Chemical Index Of Alteration Values Research Articles

Provenance and geochemistry of Lower to Middle Permian strata in the southern Junggar and Turpan basins: A terrestrial record from mid-latitude NE Pangea

The Lower to Middle Permian terrestrial successions in the southern Junggar and Turpan basins provide a wealth of information regarding the paleoclimatic and paleogeographic evolution of mid-latitude NE Pangea. Two sections, namely the Xiaolongkou section in the southern Junggar Basin and the southwest (SW) Tarlong section in the Turpan Basin, were systematically studied using detrital zircon geochronology and whole-rock geochemical analysis. Stratigraphic studies suggest that, in both sections, the Lucaogou and Hongyanchi formations are dominated by lacustrine deposits, while the Quanzijie Formation represents mixed fluvial-lacustrine and loess environments. The Lucaogou Formation displays low CIA (Chemical Index of Alteration) values and Th/U ratios, suggesting semiarid to arid conditions in both sections. The Hongyanchi Formation shows low to intermittently moderate CIA values and Th/U ratios, indicating variably semiarid-arid conditions alternating with subhumid-semiarid episodes. The lower part of the Quanzijie Formation reflects subhumid-semiarid to semiarid-arid conditions, while the upper part of the Quanzijie Formation mainly records semiarid-arid and shifts into subhumid conditions toward the top. Detrital zircon and element data indicate that, within each section, all three formations display similar zircon age distributions and geochemical characteristics, suggesting no significant changes in provenance during deposition. However, the provenance characteristics of the two sections differ from each other; the Xiaolongkou section is rich in felsic material, with two detrital zircon age peaks, while the SW Tarlong section contains more mafic components, with a single peak in detrital zircon age spectrum. This suggests that the greater Junggar-Turpan Basin was highly partitioned with several independent provenance systems active during the Early-Middle Permian. The abrupt change in depositional environments in both sections may have resulted from uplift of the North Tian Shan, which induced rapid regression of the lakeshore and uplift of sediment source and catchment areas.

The elemental geochemistry of Lower Triassic shallow-marine carbonates from central Saudi Arabia: Implications for redox conditions in the immediate aftermath of the latest Permian mass extinction

The southern margin of the Tethys Ocean was occupied by a broad, shallow continental shelf during the Permian-Triassic boundary interval, with the area of present-day Saudi Arabia located from 10° to 30° south of the paleo-equator. The strata deposited in modern Saudi Arabia in the aftermath of the latest Permian mass extinction (LPME) are dominated by oolitic microbialite limestone (OML), which are overlain by skeletal oolitic limestones (SOL) capped by dolostones and dolomitic limestones (DDL). This succession reflects changes in depositional setting, which can be potentially tied to redox conditions using redox sensitive trace elements and rare earth elements (REEs). Statistical analyses reveals that trace elements and REEs are associated with detrital material, and possibly with diagenetic minerals as well. Proxies such as the Y/Ho, Pr/Pr*, Smn/Ybn, Lan/Smn and Lan/Ybn ratios indicate that REEs do not record a seawater-like pattern, and cannot be used as redox indicator. The presence of a normal marine fauna implies oxic conditions during deposition of the DDL and SOL units. However, the OML unit, which represents the immediate aftermath of LPME, lacks both a normal marine fauna and reliable geochemical signals, making it difficult to infer redox conditions in the depositional environment. Similar to published data from sections that reflect shallow marine condition in the LPME of the Tethys Ocean, chemical index of alteration values are consistently high throughout the study succession, suggesting globally intense chemical weathering in the aftermath of the LPME. As a result, geochemical redox proxies in shallow marine carbonates of the Tethys Ocean are likely to be contaminated by detrital material that have been generated by chemical weathering, and thus, other methods are required to determine depositional redox conditions.

Fluctuations in chemical weathering on the Yangtze Block during the Ediacaran–Cambrian transition: Implications for paleoclimatic conditions and the marine carbon cycle

The Ediacaran–Cambrian transition was characterized by profound climatic, tectonic, and oceanographic fluctuations, followed by the remarkable radiation of metazoans. To further understand the paleoclimatic and paleoenvironmental conditions during this interval, comprehensive mineralogical and geochemical analyses were performed on Ediacaran–Cambrian shales (in the Doushantuo through Jiumenchong to Bianmachong formations) at Daotuo, South China. Their major and trace element compositions and ratios indicate that all of the studied shales were derived from basically uniform source rocks that were similar to granodiorite. The chemical index of alteration (CIA) values in the Doushantuo and Bianmachong formations mostly range from 65 to 80, indicating that they experienced a moderate degree of chemical weathering and were thus deposited in a warm and humid climate. In contrast, a sharp drop in CIA values (~60–70) in the lower Jiumenchong Formation, together with decreased Al/Ti ratios that are indicative of increased eolian inputs, are suggestive of a relatively cold and arid climate for the Early Cambrian Yangtze Block. In this scenario, eolian fluxes could have supplied nutrients to the marine environment; increased upwelling, along with a rising thermocline and nutricline during the extensive marine transgression, could have returned abundant nutrients to the surface seawater, thus greatly stimulating primary productivity and intensifying anoxic conditions, which would ultimately have resulted in a high degree of organic burial in the sediments. Therefore, the negative correlation between the corrected CIA values and the total organic carbon (TOC) content may imply that climate-related changes played a role in the enrichment of organic matter during the Early Cambrian.

