Geochemistry of Mansar Lake sediments, Jammu, India: Implication for source-area weathering, provenance, and tectonic setting

The sediment geochemistry, including REE, of surface and core samples from Mansar Lake, along with mineralogical investigations, have been carried out in order to understand the provenance, source area weathering, hydrolic sorting and tectonic setting of the basin. The geochemical signatures preserved in these sediments have been exploited as proxies in order to delineate these different parameters.The major element log values (Fe₂O₃/K₂O) vs (SiO₂/Al₂O₃) and (Na₂O/K₂O) vs (SiO₂/Al₂O₃) demarcate a lithology remarkably similar to that exposed in the catchment area. The chondrite normalized REE patterns of lake samples are similar to Post Archaean Australian Shale (PAAS) with LREE enrichment, a negative Eu anomaly and almost flat HREE pattern similar to a felsic and/or cratonic sedimentary source. However, the La–Th–Sc plot of samples fall in a mixed sedimentary domain, close to Upper Continental Crust (UCC) and PAAS, suggesting sedimentary source rocks for the Mansar detritus. It also indicates that these elements remained immobile during weathering and transportation. The mineralogical characteristic, REEs, and high field strength elements (HFSE), together with the high percentage of metamorphic rock fragments in the Siwalik sandstone, support a metamorphic source for lower Siwalik sediments. A very weak positive correlation between Zr and SiO₂, poor negative correlation with Al₂O₃, negative correlation of (La/Yb)_N and (Gd/Yb)_N ratios with SiO₂ and positive correlation with Al₂O₃, suggest that Zr does not dominantly control the REE distribution in Mansar sediments. The petrographic character and textural immaturity indicate a short distance transport for the detritus. The distribution of elements in core samples reflect fractionation. The higher Zr/Th and Zr/Yb ratios in coarse sediments and PAAS compared to finer grained detritus indicate sedimentary sorting. Plots of the geochemical data on tectonic discrimination diagrams suggest that the sediments derived from the lower Siwalik were originated within a cratonic interior and later deposited along a passive margin basinal setting. It therefore reveals lower Siwalik depositional history.     

Measuring and adjusting the weathering and hydraulic sorting effects for rigorous provenance analysis of sedimentary rocks: a case study from the Jurassic Ashikita Group,south‐west Japan

The present study examines the provenance of the Jurassic Ashikita Group distributed in south‐west Japan, which is composed of the Idenohana, Kyodomari and Sakamoto Formations. Two geochemical diagrams for provenance analysis were utilized, which incorporate full consideration of compositional modifications resulting from weathering (MFW diagram) and hydraulic sorting processes (SiO₂/Al₂O₃–Na₂O/K₂O diagram). The MFW diagram delineates weathering trends of sedimentary rocks and allows estimation of the original source rock composition by tracing the weathering trends backwards to an unweathered domain. Weathering trends of the Idenohana and Kyodomari Formations extend backward to the domain of intermediate and felsic igneous rocks. In contrast, sediments of the Sakamoto Formation do not fit into a linear weathering trend, indicating that the source rock cannot be approximated to igneous rocks. On the SiO₂/Al₂O₃–Na₂O/K₂O diagram, sediments are organized into compositional trends, in which the range reflects compositional variations induced by the hydraulic sorting effect. On this diagram, sediments derived from the igneous and recycled sedimentary provenances can be distinguished by reading the inclination of the trend. By utilizing this principle, source rocks of the Idenohana and Kyodomari Formations are interpreted as igneous rocks and those of the Sakamoto Formation are interpreted as recycled sedimentary rocks. Therefore, these diagrams concurrently estimate the source rock composition through quantifying and adjusting the weathering and sorting effects, and reveal a systematic transition in the provenance of the Ashikita Group. The Idenohana and Kyodomari Formations were supplied chiefly from an igneous provenance, which shifted from intermediate to felsic compositions in stratigraphic order. Whereas, sediments of the Sakamoto Formation were sourced primarily from a recycled sedimentary provenance.     

