, China were analysed for petrography, major oxides and trace elements, as well as detrital zircon U–Pb and Hf isotopes, to infer their depositional age, provenance, intensity of source-rock palaeo-weathering and depositional tectonic setting. This new information provides important evidence to constrain the tectonic evolution of the southern Lhasa subterrane during the Late Triassic – Jurassic period. The sandstones are exposed in the lower and upper sections of the Xiongcun Formation. Their average modal abundance (Q21F11L68) classifies them as lithic arenite, which is also supported by geochemical studies. The high chemical index of alteration values (77.19–85.36, mean 79.96) and chemical index of weathering values (86.19–95.59, mean 89.98) of the sandstones imply moderate to intensive weathering of the source rock. Discrimination diagrams based on modal abundance, geochemistry and certain elemental ratios indicate that felsic and intermediate igneous rocks constitute the source rocks, probably with a magmatic arc provenance. The detrital zircon ages (161–243 Ma) and εHf(t) values (+10.5 to +16.2) further constrain the sandstone provenance as subduction-related Triassic–Jurassic felsic and intermediate igneous rocks from the southern Lhasa subterrane. A tectonic discrimination method based on geochemical data of the sandstones, as well as detrital zircon ages from sandstones, reveals that the sandstones were most likely deposited in an oceanic island-arc setting. These results support the hypothesis that the tectonic background of the southern Lhasa subterrane was an oceanic island-arc setting, rather than a continental island-arc setting, during the Late Triassic – Jurassic period.

Cryptic climatic signatures and tectonic controls on Cryogenian diamictites in the NW Tarim Craton, China

Unravelling the origins of Cryogenian diamictites is vital in determining their significance in a snowball Earth context and their relationship to environmental processes. Here, we present new sedimentological and geochemical data for the Cryogenian sedimentary rocks from the Tarim Craton, NW China. The diamictite member of the East Qiaoenbrak Formation contains (1) granitic pebbles with high sphericity, (2) well-developed stratification and (3) bed-scale grain-size changes suggesting cyclicity; also, (4) ice-rafted debris is conspicuously absent. Rather than deposits of direct glacial origin, the succession is instead interpreted as a subaqueous fan complex. The major and trace element ratios and REE patterns are consistent throughout 22 sampled intervals in the Qiaoenbrak Group, suggesting a uniformly felsic source area, probably from an island arc. Chemical index of alteration (CIA) analysis indicates low CIA values (49.5–54.3, average 51.6) throughout the Qiaoenbrak Group. We suggest that both the Qiaoenbrak Group and Yuermeinak Formation were deposited during the Marinoan glaciation, in a long-lasting cold and dry interval. It is likely that the Qiaoenbrak Group was deposited in a fault-controlled basin adjacent to an active continental margin, although the material may ultimately have derived from a glacial source (e.g. outwash).

Geochemistry of the Miocene Zoumi flysch thrust-top basin (External Rif, Morocco): new constraints on source area weathering, recycling processes, and paleoclimate conditions

This study focuses on the Lower-Middle Miocene syn-orogenic flysch deposits of the Zoumi basin to infer source area paleoclimatic conditions, the intensity of source rocks paleoweathering, and mechanical sorting and recycling effects. The mudrocks are enriched in Al2O3, Fe2O3, CaO, and TiO2 relative to PAAS and depleted in the other mobile major elements. There are high positive correlations between SiO2, Al2O3, and TiO2 and negative correlations between SiO2 and CaO. Geochemically, the mudstones are mainly classified as shales, Fe-shales, and wackes. Various discriminant diagrams were used to reveal the inferred tectonics, source paleoweathering intensity, and paleoclimatic conditions. Chemical index of alteration (CIA) and chemical index of weathering (CIW) values for Lower-Middle Miocene vary from 50 to 80% indicating low to moderate degree of source area weathering compatible with non-steady-state weathering under wet and humid paleoclimatic conditions. Locally (Zoumi mid-section) CIA values are higher (> 80) reflecting intense source area weathering, which may be attributed to high tectonic impulses and more humid conditions during deposition. The combination of ICV-CIA, Al2O3-Zr-TiO2, and Th/Sc-Zr/Sc values suggests the bulk rock is chemically immature and has experienced modest physical sorting and recycling reflecting little transportation until the final deposition.