Impact of incipient weathering on uranium migration in granitic waste rock piles from former U mines (Limousin, France)

This study investigates the impact of incipient weathering on uranium (U) migration at the surface of granitic waste rock piles from former U mines in Limousin (France) operated half a century ago. We used a multi-scale mineralogical and geochemical approach to evaluate the impact of incipient weathering on uranium mobility in a chaotic waste rock pile. The surficial part of five former mines located in the “La Crouzille” and “La Creuse” mining districts was investigated. Four groups of samples were defined based on field observations and mineralogy, granites, lamprophyres, hydrothermally altered rocks and weathering products. Petrology, mineralogy and geochemistry investigations show the importance of mineralogical and geochemical transformations during incipient weathering over a few decades. The Chemical Index of Alteration (CIA) is successfully used to assess the weathering state of samples in the waste rock pile chaotic deposit. The mean CIA value is around 60 for unweathered/incipiently weathered granites whereas highly weathered samples have CIA around 70. This means that no significant major element loss occurs in the granitic rocks suffering incipient weathering. Various micron-size secondary U minerals consist in a large diversity of oxidized uranyl minerals (uranophane, uranyl sulfate, curite, autunite and other uranyl phosphates), with dominant uranyl phosphates, indicating local U mobility in the waste rock pile. Autunites micromorphology indicates different degrees of weathering of the samples in which it crystallizes. U is primarily concentrated in the clay fraction (<2μm), associated with secondary minerals from granite alteration (clay minerals such as smectite and vermiculite, Fe-oxides and phosphate minerals) and, to a limited extent, relict primary minerals. These weathering products contribute to U(VI) stabilization under oxidizing conditions.

Depositional environments, provenance and paleoclimatic implications of Ordovician siliciclastic rocks of the Thango Formation, Spiti Valley, Tethys Himalaya, northern India

Recently published findings indicate that the Ordovician period has been much more dynamic than previously anticipated thus making this period significant in geological time. The Ordovician of India can best be studied in the Spiti region because the Spiti basin records the complete uninterrupted history of excellent marine sedimentary rocks starting from Cambrian to Paleogene which were deposited along the northern margin of India. Due to these reasons the geochemical data on the Ordovician rocks from the Spiti region is uncommon. The present geochemical study on the Ordovician Thango Formation (Sanugba Group) is mainly aimed to understand the provenance and the paleoclimatic conditions. The sandstones are the dominant lithology of the Thango Formation with intercalations of minor amount of shales. Detailed petrographic and sedimentological analysis of these rocks suggest that three major depositional environments, viz., fluvial, transitional and marine prevailed in the basin representing transgressive and regressive phases. The major and trace element ratios such as SiO2/Al2O3, K2O/Na2O and La-Th- Sc discrimination diagram suggest that these rocks were deposited in passive margin tectonic settings. Various geochemical discriminants and elemental ratios such as K2O/Na2O, Al2O3/TiO2, La/Sc, Th/Sc, Cr/Th, Zr/Sc, (Gd/Yb)N and pronounced negative Eu anomalies indicate the rocks to be the product of weathering of post-Archean granites. The striking similarities of the multi-elemental spider diagrams of the studied sediments and the Himalayan granitoids indicate that sediments are sourced from the Proterozoic orogenic belts of the Himalayan region. Chemical index of alteration (CIA) values of the studied sediments (55–72) suggest that the source rocks underwent low to moderate degree of chemical weathering. The span of the CIA values (55–72) recorded in the sediments from the Spiti region may have resulted from varying degrees of weathering conditions in the source area. This, in turn, reflects variable climatic conditions prevailing during the Ordovician period, which is consistent with the global studies. The high CIA values observed by numerous studies on siliciclastic sediments may imply warm paleoclimate attributed to an increase in atmospheric carbon dioxide and/or methane levels inducing an acidic weathering environment and enhanced silicate weathering. While the low CIA values indicates prevalence of arid-glacial conditions where negligible chemical alteration is possible. This study documents the noteworthy events like fluctuating (rise and fall) atmospheric CO2 and sea levels during the Ordovician period, a conclusion which is consistent with the other such studies on the Phanerozoic glaciation (or mass extinction) events on Gondwana supercontinent.