Petrography and geochemistry of Cretaceous to quaternary siliciclastic rocks in the Tarfaya basin,SW Morocco: implications for tectonic setting,weathering, and provenance

The petrography, heavy mineral analysis, major element geochemical compositions and mineral chemistry of Early Cretaceous to Miocene–Pliocene rocks, and recent sediments of the Tarfaya basin, SW Morocco, have been studied to reveal their depositional tectonic setting, weathering history, and provenance. Bulk sediment compositional and mineral chemical data suggest that these rocks were derived from heterogeneous sources in the Reguibat Shield (West African Craton) including the Mauritanides and the western Anti-Atlas, which likely form the basement in this area. The Early Cretaceous sandstones are subarkosic in composition, while the Miocene–Pliocene sandstones and the recent sediments from Wadis are generally carbonate-rich feldspathic or lithic arenites, which is also reflected in their major element geochemical compositions. The studied samples are characterized by moderate SiO₂ contents and variable abundances of Al₂O₃, K₂O, Na₂O, and ferromagnesian elements. Binary tectonic discrimination diagrams demonstrate that most samples can be characterized as passive continental marginal deposits. Al₂O₃/Na₂O ratios indicate more intense chemical weathering during the Early Cretaceous and a variable intensity of weathering during the Late Cretaceous, Early Eocene, Oligocene–Early Miocene, Miocene–Pliocene and recent times. Moreover, weathered marls of the Late Cretaceous and Miocene–Pliocene horizons also exhibit relatively low but variable intensity of chemical weathering. Our results indicate that siliciclastics of the Early Cretaceous were primarily derived from the Reguibat Shield and the Mauritanides, in the SW of the basin, whereas those of the Miocene–Pliocene had varying sources that probably included western Anti-Atlas (NE part of the basin) in addition to the Reguibat Shield and the Mauritanides.     

Provenance,tectonic setting and source-area weathering of the lower Cambrian sediments of the Parahio valley in the Spiti basin,India

The geochemical study of siliciclastic rocks from the Lower Cambrian of Parahio Valley has been studied to describe the provenance, chemical weathering and tectonic setting. The K₂O/Al₂O₃ ratio and positive correlation of Co (r=0.85), Ni (r=0.86), Zn (r=0.82), Rb (r=0.98) with K₂O reflects that the presence of clay minerals control the abundances of these elements and suggests a warm and humid climate for this region. The chondrite normalized REE pattern of the samples is equivalent to upper continental crust, which reflects enriched LREE and flat HREE with negative Eu anomaly. The tectonic setting discriminant diagram log[K₂O/Na₂O] vs. SiO₂; [SiO₂/Al₂O₃] vs. log[K₂O/Na₂O]; [SiO₂/20] – [K₂O+Na₂O] – [TiO₂+Fe₂O₃+MgO] indicates transitional tectonic setting from an active continental margin to a passive margin. The discriminant function plot indicates quartzose sedimentary provenance, and to some extent, the felsic igneous provenance, derived from weathered granite, gneissic terrain and/or from pre-existing sedimentary terrain. The CIA value indicates low to moderate degree of chemical weathering and the average ICV values suggests immature sediments deposited in tectonically active settings. The A–CN–K diagram indicates that these sediments were generated from source rocks of the upper continental crust.     

Lower Palar River Sediments,Southern Peninsular,India: Geochemistry,Source-Area Weathering,Provenance and Tectonic Setting

This present study describes the geochemistry of fluvial sediments of the Palar river (lower reaches), Southern Peninsular India, with an aim to evaluate their provenance, weathering and tectonic setting. The bulk sediment chemistry is influenced by grain size. The river sediments are enriched with SiO₂ and depleted in Al₂O₃, K₂O, CaO, Na₂O, MgO, P₂O₅, MnO, Fe₂O₃ as compared with UCC values. Geochemical classification indicate that the sediments are mainly arkose, wacke and shale in composition. Discriminant diagrams together with immobile element ratio plots reveal that, the Palar river sediments are mostly derived from rocks formed in an active continental margin. Additionally, the rare earth element ratios as well as chondrite-normalized REE patterns with flat HREE, LREE enrichment, and negative Eu anomalies indicate felsic rock sources. The chemical indices of alteration suggest that Palar river sediments are chemically immature and have experienced low chemical weathering effects. This is further supported by the Th/U Rb/Sr ratio and A-CN-K ternary diagram, with most of the sample data points falling close to the plagioclase-smectite line. The bivariate plot of Th/Sc versus Zr/Sc suggest a moderate recycled origin of the sediments.     