Large Ca stable isotopic (δ44/40Ca) variation in a hand-specimen sized spheroidally weathered diabase due to selective weathering of clinopyroxene and plagioclase

Calcium stable isotopic compositions (delta Ca-44/40(SRM 915a)) as well as selected major and trace element concentrations are reported for micro-drilled samples of a hand-specimen sized spheroidally weathered diabase (synonymously used as dolerite or microgabbro) from southern India. A sample of the similar to 2.37 Ga old unweathered diabase (UW) dike was also analyzed. X-ray micro-CT imaging of a representative portion of the weathered sample shows the presence of two dominant mineral phases, plagioclase and clinopyroxene, with minor proportions of an iron-bearing phase (possibly haematite or ilmenite). Two different generations of crosscutting micro-fractures are identified from the micro-CT image. The older fracture is sealed with secondary mineral deposits. The delta Ca-44/40 values of the weathered samples range from 0.42 parts per thousand to 0.84 parts per thousand with samples showing both higher and lower values than that of the un-weathered diabase dike which shows a delta Ca-44/40 value of 0.65 parts per thousand. The variation in delta Ca-44/40 in the weathered samples is significantly higher than the external reproducibility of our measurements (< 0.1 parts per thousand) based on multiple measurements of Ca isotopic standards SRM 915a, SRM 915b and seawater (NASS-6). Mass balance calculations based on Sr/Ca and delta Ca-44/40 values rule out the contributions of secondary silicate as well as carbonate minerals in causing the variability of delta Ca-44/40 in the weathered samples. The positive correlations between delta Ca-44/40 and Mg/Al, Mg/Na in the weathered samples further negate the possible contribution of secondary carbonates in causing the variability in delta Ca-44/40, as Al and Na are unlikely to be present in carbonates. The samples with lower delta Ca-44/40 show higher CIA (Chemical Index of Alteration) values and Al/Ca than the UW diabase while samples having higher delta Ca-44/40 show lower CIA and Al/Ca than the UW diabase. The weathered sample having the lowest delta Ca-44/40 (close to the delta Ca-44/40 of plagioclase) exhibits the highest value of europium anomaly (Eu/Eu*). These observations are explained by the presence of variable relative proportions of residual clinopyroxene and plagioclase in the weathered diabase, due to selective weathering of these minerals. Since plagioclase and clinopyroxene could not be separated from the diabase, we measured the Ca isotopic compositions of a clinopyroxene (delta Ca-44/40= 1.06 parts per thousand) from the San Carlos mantle peridotite and plagioclase from the Chilka anorthosite complex (delta Ca-44/40= 0.40 parts per thousand) from the Eastern Ghats and used these isotopic values as representative of the compositions of these minerals in the diabase. Based on mixing models, using delta Ca-44/40, Al/Ca and Sr/Ca of the plagioclase and clinopyroxene end-members, we estimate that the delta Ca-44/40 of the UW diabase can be explained by 31 parts per thousand clinopyroxene and 69 parts per thousand plagioclase, which is consistent with the modal proportion of these minerals in an average unweathered diabase. Based on mixing calculations, the delta Ca-44/40, Sr/Ca and Al/Ca variability of the micro-drilled samples of the weathered diabase are explained by the presence of varying proportions of residual clinopyroxene (20-65 parts per thousand) and plagioclase (80-35 parts per thousand) in the weathered rock. This study demonstrates that selective weathering of major rock-forming minerals in nature can result in significant variation in delta Ca-44/40 in weathered rocks and that selective weathering of rock-forming minerals should be considered as an additional mechanism to explain the delta Ca-44/40 variability in rivers.