Geochemical and Nd-isotopic compositions of juvenile-type Paleoproterozoic Birimian sedimentary rocks from southeastern West African Craton (Ghana): Constraints on provenance and tectonic setting

The Paleoproterozic Birimian Supergroup consists of sedimentary basins composed predominantly of wackes and argillites, and volcanic belts composed predominantly of basaltic rocks. Whole rock geochemical and Nd-isotopic studies were carried out on fine-grained sedimentary rocks (shales and fine-grained sandstones) from one of the Birimian sedimentary basins, the Kumasi basin. The sedimentary rocks are immature in nature, with very little recycled materials. Relative to average early Proterozoic upper continental crust, the Birimian sedimentary rocks are depleted in the alkalis and alkaline earth elements and the high field strength elements, and enriched in transition metals. Chondrite-normalized rare earth element (REE) plots for both the shales and sandstones show fractionated LREE patterns (average LaN/SmN=3.45), with slight negative Eu-anomalies (average Eu/Eu∗=0.79) and a slightly fractionated HREE patterns (average GdN/YbN=1.84). Low to moderate degree of chemical weathering in the sediment source area is indicated by Chemical Index of Alteration values of between 52 and 80. Elemental ratios such as La/Co, Th/Co, Th/Cr, and Eu/Eu∗ that are critical to provenance suggests that the sediments were derived from rocks of mixed felsic and mafic compositions. Mixing calculations based on the REEs indicate that, the shale is best modeled with a mixture having 62% TTG, 16% granite and 22% basalt whereas the sandstone is almost 100% TTG. The εNd values for the analyzed samples, calculated at the age of deposition (i.e., 2.1Ga), range from −1.69 to 1.07, suggesting derivation from an undifferentiated source. The Nd-isotopic compositions, together with Eu/Eu∗ and Th/U values and other trace element characteristics suggest Young Undifferentiated Arc provenance, and this is in agreement with previous studies that show that the Birmian crust is juvenile and little amount of Archean crust is present in the sedimentary rocks. The geochemical and Nd-isotopic systematics further suggest that the bulk of detritus was supplied by adjacent Paleoproterozoic Birimian greenstone belts with only 10% or less contribution from the Archean crust. Trace element characteristics further suggest that deposition took place in an island arc setting.

Evidence for a chondritic impactor, evaporation-condensation effects and melting of the Precambrian basement beneath the ‘target’ Deccan basalts at Lonar crater, India

The ∼1.88km diameter Lonar impact crater formed ∼570ka ago and is an almost circular depression hosted entirely in the Poladpur suite of the ∼65Ma old basalts of the Deccan Traps. To understand the effects of impact cratering on basaltic targets, commonly found on the surfaces of inner Solar System planetary bodies, major and trace element concentrations as well as Nd and Sr isotopic compositions were determined on a suite of selected samples composed of: basalts, a red bole sample, which is a product of basalt alteration, impact breccia, and impact glasses, either in the form of spherules (<1mm in diameter) or non-spherical impact glasses (>1mm and <1cm). These data include the first highly siderophile element (HSE) concentrations for the Lonar spherules. The chemical index of alteration (CIA) values for the basalts and impact breccia (36.4–42.7) are low while the red bole sample shows a high CIA value (55.6 in the acid-leached sample), consistent with its origin by aqueous alteration of the basalts. The Lonar spherules are classified into two main groups based on their CIA values. Most spherules show low CIA values (Group 1: 34.7–40.5) overlapping with the basalts and impact breccia, while seven spherules show significantly higher CIA values (Group 2: >43.0). The Group 1 spherules are further subdivided into Groups 1a and 1b, with Group 1a spherules showing higher Ni and mostly higher Cr compared to the Group 1b spherules. Iridium and Cr concentrations of the spherules are consistent with the admixture of 1–8wt% of a chondritic impactor to the basaltic target rocks. The impactor contribution is most prominent in the Group 1a and Group 2 spherules, which show higher Ni/Co, Ni/Cr and Cr/Co ratios compared to the target basalts. In contrast, the Group 1b spherules show major and trace element compositions that overlap with those of the impact breccia and are characterized by high EFTh (Enrichment Factor for Th defined as the Nb-normalized concentration of Th relative to that of the average basalt) as well as fractionated La/Sm(N), and higher large ion lithophile element (LILE) concentrations compared to the basalts. The relatively more radiogenic Sr and less radiogenic Nd isotopic composition of the impact breccia and non-spherical impact glasses compared to the target basalts are consistent with melting and mixing of the Precambrian basement beneath the Deccan basalt with up to 15wt% contribution of the basement to these samples. Variations in the moderately siderophile element (MSE) concentration ratios of the impact breccia as well as all the spherules are best explained by contributions from three components – a chondritic impactor, the basaltic target rocks at Lonar and the basement underlying the Deccan basalts. The large variations in concentrations of volatile elements like Zn and Cu and correlated variations of EFCu-EFZn, EFPb-EFZn, EFK-EFZn and EFNa-EFZn, particularly in the Group 1a spherules, are best explained by evaporation-condensation effects during impact. While most spherules, irrespective of their general major and trace element composition, show a loss in volatile elements (e.g., Zn and Cu) relative to the target basalts, some spherules, mainly of Group 1, display enrichments in these elements that are interpreted to reflect the unique preservation of volatile-rich vapour condensates resulting from geochemical fractionation in a vertical direction within the vapour cloud.