Geochemical signatures of Mesoproterozoic siliciclastic rocks of the Kaimur Group of the Vindhyan Supergroup,Central India

The Upper Kaimur Group of the Vindhyan Supergroup in Central India, primarily consists of three rock types-DhandraulSandstone, Scarp Sandstone and Bijaigarh Shale. Mineralogically and geochemically, they are quartz arenite, sublitharenite to litharenite and litharenite to shale in composition, respectively. The A-CN-K ternary plot and CIA and ICV values suggest that the similar source rocks suffered severe chemical weathering, under a hot-humid climate in an acidic environment with higher P _{CO 2}, which facilitated high sediment influx in the absence of land plants. Various geochemical discriminants, elemental ratios like K₂O/Na₂O, Al₂O₃/TiO₂, SiO₂/MgO, La/Sc, Th/Sc, Th/Cr, Gd_N/Yb_N and pronounced negative Eu anomalies indicate the rocks to be of post-Archean Proterozoic granitic source, with a minor contribution of granodioritic input, in a passive margin setting. The sediments of the Upper Kaimur Group were probably deposited in the interglacial period in between the Paleoproterozoic and Neoproterozoic glacial epochs.     

Geochemical Signatures of Karlad Lake Sediments,North Kerala: Source Area Weathering and Provenance

In the present study, the lake floor sediments of the Karlad lake, located at higher elevation in Wayanad region of north Kerala, were analyzed for textural characteristics, organic matter, calcium carbonate, major oxides and trace elements. This study was carried out to infer the chemical composition, provenance and intensity of chemical weathering of the source rocks in the lake catchment area. Textural studies signify that lake floor sediments are predominantly as clays (38.75%) followed by sand (36.36%) and silt (25.19%) fractions. The C/N ratio of the lake sediments signify that the sediments are both autochthonous and allochthonous in origin. The major oxides average content reveals the order of abundance as follows; SiO₂ > Al₂O₃ > Fe₂O₃ > TiO₂ > MgO > CaO > K₂O > P₂O₅ > Na₂O > MnO. Moreover, the various weathering indices such as Chemical Index of Alteration (CIAAvg. 93.5%), PlagioclaseIndex of Alteration (PIA- Avg. 95.6%) and Chemical Index of Weathering (CIW- Avg. 95.76%) suggest an intense chemical weathering of the source area. The A-CN-K diagram is also corroborating the same. Various provenance discrimination diagrams reveal that the sediments are derived from the mafic source rocks.     

Geochemical comparison between Archaean and Proterozoic orthogneisses from the Nagssugtoqidian orogen,West Greenland

In the Palaeoproterozoic Nagssugtoqidian orogen of West Greenland reworked Archaean and juvenile Proterozoic orthogneisses occur side by side and are difficult to differentiate in the field. Archaean gneisses have tonalitic to trondhjemitic compositions with relatively low Al₂O₃ and Sr, and may have been derived from magmas formed by melting of basaltic or amphibolitic rocks at moderate pressures. The Proterozoic rocks are on average more mafic, and it is likely that they crystallised from mantle-derived magmas. Felsic varieties of the Proterozoic igneous suite probably formed from the original magma by fractional crystallisation, in which hornblende played an important role, and at SiO₂ > 65% Archaean and Proterozoic rocks have very similar major and trace element compositions (including REE), illustrating that different modes of origin may lead to very similar results.     

Petrographical and geochemical signatures of Jurassic rocks of Chari Formation,Western India: implications for provenance and tectonic setting

