Average Upper Continental Crust Research Articles

Comprehensive assessment of heavy metal pollution of agricultural soils impacted by the Kalsaka abandoned gold mine and artisanal gold mining in northern Burkina Faso.

In this study, the environmental quality of agricultural soils around the Kalsaka abandoned gold mine was evaluated. A total of 117 samples including industrial mine wastes, topsoil, and control soil were collected in and around the mine, and their heavy metal concentrations were determined using ICP-MS after aqua regia digestion. Except for Zn, the average concentrations of the metals were higher in mine wastes and the agricultural soils than their average upper continental crust (UCC) counterpart, whereas those of Ag, As, and Hg exceeded the UCC in the control soils. The control soils had the lowest contaminations and the lowest pollution levels for all metals except for Hg. Based on the average concentrations and coefficients of variation, it can be concluded that Cr, Ni, and Zn abundance in the agricultural soils was controlled by natural factors, whereas that of Ag, As, Hg, Co, Cu, and Mn was mainly associated with mining. The absence of Hg in industrial mine wastes and its high contents in agricultural and control soils reflected the artisanal gold mining source of this metal. Thus, single and integrated pollution indices showed that ecological risk and toxicity are much higher when Hg is included in the index calculation, suggesting cumulative effects of industrial and artisanal gold mining on the agricultural soil quality. The results also showed that Hg and As are the pollutants of major concern in the Kalsaka abandoned gold mine. Because of its proximity to human settlements, the Kalsaka abandoned gold mine necessitates an immediate rehabilitation.

Tungsten stable isotope composition of the upper continental crust

Tungsten (W) stable isotope data for twenty-four composites made from the fine-grained matrix of glacial diamictites, deposited between the Mesoarchean and the Paleozoic, as well as two Archean tonalite-trondhjemite-granodiorites (TTG) are used to refine the W isotope compositions (expressed as δ186W deviations from the NIST 3163 W standard) of the upper continental crust (UCC). To do this we evaluate W isotope fractionation in the diamictites as the result of both magmatic and low-temperature processes. The δ186W values of the diamictites are heterogeneous (0.017 ± 0.011 ‰, 2SE to 0.182 ± 0.007 ‰, 2SE), encompassing the range of previously published values for igneous rocks. We find that δ186W correlates positively with the diamictite’s chemical index of alteration (CIA), a measure of the degree of chemical weathering in the provenance, suggesting that the continental regolith (i.e., surface material composed of weathered residues) is isotopically heavy for W. Since W in rivers and ocean water is also isotopically heavy, this suggests that equilibration with Fe-Mn-oxides controls the W isotope composition of surface waters and that such oxides are not important constituents of the regolith, whose W is likely accommodated in clays. Using diamictites with low CIA (<60, n = 9), we calculate an average δ186W of 0.046 ± 0.036 ‰ (2SD) for the UCC, from 2.3 to 0.3 Ga. This average is combined with the TTG data of this study, as well as data from the literature for upper crustal rocks to obtain a representative δ186W value of the UCC of 0.046 ± 0.046 ‰ (2SD). The W stable isotope composition of the UCC is lighter than that of mantle-derived melts (MORB and OIB, with an average δ186W = 0.082 ± 0.026 ‰, 2SD), and much lighter than the weighted average δ186W of intra-oceanic arcs (δ186W = 0.104 ± 0.052 ‰, 2SD), though the spread in the arc rocks is large. The significant difference in δ186W between average intra-oceanic island arc lavas and UCC is intriguing given that continental crust is thought to form primarily via arc magmatism. This difference suggests that not all modern intra-oceanic arcs are representative of those that formed the continental crust. Rather, only arcs that are enriched in incompatible trace elements, which also tend to have lighter W isotope compositions, may have been involved in making new continental crust. Alternatively, there may have been a secular change in the W isotope composition of arc magmas.