Mixed-layer illite-vermiculite as a paleoclimatic indicator in the Pleistocene red soil sediments in Jiujiang, southern China

The mineralogy and chemical composition of the Jiujiang red soil sediments were investigated using X-ray diffraction (XRD), X-ray fluorescence (XRF), and inductively coupled plasma mass spectrometry (ICP-MS). Mixed-layer illite–vermiculite (I/V) was confirmed based on the conditions that a broad peak of 1.0 to 1.4nm appeared in XRD after Mg-saturation, and its d-spacing did not change after glycol treatment, but collapsed to 1.0nm after heating, owing to the loss of water and hydroxy-cations from its interlayer. The mixed-layer I/V contents fluctuate throughout the section but show a decreasing trend upwards, suggesting decreased weathering towards the surface of the section. The chemical index of alteration (CIA) values and Rb/Sr ratios also exhibit upward decreasing trends, jointly revealing the climate shift from warm/humid to cool/dry from the Middle to the Late Pleistocene. Although the general climate evolution can be evaluated by CIA values and Rb/Sr ratios, sub-climatic events within the Jiujiang red soil sediments cannot be identified. The SiO2/Al2O3, Al2O3/Fe2O3, and SiO2/Fe2O3 ratios clearly fluctuate along the section, especially within the net-like red soil sediments, but they do not reveal multiple climate cycles. This is because of an inhomogeneous distribution of SiO2, Al2O3, and Fe2O3 in the white veins and the red matrix and also non-uniform sampling. The variation in the content of mixed-layer I/V show a strong positive correlation with changes in magnetic susceptibility of loess-paleosol sediments and in marine oxygen isotope ratios, revealing multiple sub-climate cycles on a global scale since the Middle Pleistocene. Therefore, the mixed-layer I/V contents not only reveal the whole climate trend documented in the red soil sediments since the Middle Pleistocene, but also disclose the sub-climate cycles related to the deposit-pedogenesis process.

Geochemistry, environmental and provenance study of the Middle Miocene Leitha limestones (Central Paratethys)

Abstract Mineralogical, major, minor, REE and trace element analyses of rock samples were performed on Middle Miocene limestones (Leitha limestones, Badenian) collected from four localities from Austria (Mannersdorf, Wöllersdorf, Kummer and Rosenberg quarries) and the Fertőrákos quarry in Hungary. Impure to pure limestones (i.e. limited by Al2O3 contents above or below 0.43 wt. %) were tested to evaluate the applicability of various geochemical proxies and indices in regard to provenance and palaeoenvironmental interpretations. Pure and impure limestones from Mannersdorf and Wöllersdorf (southern Vienna Basin) show signs of detrital input (REEs = 27.6 ± 9.8 ppm, Ce anomaly = 0.95 ± 0.1 and the presence of quartz, muscovite and clay minerals in impure limestones) and diagenetic influence (low contents of, e.g., Sr = 221 ± 49 ppm, Na is not detected, Ba = 15.6 ± 8.8 ppm in pure limestones). Thus, in both limestones the reconstruction of original sedimentary palaeoenvironments by geochemistry is hampered. The Kummer and Fertőrákos (Eisenstadt–Sopron Basin) comprise pure limestones (e.g., averages Sr = 571 ± 139 ppm, Na = 213 ± 56 ppm, Ba = 21 ± 4 ppm, REEs = 16 ± 3 ppm and Ce anomaly = 0.62 ± 0.05 and composed predominantly of calcite) exhibiting negligible diagenesis. Deposition under a shallow-water, well oxygenated to intermittent dysoxic marine environment can be reconstructed. Pure to impure limestones at Rosenberg–Retznei (Styrian Basin) are affected to some extent by detrital input and volcano-siliciclastic admixture. The Leitha limestones at Rosenberg have the least diagenetic influence among the studied localities (i.e. averages Sr = 1271 ± 261 ppm, Na = 315 ± 195 ppm, Ba = 32 ± 15 ppm, REEs = 9.8 ± 4.2 ppm and Ce anomaly = 0.77 ± 0.1 and consist of calcite, minor dolomite and quartz). The siliciclastic sources are characterized by immobile elemental ratios (i.e. La/Sc and Th/Co) which apply not only for the siliciclastics, but also for marls and impure limestones. At Mannersdorf the detrital input source varies between intermediate to silicic igneous rocks, while in Kummer and Rosenberg the source is solely silicic igneous rocks. The Chemical Index of Alteration (CIA) is only applicable in the shale-contaminated impure limestones. CIA values of the Leitha limestones from Mannersdorf indicate a gradual transition from warm to temperate palaeoclimate within the limestone succession of the Badenian.