The sandstones of the Ridge and Athleta members of Chari Formation (Callovian-Oxfordian) exposed at Jara have been analyzed for their petrographical and geochemical studies. Texturally, these sandstones are medium to coarse grained, poorly to well sorted, sub-angular to sub-rounded, and show low to medium sphericity. These sandstones were derived from a mixed provenance including granites, granite-gneisses, low and high-grade metamorphic, and some basic rocks of Aravalli range and Nagarparkar massif. The petrofacies analysis reveals that these sandstones belong to the continental block and recycled orogen tectonic regime. The studied sandstones are modified by paleoclimate, distance of transport, and diagenesis. Mineralogically and geochemically, sandstones are classified as quartzarenite, subarkose, arkose, sublithic arenite, and wacke, respectively. The A-CN-K ternary plot and CIA, CIW, PIA, and ICV values suggest that the similar source rocks suffered moderate to high chemical weathering under a hot-humid climate in an acidic environment with higher \({\text{P}}_{{{\text{CO}}_{ 2} }}\). Generally good to strong correlations between Al₂O₃ and other oxides in these sediments indicate clay mineral control. The K₂O/Na₂O versus SiO₂ diagram indicates that the studied samples occupy passive margin fields but the SiO₂/Al₂O₃ versus K₂O/Na₂O plot suggests that the Athleta Sandstone and Ridge Sandstone fall within the passive margin field, while Ridge Shale falls within the active continental margin field.     

Provenance and weathering history of Mesoproterozoic clastic sedimentary rocks from the basal Gulcheru Formation,Cuddapah Basin

Geochemical analysis for the Mesoproterozoic clastic sedimentary rocks of the basal Gulcheru Formation of the Cuddapah Basin in the Gugudu-Dadithota-Parnapalle-Palkonda region (extending atleast 40 km) have been performed to highlight their provenance and weathering history. The low K₂O/Al₂O₃ ratios of the representative samples points to the recycled nature of sediments and illite clay in the sediments indicate an overall cold climate (low chemical weathering degree). All the provenance diagrams indicate a mixed source of the Gulcheru sediments with felsic dominancy and Upper continental crust (UCC) signature.     

Provenance, tectonics and source weathering of modern fluvial sediments of the Brahmaputra–Jamuna River, Bangladesh: Inference from geochemistry

This present study describes the elemental geochemistry of fluvial sediments in the Kurigram (upstream) to Sirajganj–Tangail (downstream) section of the Brahmaputra–Jamuna River, Bangladesh, with the aim of evaluating their provenance, weathering and tectonic setting. Petrographically, the sediments are rich in quartz (68%), followed by feldspars (8.5%) and lithic grains (7%). The bulk sediment chemistry is influenced by grain size. Concentrations of TiO₂, Fe₂O₃, MgO, K₂O, P₂O₅, Rb, Nb, Cr, V, Y, and, Ce, Th and Ga slightly decrease with increasing SiO₂/Al₂O₃ and grain size, suggesting clay matrix control. In contrast, concentrations of CaO, Na₂O, Sr and Pb increase with increasing SiO₂/Al₂O₃ and grain size, suggesting residence of these substances in feldspar. Decrease in Zr as grain size increases is likely controlled both by clay matrix and heavy minerals. In addition, heavy minerals' sorting also influences Ce, Th, Y and Cr abundances in some samples. The sediments are predominantly quartzose in composition with abundant low-grade metamorphic and sedimentary lithics, low feldspars and trace volcanic detritus, indicating a quartzose recycled orogen province as a source of the sediments. Discriminant diagrams together with immobile element ratio plots show that, the Brahmaputra–Jamuna River sediments are mostly derived from rocks formed in an active continental margin. Moreover, the rare earth element ratios as well as chondrite-normalized REE patterns with flat HREE, LREE enrichment, and negative Eu anomalies indicate derivation of the sediments of Brahmaputra–Jamuna River from felsic rock sources of upper continental crust (UCC). The chemical indices of alteration suggest that Brahmaputra–Jamuna River sediments are chemically immature and experienced low chemical weathering effects. In the A–CN–K ternary diagram, most of the samples close to the plagioclase–K-feldspar join line and to the UCC plot, and in the field of various lithologies of Higher Himalayan Crystalline Series, suggesting that rocks in these series are likely source rocks. Therefore, the elemental geochemistry of the Brahmaputra–Jamuna River sediments is controlled mostly by mechanical breakdown of lithic fragments and subsequent preferential attrition of muscovite > albite > quartz.     