Whole‐rock geochemistry and zircon Hf isotope of Late Carboniferous–Triassic sediments in the Bogda region, NW China: Clues for provenance and tectonic setting

Detailed petrological, whole‐rock geochemical, and zircon Hf isotopic researches were conducted on the Late Carboniferous–Triassic sedimentary rocks in the Bogda region to evaluate the effects of weathering, sorting, and alteration, as well as to understand their provenance and tectonic setting. These rocks show variable major element compositions, but most are generally similar to post‐Archean Australian shales. The relatively narrow range of TiO2–Zr variation and good correlation between Th/Sc and Zr/Sc signify no obvious sorting and recycling of the sedimentary rocks. The relatively high index of compositional variability (0.8–3.1) and low chemical index of alteration values (44.4–76.4) of the sedimentary rocks indicate that they are immature and probably undergo weak to moderate chemical weathering. In the chondrite‐normalized diagrams, almost all the samples are distinguished by moderately enriched light rare earth element and flat heavy rare earth element patterns with negative Eu anomalies (Eu/Eu* = 0.5–0.8). In conjunction with their Th/Sc, Zr/Sc, La/Th, Zr/Al2O3, TiO2/Zr, Co/Th, and La/Sc ratios, we infer that the major sources of these sedimentary rocks were the intermediate–acidic igneous rocks from the North Tianshan and Yili–Central Tianshan. Combining the orientation of the Bogda region in the Palaeozoic that was roughly perpendicular to the Tianshan and/or East Junggar orogens with the sedimentologic, petrologic, and tectonic researches, we suggest that the Bogda region was a rift basin that occurs at high angles to the orogenic belt. On the other hand, the zircons from these sediments have minor Precambrian grains (&lt;0.2%), positive εHf(t) values (mostly +7.1–+15.0) and young 2‐stage Hf model ages (major peak &lt;1,000 Ma), suggesting that a juvenile continental crust exists as the basement of the Junggar Basin.

Geochemistry of sediments of the Peninsular rivers of the Ganga basin and its implication to weathering, sedimentary processes and provenance

Abstract Geochemistry of the Himalayan rivers has been a major point of contention in sedimentary geochemistry because of its global impact on ocean chemistry and climate. Among the Himalayan rivers, the Ganga and its Himalayan catchment have been studied extensively. However, the Peninsular rivers constituting a significant part of the Ganga basin have not received much attention, heretofore. In the present geochemical study of the Peninsular river's sediments, we have found that the initial chemical weathering of rocks in the catchment produces texturally, mineralogically and geochemically diverse sediments and fluvial processes further segregate them into the geochemically contrasting coarse channel and fine-grained suspended sediments. The overbank sediments possess an intermediate texture and geochemistry, but more towards the suspended sediments. The sediment geochemistry data indicates that the Peninsular river's sediments show an increase in the weathering intensity from the west (Banas river) to the east (Son river) along the climate gradient, i.e., from the arid-semiarid to the sub-tropical condition. The higher chemical index of alteration (CIA) values of the Peninsular river's sediments than the Yamuna and Ganga river sediments reflect the control of weatherable lithology in the tectonically stable, low relief and subtropical climate condition. The Yamuna and Ganga river sediments with lower CIA values show more physical weathering in the tectonically active Himalaya. The suspended and overbank sediments of the Yamuna and Ganga rivers show an increase in their CIA values after the confluence of the Peninsular rivers, indicating the contribution of more weathered materials by the Peninsular rivers. It has been found that there is a geochemical split between the channel and suspended sediments of the Peninsular rivers for sharing different sources, i.e., the dominant contribution of felsic sources to the channel and mafic sources to the suspended sediments. The weathering geochemistry, trace element systematics, and REE patterns suggest that the dominant sources of the channel sediments of the Peninsular rivers are the felsic crystallines and sedimentary lithologies of the Aravalli range, Bundelkhand and Chhotanagpur granite and gneisses, and Vindhyan sandstones. Whereas, the mafic lithologies such as the Deccan traps and mafic components of the felsic lithologies appear to be the major sources of the suspended sediments. Within the range of intermediate composition, the overbank sediment chemistry reflects more contribution of the mafic sources except for the Ken which shows a higher contribution of the felsic sources. The geochemical split between the channel and suspended sediments of the Peninsular river's sediments for the different provenances indicates differential weathering of the Deccan basalts, Bundelkhand crystalline, and Vindhyan sedimentary rocks; and also the hydrodynamic control of the Peninsular rivers during erosion, transport, and deposition. However, the Himalayan rivers do not show such contrast between the channel and suspended sediments because of the higher physical weathering and sediment mixing in the high gradient topography of the Himalayan catchment.