Zircon U–Pb dating and whole-rock elemental geochemistry of the Shazi anatase deposit in Qinglong, Western Guizhou, SW China

The Shazi deposit is a large-scale anatase deposit in Qinglong, Guizhou Province. Zircon grains from this deposit yielded a zircon U–Pb age of ~259 Ma, representing the formation age of the deposit’s parent rocks. This age is identical to the eruption age of the Emeishan large igneous province, indicating a synchronous magmatic event. The rare-earth-element patterns of laterite samples were similar to those of the weathered basalt sample, and sub-parallel to those of the Emeishan high-Ti basalts, implying a genetic relationship between the laterite and the basalt. The Chemical Index of Alteration values of laterite ranged from 96 to 98, suggesting a high degree of weathering. SiO2, MgO, and alkaline metal elements decreased with increasing degree of weathering, while Al2O3, Fe2O3, and TiO2 increased. We found the highest TiO2 in laterite and the lowest in pillow basalts, indicating that Ti migrated from basalt to laterite. Our U–Pb dating and whole-rock elemental geochemistry analyses suggest that the Emeishan basalt is the parent rock of the Shazi anatase ore deposit. Based on our analysis, we propose a metallogenic model to explain the ore-forming process, in which the karst terrain formed by the Emeishan mantle plume and the subsequent basaltic magma eruption were the key factors in the formation the Shazi anatase ore deposit.

High-resolution clay mineral and major elemental characterization of a Permian-Triassic terrestrial succession in southwestern China: Diagenetic and paleoclimatic/paleoenvironmental significance

Relatively high-resolution clay mineralogical and chemical index of alteration (CIA) analyses were conducted on the terrestrial Lubei section in southwestern China, to gain a better understanding of the paleoclimatic/paleoenvironmental evolution on land during the Permian-Triassic transition (P-Tr transition). Clay-mineral species include illite-smectite mixed layers (I/S), illite, irregular chlorite-expandable mixed layers (C/E), chlorite, kaolinite, vermiculite, hydroxyl-interlayered mineral (HIM), and corrensite. CIA values are as high as approaching 100 in the Permian and show a gradual and subtle decrease towards the lowermost Triassic, where a sudden decrease to persistently <60 occurs with only periodic positive excursions. Diagenetic alteration is important at the study section and include chloritization, illitization of smectite in the Permian-lowest Triassic, transformation of smectite to corrensite in the upper sampled Triassic and potassium-addition induced decrease of CIA values throughout the section. However, with the integration of climate-sensitive sedimentary features, paleoclimatic/paleoenvironmental information was extracted from both clay-mineral composition and CIA. The dominance of precursor smectite together with the occurrence of kaolinite, Gigantopteris-type flora and coal seams, indicates an overall warm and semi-humid to semi-arid climate for the Permian part, and a general drying-out trend is suggested by the gradual decrease and final disappearance upsection of kaolinite, Gigantopteris-type flora and coal seams, as well as decreasing CIA values. Chemical weathering decrease is minor up to the lowest Triassic, which is mainly manifested by the subtle decrease of CIA values and which is probably partly due to the augmentation by acidification as indicated by the presence of vermiculite and HIM clay minerals. The short-term lingering (preliminarily estimated as <0.08Myr) of relatively intense chemical weathering after the extinction event was followed by an abrupt drop of CIA values to <60, which indicates the onset of a rapid and enhanced dry-out condition unfavorable for chemical weathering. In association with the general drying-out trend is an increase in climate periodicity and/or seasonality, which is manifested by increasingly prominent laminations, lithological alternation and intermittent large excursions of CIA values, suggesting a more complicated climate condition during the Early Triassic than previously recognized. While drying-out rather than acidification is suggested in this study to have been a killer agent to land plant at the Lubei section, the real relationship between environmental deterioration and the end-Permian mass extinction is still enigmatic and needs further investigation. This is the first relative high-resolution clay mineralogical and CIA investigation on a well-preserved and continuous terrestrial Permian-Triassic section.

Physicochemical, Geochemical and Mineralogical Aspects of Agricultural Soils in Limpopo Province, South Africa

ABSTRACTSoil samples were collected from three selected sites (uncultivated, cultivated and grazing land) at University of Limpopo Experimental Farm (Syferkuil) in Mankweng area to characterise their physicochemical, geochemical and mineralogical properties and their influence on soil fertility for agriculture. Soil textural triangle using Texture Auto Lookup Software (TAL 4.2) Package was used to determine particle sizes of samples which were all dominated by a loamy texture. Chemical characterisation carried out included determination of exchangeable cations, major elemental oxides and trace elements. Silica was the dominant oxide with the highest weight percentage values ranging from 75.99 to 83.45. Chemical index of alteration values of 66 to 75 depicted moderate silica weathering with depletion of soil nutrients due to leaching. Seven minerals were identified i n soil samples which were dominated by kaolinite. The presence of mica and smectites in the soils displayed properties typical of soils rich ...