Palaeoweathering,composition and tectonics of provenance of the Proterozoic intracratonic Kaladgi–Badami basin,Karnataka, southern India: Evidence from sandstone petrography and geochemistry

Petrographic and geochemical data on the sandstones of the Proterozoic intracratonic Kaladgi–Badami basin, southern India are presented to elucidate the palaeoweathering pattern, and composition and tectonics of their provenance. The Kaladgi–Badami basin, hosting the Kaladgi Supergroup, occupies an E–W trending area. The Supergroup unconformably overlies Archaean basement TTG gneisses, granites and greenstones, comprises a cyclic arenite–pelite–carbonate association and is divided into the Bagalkot and Badami Groups. The immature arkosic character of the basal Saundatti Quartzite Member (Bagalkot Group) containing fresh and angular feldspars, along the northern margin of the basin, suggests that during the initial stage of deposition, this part of the basin received sediments from a restricted, uplifted and less weathered source dominated by K-rich granites occurring to the north. In contrast, the Saundatti Quartzite along the southern margin displays a mostly mature, quartz-rich character with less abundant but severely weathered feldspars, and higher SiO₂ and CIA but lower Al₂O₃, TiO₂, Rb, Sr, Ba, K₂O, K₂O/Na₂O, Zr/Ni and Zr/Cr. This is interpreted in terms of a tectonically stable, considerably weathered mixed source (Archaean gneisses, granites and greenstones) along the southern fringe of the basin. The highly mature (quartz arenite) Muchkundi Quartzite Member (also of the Bagalkot Group), occurring higher up in the succession, exhibits minor but severely altered feldspars, and higher SiO₂, Na₂O, CIA, Cr and Ni with lower K₂O, Al₂O₃, TiO₂ and K₂O/Na₂O. This reflects that with the passage of time the source evolved to a uniform, extensively weathered, tectonically stable peneplained provenance which consisted of less evolved TTG gneisses and greenstones. This was followed by closure, deformation and upliftment of the basin hosting the Bagalkot Group and subsequent deposition of the Badami Group. Sandstone Members of this younger Group (Cave-Temple Arenite and Belikhindi Arenite) range widely in mineralogy (quartz arenite to arkose) and chemistry (including CIA), and point to a source that varied from uplifted, less weathered K-rich granites to less evolved, peneplained TTG gneisses and greenstones or even Bagalkot sediments. Variable alteration of feldspars in the Kaladgi sandstones and severe depletion of Ca, Na and Sr in the associated shales indicate a humid tropical (tropical and subtropical) climate facilitating chemical weathering.     

Geochemistry of Renuka Lake and wetland sediments,Lesser Himalaya (India): implications for source-area weathering,provenance, and tectonic setting

The geochemical investigation of sediments deposited in the Renuka Lake basin and its adjoining wetland has shown variation in the distribution and concentration of major, trace and REEs. The major elements are depleted in the lake in relation to wetland and that of Post Archaean Australian, Shale (PAAS), except for CaO which is strikingly in excess and has a dilution effect on SiO₂ and other oxides and trace elements. The Wetland sediments, on the other hand, are enriched in Al₂O₃, Fe₂O₃, K₂O and TiO₂ and the latter three show a positive correlation with Al₂O₃ in both wetland and lake sediments suggesting their association with phyllosilicates and similar source rocks. The enrichment of Y, Zr, Ni, Th, U and Nb in wetland compared to lake and their similarity with PAAS in the former, suggests more clay fractions in the wetland. A high Zr/Hf ratio in wetland and lake sediments and a positive correlation of Zr with Y and HREE indicate Zr control on HREEs. However, higher Zr/Yb and Zr/Th ratios in wetland compared to lake indicate mineral sorting during the process of lighter particles (clays) being trapped in wetland soil. This is also reflected from negative correlation of Gd_N/Yb_N with Al₂O₃ and a strong positive correlation with SiO₂ in wetland sediments. The wetland in this context has a control on lake sediment chemistry. The chondrite normalized REE patterns are essentially the same for lake as well as wetland sediments but abundance decreases in the former. The similarity of pattern with that of PAAS and negative Eu anomaly indicates a cratonic source of sediments. In a plot of the individual samples, wetland samples cluster while lake samples are separated indicating fractionation of lake sediments. A strong positive correlation of La_N/Yb_N with Al₂O₃ and a positive correlation of Zr-∑LREE and Zr-La_N/Yb_N suggest that LREEs are controlled by both phyllosilicates and zircon. The chemical index of alteration (CIA) indices in lake sediments and in wetland are higher than PAAS indicating moderate chemical weathering in the source area. The petrography, lack of felsic magmatic rock fragments, and negative correlation between Zr-(Gd/Yb)_C indicate sedimentary source rocks for the detritus. This is in conformity with the Lesser Himalayan sedimentary sequence belonging to neo-Proterozoic–Proterozoic age and constituting lake catchment of Renuka. The tectonic delineation and discriminant function plots of lake and wetland sediments indicate their cratonic and/or quartzose sedimentary orogenic terrain source that has been deposited in a passive margin setting.     