Rubidium isotope compositions of the average upper continental crust and the Himalayan leucogranites: Implications for magmatic-fluid interaction

Rubidium is a highly fluid-mobile and incompatible element. Rb isotopes have great potential to trace the migration and enrichment of Rb in magmatic and fluid-related processes. In order to decipher the behavior of Rb isotopes during magmatic-fluid interaction, this study presents an estimate of average δ87Rb of the upper continental crust (UCC) and Rb isotope data for highly evolved leucogranites from Kampa, Himalayan orogen. The samples used to estimate the average UCC include 21 granites, 42 loess, and 10 sediments, which show small variations from −0.27‰ to 0.02‰, −0.17‰ to −0.07‰, and −0.31‰ to 0.07‰, respectively. Based on the weighted average δ87Rb of these samples, the estimate of average Rb isotope composition of UCC is −0.14 ± 0.01‰ (2SD/√n, n = 73), which is consistent with the δ87Rb of the bulk BSE (−0.13 ± 0.01‰).A comparison of the Kampa leucogranites with the UCC indicates that the δ87Rb of the Kampa leucogranites (−0.29‰ to 1.20‰) is significantly higher than the average value of the UCC. Such heavy Rb isotope composition cannot be due to chemical weathering, source heterogeneity, fractional crystallization, or partial melting. Instead, because the Kampa leucogranites have been modified by magmatic fluids during magma evolution, the high δ87Rb most likely reflects the signature of magmatic fluids derived from deeper magma. Our study for the first time indicates that the heavy Rb isotope characteristics of highly fractionated leucogranites provide a novel tracer for processes involving fluid activity. This could be useful for understanding the enrichment and mineralization of rare metals related to magmatic fluids.

Petrographic and geochemical characteristics of selected coal seams from the Late Cretaceous-Paleocene Guaduas Formation, Eastern Cordillera Basin, Colombia

Petrographic, geochemical and carbon isotopic determinations were conducted on five samples from four seams within the Late Cretaceous-Paleocene of the Guaduas Formation in Colombia. The individual coal seams are thin, ranging from 0.59 to 1.2 m, and occur over a 157 m interval. The average weighted ash yield is 13.7% (dry basis), although one sample presented values >35%. Vitrinite reflectance increases from 1.33% in the stratigraphically uppermost seam (La Cuarta) to 1.44% in the lower most coal seam (Cisquera), indicating their rank to be medium volatile bituminous.The studied sequence of the Guaduas Formation represents a peat environment influenced by shifting depositional settings, particularly in relation to the proximity of marine/brackish water environments as indicated by Sr/Ba values, sulfur, pyrite, δ13C and the proportion of telinite. A decrease in inertinite indicates that the peat mires were becoming relatively ‘wetter’ stratigraphically upwards. This trend, unlike that of Sr/Ba and telinite, may be driven by climatic shifts rather than only changes in depositional environment.Both trace and rare earth elements, plus yttrium (REY) are related to the inorganic fraction of the coal. All rare earth elements are depleted relative to average worldwide hard coals (for trace elements) and average upper continental crust (for REY) with the notable exception of Li and Sb for the high ash yield sample (the La Cuarta lower coal seam). The abundance of some REY elements (i.e. Eu, Gd, Tb, Dy, Y, Ho, Er, Tm, Yb and Lu) may be more influenced by the presence of illite rather than just the quantity of total ash yield. Based on the relationship between SiO2 and Nb/Y, the provenance of the inorganics changed from a rhyolitic source at the lowest seam (Cisquera) to a dacite source in the middle layer (El Tesoro) to an andesite source in the stratigraphically uppermost seams (La Gemela and La Cuarta).

Geochemistry and mineral chemistry of quartz mica schists within Iseyin-Oyan Schist Belt, Southwestern Nigeria