Varying Contents of Sources Affect Tectonic-Setting Discrimination of Sediments: A Case Study from Permian Sandstones in the Eastern Tianshan, Northwestern China

AbstractThis article reports new geochemical data from Permian sandstones, most probably deposited during a period of postcollisional extension, from the Yamansu, Shaquanzi, and Xingxingxia areas in the Eastern Tianshan, northwestern China, to determine and characterize their provenance and source nature. The medium- to coarse-grained sandstones are composed mainly of angular to subangular quartz and volcanic fragments, with minor plagioclase and/or microcline, suggesting proximal deposition. Although the samples show relatively large variations in SiO2 (59.7–70.8 wt%), Al2O3 (9.0–14.9 wt%), and Fe2O3T + MgO (2.3–5.2 wt%) contents, they yield low chemical index of alteration values (41–55) but high index of compositional variability values (1.06–1.78), suggestive of their derivation mainly from compositionally immature sources with weak chemical weathering. Their relatively low Zr concentrations (99–225 ppm) and GdN/YbN ratios (1.20–1.53) manifest insignificant zircon and monazite enrichments. Most of the...

Late Triassic sedimentary records in the northern Tethyan Himalaya: Tectonic link with Greater India

The Upper Triassic flysch sediments (Nieru Formation and Langjiexue Group) exposed in the Eastern Tethyan Himalayan Sequence are crucial for unraveling the controversial paleogeography and paleotectonics of the Himalayan orogen. This work reports new detrital zircon U-Pb ages and whole-rock geochemical data for clastic rocks from flysch strata in the Shannan area. The mineral modal composition data suggest that these units were mainly sourced from recycled orogen provenances. The chemical compositions of the sandstones in the strata are similar to the chemical composition of upper continental crust. These rocks have relatively low Chemical Index of Alteration values (with an average of 62) and Index of Compositional Variability values (0.69), indicating that they experienced weak weathering and were mainly derived from a mature source. The geochemical compositions of the Upper Triassic strata are similar to those of graywackes from continental island arcs and are indicative of an acidic-intermediate igneous source. Furthermore, hornblende and feldspar experienced decomposition in the provenance, and the sediment became enriched in zircon and monazite during sediment transport. The detrital zircons in the strata feature two main age peaks at 225–275 Ma and 500–600 Ma, nearly continuous Paleoproterozoic to Neoproterozoic ages, and a broad inconspicuous cluster in the Tonian–Stenian (800–1200 Ma). The detrital zircons from the Upper Triassic sandstones in the study area lack peaks at 300–325 Ma (characteristic of the Lhasa block) and 1150–1200 Ma (characteristic of the Lhasa and West Australia blocks). Therefore, neither the Lhasa block nor the West Australia blocks likely acted as the main provenance of the Upper Triassic strata. Newly discovered Permian–Triassic basalt and mafic dikes in the Himalayas could have provided the 225–275 Ma detrital zircons. Therefore, Indian and Himalayan units were the main provenances of the flysch strata. The Tethyan Himalaya was part of the northern passive margin and was not an exotic terrane separated from India during the Permian to Early Cretaceous. This evidence suggests that the Neo-Tethyan ocean opened prior to the Late Triassic and that the Upper Triassic deposits were derived from continental crustal fragments adjacent to the northern passive continental margin of Greater India.

Geochemistry of Oligocene Huagang Formation clastic rocks, Xihu Sag, the East China Sea Shelf Basin: Provenance, source weathering, and tectonic setting

The East China Sea Shelf Basin (ECSSB) is an important tectonic unit of the western Pacific active continental margin. It is divided further into a series of sags and basement ridges. The Xihu Sag on the east margin of the ECSSB is an important hydrocarbon exploration area and contains significant petroleum resources. The Huagang Formation is the major reservoir in Xihu Sag, and its provenance has been the focus of debate at all times. The major and trace elements (including rare earth elements) compositions were analyzed for the Oligocene Huagang Formation sandstone and mudstone samples in the Xihu Sag. The provenance and tectonic setting of these clastic rocks are discussed. The Chemical Index of Alteration value and A–CN–K diagrams suggest low to moderate weathering in the source areas of the Huagang Formation. The mudstones have lower index of compositional variation values (&lt;1), indicating that the mudstone samples are compositionally mature, whereas the index of compositional variation values for the sandstone samples (0.59 to 1.87) indicate that the sandstones are compositionally immature and mature. The major and trace element composition and ratios of the clastic rock samples indicate that mudstones are mainly from Late Mesozoic igneous rocks in SE China, which is dominated by the acidic volcanic rock. The provenance of sandstones is from two different directions, the Central Uplift Group of ECSSB and the Taiwan–Sinzi Fold Belt with more mafic volcanic rock. Based on the geochemical analysis of the clastic rock samples, the tectonic setting of Xihu Sag was Japanese‐type continental margin during the Huagang Formation deposition time (Oligocene).