Geochemical characteristics of modern river sediments in Myanmar and Thailand: Implications for provenance and weathering

The elemental composition of organic matter and the major and trace element compositions of stream sediments from Myanmar (Ayeyarwady and Sittaung rivers) and Thailand (Mekong and Chao Phraya rivers, and their tributaries) were determined to examine their distributions, provenance, and chemical weathering processes. Higher total organic carbon (TOC) and total nitrogen (TN) contents in the finer grained sediments indicate hydrodynamic energy may control their distributions. TOC/TN ratios indicate inputs of both aquatic macrophyte and higher vascular plant material to the river sediments. The major element abundances of the sediments are characterized by predominance of SiO₂ in coarser fractions and a marked negative correlation with Al₂O₃, representing primary grain size primarily control on SiO₂ content. Marked depletion of most labile elements (Na₂O, CaO, K₂O, Ba and Sr) relative to UCC (upper continental crust), indicate destruction of feldspar during chemical weathering in the source area or during transport. However, enrichment of some high field strength elements (Zr, Th, Ce and Y) relative to UCC and higher Zr/Sc ratios indicate moderate concentration of resistant heavy minerals in finer-grained samples. Discriminant diagrams and immobile trace element characteristics indicate that the Mekong, and Chao Phraya river sediments were largely derived from felsic sources with compositions close to typical rhyolite, dacite/granodiorite, UCC, I- and S-type granites. Relative enrichment of ferromagnesian elements (e.g. MgO, Cr, Ni) and high Cr/V and low Y/Ni ratios in Ayeyarwady and Sittaung sediments indicate the presence of a mafic or ultramafic component in their sources. The ICV (Index of Compositional Variability), CIA (Chemical Index of Alteration), PIA (Plagioclase Index of Alteration), α^Al, Rb/Sr and K₂O/Rb ratios indicate that the Ayeyarwady and Sittaung sediments record low to moderate degrees of chemical weathering in their source, compared to moderate to intense chemical weathering in the Mekong and Chao Phraya river basins. These results are compatible with existing major ion data for river waters collected at the same locations.     

Geochemistry of shales from the Upper Miocene Samh Formation,north Marsa Alam,Red Sea,Egypt: implications for source area weathering,provenance, and tectonic setting

The Upper Miocene shales of the Samh Formation, North Marsa Alam along the Egyptian Red Sea coastal plain were analyzed for major and selected trace elements to infer their provenance, weathering intensity, and tectonic setting. The Samh Formation consists of sandstone underlies by shale and marl intercalations. The Samh shales are texturally classified as mudstones. Mineralogically, these shales consist mainly of smectite and kaolinite, associated with non clay minerals (abundant quartz and trace of plagioclase, microcline, and halite). Compared to post-Archaean Australian shales (PAAS), the Samh shales are highly enriched in SiO₂, Al₂O₃, and Fe₂O₃ and depleted in TiO₂, P₂O₅, Na₂O, MgO, and K₂O contents. The K₂O/Al₂O₃ ratio values indicate predominance of clay minerals over K-bearing minerals. Trace elements like zirconium (Zr), Cr, Pb, Sc, Rb, and Cs are positively correlated with Al₂O₃ indicating that these elements are likely fixed in K-feldspars and clays. The Chemical Index of Alteration (CIA), Plagioclase Index of Alteration (PIA), and Chemical Index of Weathering (CIW) values indicate moderate to intense weathering of the source material in a semiarid climate. The geochemistry results suggest that the Samh shales were deposited in a passive margin of a synrift basin and derived from felsic (granitic) source rocks. The inferred tectonic setting for the Upper Miocene Samh shales in Marsa Alam is in agreement with the tectonic evolutionary history of the Eastern Desert of Egypt during the Upper Miocene.     

Late Vendian unfolded Molasse in the northeastern, eastern, and southwestern peripheries of the east European platform: Distinctive lithogeochemical features

The paper reports data on the lithogeochemistry of sandstones and silty mudstones from Upper Vendian sedimentary sequences in the northeastern, eastern, and southwestern peripheries of the East European Platform belonging to the so-called unfolded molasse. The sequences are dominated by wackes, arkoses, subarkoses, litharenites, and sublitharenites, i.e., chemically immature and moderately mature psammites, and can be classed with rocks produced by clastic material brought from orogens surrounding the platform. The higher TiO₂, Al₂O₃, FeO_tot, MgO, Na₂O, and K₂O concentrations of the psammites than those in the average cratonic Phanerozoic sandstone (APhSa) testify that the chemical maturing of the rocks was not completed. The silty mudstones accompanying the sandstones have a composition closer to those of the average cratonic Phanerozoic shale (APhSh), but this is likely explained by the fact that the rocks were produced of material brought from erosion territories of much greater area. The lithogeochemical data generally indicate that these territories were dominated by acid and intermediate magmatic rocks with variable fraction of sedimentary rocks when the Late Vendian sedimentary associations in question were produced. The distribution of certain indicator trace elements in the sandstones and silty mudstones show that the average composition of the eroded complexes was close to the composition of the post-Archean upper continental crust, but the erosion areas occasionally (in the Vychegorskii trough and the Shkapovsko-Shikhanskaya depression) contained relatively primitive source rocks. The data points of the great majority of the sandstones and silty mudstones plot in the SiO₂-K₂O/Na₂O and F1–F2 diagrams in the fields of sediments typical of the environments of active continental margins, which is consistent with the arrangement of the data points of these rocks in the La-Th-Sc, Th-Sc-Zr/10, and Th-Co-Zr/10 diagrams. All of these features confirm that the sedimentary rocks in question affiliate with rock associations produced at an active tectonic regime.     

Zircon U–Pb SHRIMP ages of weakly to unmetamorphosed granitoids of the Yangtze basement outcrop in Dabieshan, central China

The Qichun granitoids exposed in the Dabie Orogen of China are composed of two types of rocks: porphyritic monzogranite (with variable schistosity) and syenogranite (without schistosity). The two types show large differences in geochemical characteristics. The porphyritic monzogranite is characterized by high Al₂O₃ content (15.73%), relatively high CaO (2.46%) and Na₂O contents (Na₂O/K₂O=1.27), strong depletion in HREE and strong fractionation between LREE and HREE ((La/Yb)_N=46.8), similar to some high Al₂O₃ Archaean TTG gneisses. Conversely, the syenogranite is characterized by relatively low Al₂O₃ (14.05%) and CaO (0.82%) contents, and higher K₂O than Na₂O (Na₂O/K₂O=0.81). The degree of fractionation between LREE and HREE is minor. The U–Pb SHRIMP zircon age of the porphyritic monzogranite is 841±15 and 824±27 Ma for the syenogranite. These ages are similar to the protolith emplacement ages of granitic gneisses in the Dabie Orogenic Belt. The existence of weakly to unmetamorphosed granitoids in the Dabie Orogen shows that the granitoids were situated in the back part of the subducted plate during collision and subduction between the Yangtze and the North China cratons, and may represent outcrops of the Yangtze basement.     

Geochemical characteristics of stream sediments,sediment fractions,soils, and basement rocks from the Mahaweli River and its catchment,Sri Lanka