Sorting out compositional trends in sedimentary rocks of the Bradbury group (Aeolis Palus), Gale crater, Mars

AbstractSedimentary rocks are composed of detrital grains derived from source rocks, which are altered by chemical weathering, sorted during transport, and cemented during diagenesis. Fluvio‐lacustrine sedimentary rocks of the Bradbury group, observed on the floor of Gale crater by the Curiosity rover during its first 860 Martian solar days, show trends in bulk chemistry that are consistent with sorting of mineral grains during transport. The Bradbury group rocks are uniquely suited for sedimentary provenance analysis because they appear to have experienced negligible cation loss (i.e., open‐system chemical weathering) at the scale of the Alpha Particle X‐ray Spectrometer bulk chemistry analyses based on low Chemical Index of Alteration values and successful modeling of &gt;90% of the (volatile‐free) targets as mixtures of primary igneous minerals. Significant compositional variability between targets is instead correlated to grain‐size and textural characteristics of the rocks; the coarsest‐grained targets are enriched in Al2O3, SiO2, and Na2O, whereas the finer‐grained targets are enriched in mafic components. This is consistent with geochemical and mineralogical modeling of the segregation of coarse‐grained plagioclase from finer‐grained mafic minerals (e.g., olivine and pyroxenes), which would be expected from hydrodynamic sorting of the detritus from mechanical breakdown of subalkaline plagioclase‐phyric basalts. While the presence of a distinctive K2O‐rich stratigraphic interval shows that input from at least one distinctive alkali‐feldspar‐rich protolith contributed to basin fill, the dominant compositional trends in the Bradbury group are consistent with sorting of detrital minerals during transport from relatively homogeneous plagioclase‐phyric basalts.

Discussion of Suitable Chemical Weathering Proxies in Sediments by Comparing the Dissolution Rates of Minerals in Different Rocks

In studies of paleoclimate change, variations in major and minor elements are often used as proxies for chemical weathering in soil or lake sediments. To prove the suitability of chemical weathering proxies and further discuss the impact of mineral dissolution on element migrations, various parent rocks in Taiwan were chosen to react with sulfuric acid for discussing mineral dissolution rankings and element leaching during chemical weathering processes. The parent rocks include granite, andesite, actinolite schist, slate, quartz sandstone, calcium sandstone, and mudstone. Experimental results show that dissolution rates are mainly controlled by mineral structures and the chemical compositions of rocks. The ranking of rock dissolution rates is as follows: mudstone > slate > actinolite schist > quartz sandstone > granite. Dissolution rates of andesite and calcium sandstone cannot be compared due to secondary minerals being produced in experiments. Moreover, the dissolution sequences of minerals are of the following order: calcite > chlorite > muscovite and illite > augite > hornblende ≈ feldspar > quartz. This result is consistent with the stability of silicate polymerization. Most notable among the results was that the dissolution rates of clay minerals were faster than those of feldspar. Element migration ranking in dissolution processes is as follows: Mg > Ca ≈ Rb > K > Na > Sr > Al > Zr > Fe ≈ Ti. The dissolution rates of univalent elements follow the sequence of bonding forces; however, those of divalent elements are less ordered. This is due to divalent elements being rich in different mineral phases. Therefore, the study suggests that univalent elements are more suitable as chemical weathering proxy candidates. Moreover, whether ratios of Rb/Sr and K/Rb or chemical index of alteration values in sediments are suitable proxies of paleorainfall intensity is discussed in detail.

Geochemistry of Palaeoproterozoic Rocks of Aravalli Supergroup: Implications for Weathering History and Depositional Sequence

The Paleoproterozoic Aravalli Supregroup of rocks, hosted in Aravalli Craton of NW shield, is deposited in shallow and deep water conditions. The major lithologies are phyllites and quartzites with significant components of greywacks and dolomite. Geochemical indices in particular, CIA (Chemical Index of Alteration) values (avg. phyllites: 51.6 - 81.5, avg. quartzites: 57.4 - 95.5) calculated from the data of clastic rocks of ASG suggest minimum to highly intense weathering in the source region. Other indices including PIA, CIW and ICV along with plot patterns on the A-CN-K diagram also nearly endorse CIA based interpretation. These rocks possess relatively high Th/U ratios compared to that found in fresh igneous rocks or their high grade metamorphic equivalents. This high Th/U ratio is neither a source inheritance nor a result of oxidation state rather a manifestation of Th hosting mineral accumulation through sorting. Viewed in the context of present stratigraphic succession, the weathering history adduced from geochemistry does not seem compatible but matches well with earlier classification scheme wherein the evolution of Aravalli Supergroup was considered episodic.