Geochemical variations in stream sediments (n = 54) from the Mahaweli River of Sri Lanka have been evaluated from the viewpoints of lithological control, sorting, heavy mineral concentration, influence of climatic zonation (wet, intermediate, and dry zones), weathering, and downstream transport. Compositions of soils (n = 22) and basement rocks (n = 38) of the catchment and those of <180 μm and 180–2000 μm fractions of the stream sediments were also examined. The sediments, fractions, soils and basement rocks were analyzed by X-ray fluorescence to determine their As, Pb, Zn, Cu, Ni, Cr, V, Sr, Y, Nb, Zr, Th, Sc, Fe₂O₃, TiO₂, MnO, CaO, P₂O₅ and total sulfur contents. Abundances of high field strength and ferromagnesian elements in the sediments indicate concentration of durable heavy minerals including zircon, tourmaline, rutile, monazite, garnet, pyriboles, and titanite, especially in <180 μm fractions. The sediments show strong correlation between Ti and Fe, further suggesting presence of heavy mineral phases containing both elements, such as ilmenite and magnetite. The basement rocks range from mafic through to felsic compositions, as do the soils. The river sediments lack ultrabasic components, and overall have intermediate to felsic compositions. Elemental spikes in the confluences of tributary rivers and high values in the <180 μm fractions indicate sporadic inputs of mafic detritus and/or heavy minerals to the main channel. Al₂O₃/(K₂O + Na₂O) and K₂O/Na₂O ratios of the sediments and LOI values of the soils correlate well with the climatic zones, suggesting intense weathering in the wet zone, lesser weathering in the intermediate zone, and least weathering in the dry zone. Low Sr and CaO contents and Cr/V ratios in stream sediments in the wet zone also suggest climatic influence. Fe-normalized enrichment factors (EFs) for As, Pb, Zn, Cu, Ni and Cr in stream sediments in the main channel are nearly all <1.5, indicating there is no significant environmental contamination. The chemistry of the sediments, rocks and the soils in the Mahaweli River are thus mainly controlled by source lithotype, weathering, sorting, and heavy mineral accumulation.     

Major, trace and REE geochemistry of the Ganga River sediments: Influence of provenance and sedimentary processes

The sediments of the Ganga River from different depositional regimes in the Plain region such as the river channel, active flood-plain and the older flood-plain sediments from the inter-fluve region were analysed for major, trace and the rare earth elements (REEs). These are compared with catchment zone sediments of the river and probable source rocks in the Himalaya. The lower CIA values between 48 and 54.7 for the catchment sediments indicates that the sediments supplied to the Ganga Plain are chemically immature and subjected mostly to physical weathering due to higher erosion rates in the Himalaya. The CIA values ranging between 55 and 74, with average value of 59, 61.4 and 67 for sediments from the Plain's bed-load, active flood-plain and older flood-plain from the inter-fluve region indicates that silicate weathering of Ganga River sediments has occurred only after entering into the plains. This is likely because of higher residence time and change in the climate from cold-frigid in the Himalaya to tropical sub-humid in the plains. Therefore, the use of geochemical data on ancient system to infer climate in their source region may not always be true. Although the CIA values indicate a moderate chemical weathering in the plains, it is far from impressive. Dominance of physical weathering in the catchment region and lower degree of chemical weathering in the Plains indicate that weathering of sediments supplied by Himalayan Rivers, particularly the Ganga River may not have affected the atmospheric CO₂ to a significant level as is generally believed. Thus the net effect of the Himalaya on the CO₂ sequestration and consequent global cooling needs a re-evaluation.The plots of sediments in ternary diagram among La, Th, Sc and ratios involving Co/Th, La/Sc and Sc/Th indicate granitic to granodioritic source rocks to the sediments. The ratio plots involving relatively immobile Al₂O₃, TiO₂ and FeO along with REE plots suggest that out of the major Himalayan lithologies, gneisses and Cambro-Ordovician granites of HHCS have acted as the dominant source to the sediments.The plots of LogNa₂O/K₂O vs. LogSiO₂/Al₂O₃ and FeO/SiO₂ vs. Al₂O₃/SiO₂ diagrams show that the combination of processes including erosion, weathering, sorting and aeolian activity has together played a major role in progressively changing the chemistry from source rock to catchments bed-load to Plains bed-load, active flood-plains and the older inter-fluve sediments in the Ganga River system. The above plots demonstrate that as a result of above processes the ratios between the elements generally thought to be immobile and used in provenance studies does not always remain invariant and the linear trend line in the scatter gram between the two immobile elements show rotation around the fine grained end member.     

黔北务川瓦厂坪铝土矿床元素迁移规律研究

黔北务正道地区铝土矿矿床属古风化壳沉积型,成矿母岩具多源性,但主要来源于下覆的中下志留统韩家店组。本文依据Grant提出的质量平衡方程和图解法,对瓦厂坪矿床在成矿过程中元素迁移的富集、贫化（亏损）规律进行了定量研究。结果表明,主要成矿母岩韩家店组砂、页岩一中间产物铝土质页岩、粘土岩是主要元素A12O3、TiO2显著富集...