Geochemistry and mineralogy of the Al-rich shale from Baluti formation, Iraqi Kurdistan region: implications for weathering and provenance

The mineralogical and geochemical characteristics of the Upper Triassic Baluti shale from the Northern Thrust Zone (Sararu section) and High Folded Zone (Sarki section) Kurdistan Region, Iraq, have been investigated to constrain their paleoweathering, provenance, tectonic setting, and depositional redox conditions. The clay mineral assemblages are dominated by kaolinite, illite, mixed layers illite/smectite at Sararu section, and illite > smectite with traces of kaolinite at Sarki. Illite, to be noted, is within the zone of diagenesis. The non-clay minerals are dominated by calcite with minor amounts of quartz and muscovite in Sararu shale; and are dominated by dolomite with amounts of calcite and quartz in Sarki shale. Baluti shale is classified as Al-rich based on major and minor elements. The chemical index of alteration (CIA) is significantly higher in the Sararu than the Sarki shales, suggesting more intense weathering of the Sararu than the Sarki shales. The index of compositional variability (ICV) of the Sararu shale is less than 1 (suggesting it is compositionally mature and was deposited in a tectonically quiescent setting). More than 1 for Sarki shales (suggest it is less mature and deposited in a tectonically active setting). Most shale of the Baluti plot parallel and along the A-K line in A-CN-K plots suggest intense chemical weathering (high CIA) without any clear-cut evidence of K-metasomatism. Clay mineral data, Al enrichment, CIA values, and A-CN-K plot suggest that the source area experienced high degree of chemical weathering under warm and humid conditions, especially in Sararu. Elemental ratios critical of provenance (La/Sc, Th/Sc, Th/Cr, Th/Co, Ce/Ce*PN, Eu/Eu*PN, and Eu/Eu*CN) shows slight difference between the Sararu and Sarki shales; and the ratios are similar to fine fractions derived from the weathering of mostly felsic rocks. The Eu/Eu*CN, Th/Sc, and low K2O/Al2O3 ratios of most shales suggest weathering from mostly a granodiorite source rather than a granite source, consistent with a source from old upper continental crust. Discrimination diagrams based on major and trace element content point to a role of the felsic-intermediate sources for the deposition of Baluti Formation, and probably mixed with mafic source rocks at Sararu section. The chondrite-normalized rare earth elements (REE) patterns are similar to those of PAAS, with light REE enrichment, a negative Eu anomaly, and almost flat heavy REE pattern similar to those of a source rock with felsic components. The source of sediments for the Baluti Formation was likely the Rutba Uplift and/or the plutonic-metamorphic complexes of the Arabian Shield located to the southwest of the basin; whereas the Sararu shale was affected by the mafic rocks of the Bitlis-Avroman-Bisitoun Ridge to the northeast of Arabian Plate. The tectonic discrimination diagrams, as well as critical trace and REE characteristic parameters imply rift and active setting for the depositional basin of the shale of Baluti Formation. The geochemical parameters such as U/Th, V/Cr, V/Sc, and Cu/Zn ratios indicate that these shales were deposited under oxic environment and also show that Sarki shale was deposited under more oxic environment than Sararu.

Mid- to late Holocene climate response from the Triloknath palaeolake, Lahaul Himalaya based on multiproxy data

A 2.13-m-thick palaeolake sequence deposited between the right lateral moraine and the valley wall of Triloknath glacier, has been studied at high resolution to understand the past climatic changes in Lahaul Himalaya. The OSL and AMS14C chronology suggests that the lake building activity was initiated post-cooling phase since ~6484cal.YBP. The combination of sedimentological, geochemical, and environmental magnetic parameters helped to infer five first-order climatic zones. The influx of magnetic minerals along with a notable increase in antiferromagnetic mineralogy (appearance of hematite) and higher chemical index of alteration (CIA) values during ~5925cal.YBP to 5676cal.YBP suggests warm and wet conditions owing to enhanced Indian Summer Monsoon (ISM) precipitation corresponding to the mid-Holocene warming. Further, reduction in the magnetic mineral concentration with few peaks of antiferromagnetic mineralogy and higher CIA values suggests warmer episodes within an overall cold climate, and the finer ferrimagnetic grain size suggests intermittent low energy and ponding conditions during ~3904cal.YBP to 2365cal.YBP. This was followed by fluctuating cold arid conditions in response to reduced ISM precipitation during ~2365cal.YBP and 1758cal.YBP. Further, between 1758cal.YBP to the present, episodic warming, pedogenic enhancement, and increased monsoon precipitation have been recorded. A major warm episode (Medieval warm period) as depicted from enhanced antiferromagnetic mineralogy and higher CIA values has been recorded between ~742cal.yrs. BP and 593cal.YBP. The sudden drop in the magnetic mineral concentration as well as the CIA value, around ~300cal.YBP (1650 CE) is likely because of colder conditions prevailing during the Little Ice Age (LIA). The study illustrates high sensitivity of the lake sediment sequence to witness the climatic episodes experienced in the glaciated regime of the Triloknath glacier.