Background/Objectives: The Iseyin-Oyan schist belt is made up metasedimentary rocks, gneisses, granites and pegmatite intrusions. The study was aimed at identifying the schist within this belt and assess their metamorphism, geochemical characteristics and tectonic origin. Methods: Detailed geologic field mapping was undertaken where rocks were located, studied in-situ and identified. Samples of the schist were prepared for petrographic studies. Mineralogical contents were determined using X-Ray Diffraction technique. Polished sections were studied for mineral chemistry using Scanning Electron Microscope-Energy Dispersive Spectroscopy. Rock samples were analysed using X-Ray Fluorescence Spectroscopy and Inductively Coupled Plasma Emission Spectrometry. Geochemical data were elucidated using diverse geochemical discrimination diagrams. Findings: The schists are quartz mica schists and occur in close association with amphibolites, intrusive granitoids and pegmatites. The Mineral assemblage indicates upper (at the western part) to lower (at the central part) amphibolite facies grade metamorphism in the area. Pyrope-almandine garnets occur in quartz mica schist at the western parts reinforcing higher pressure-temperature metamorphic conditions. The concentration (in %) of SiO2 ranged from 56.4-71.6; Al2O3, 13.7-21.1; Fe2O3, 2-8; MgO, 0.7-2.4; and K2O, 2.1-5.5 supporting the evidence for differential degrees of metamorphism. Large iron lithophile and high field strength elements are similar to the average upper continental crust. Pronounced negative Europium anomaly pointed to the major roles played by feldspars during the geological processes. Plagioclase ranged from albite-oligoclase and oligoclase- andesine. The precursors of the quartz mica schist are possibly arkosic and greywacke sands deposited within the active continental margins. Evidence of uplift and overturning suggested for the differential metamorphism may be due to these events usually associated with active continental margins. Applications: This study has identified the once named undifferentiated schist in the study area to be quartz mica schist with details in their grades of metamorphism elucidated. Keywords: Quartz mica schist; geochemistry; mineral chemistry; Iseyin-Oyan schist belt; precambrian basement complex

Petrography and geochemistry of the Lower Carboniferous Abu Durba sandstones, West Central Sinai, Egypt

Petrographically, the Lower Carboniferous Abu Durba sandstones are classified as quartz arenites reflecting their high maturity and have an average framework composition of Q96F2L2. The sandstones are rich in SiO2, Zr, Sr, and Ba and depleted in most other major and trace elements, compared with the average upper continental crust. The positive correlation between Al2O3 and most trace elements signifies that trace elements are associated with clay minerals rather than with source rocks. Provenance discrimination diagrams, major elemental ratios (Al2O3/TiO2 and K2O/Na2O), and SiO2, Cr, Ni, V, and Zr contents in sandstones reveal that the sandstones are mainly derived from quartzose recycled sedimentary rocks with subordinate felsic plutonic sources. The chemical index of alteration (avg. 88.21) and Rb/Sr ratio (> 0.8) values for the Abu Durba Sandstone indicate an intensive degree of weathering either of the original source or during transport before deposition, and may reflect intense recycling in the source area. Also, low index of compositional variability values and high SiO2/Al2O3 ratios indicate high mineralogical maturity of Abu Durba Sandstone. The discriminant function-based multidimensional diagrams reveal that the Abu Durba Sandstone was deposited in a rift basin, which is consistent with the general geology of West Central Sinai during the Lower Carboniferous. The older sandstones of Naqus (Lower Cambrian) and Abu Thora (Lower Carboniferous) formations in addition to the Precambrian basement rocks are probably the sources of the Abu Durba Sandstone.

Geochemical characteristics of Gondwana shales from the Barapukuria basin, Bangladesh: implications for source-area weathering and provenance

The major, trace, and rare earth element (REE) compositions of shales from the drill hole GDH-40, Barapukuria basin, northwestern Bangladesh have been examined to evaluate their compositional variations, source rock weathering, and provenance. The shale samples are characterized by very low to moderate abundance of SiO2 (38.84–62.91 wt.%) and high Al2O3 (19.08–30.36 wt.%). The Gondwana shales are mainly enriched in Al2O3, TiO2, Th, Ce, Zr, Nb, Pb, Sc, Y, Ni, Cr, and V relative to average upper continental crust (UCC) and marked depleted in CaO, Na2O, K2O, MgO, and Ba. Depletion of these labile elements relative to UCC indicates destruction of feldspar during fluvial transportation. The Index of Compositional Variability (ICV) values range from 0.08 to 0.21, indicating high compositional maturity. Chondrite-normalized REE patterns for the shales are very similar to the average UCC, with significant LREE enrichment (LaN/YbN 10.03–13.58), flat HREE, and negative Eu anomalies (Eu/Eu* 0.36–0.73) representing a felsic source-rock provenance. The Chemical Index of Alteration (CIA), Plagioclase Index of Alteration (PIA), K2O/Rb, and Rb/Sr ratios indicate that the Gondwana shales were largely derived from deeply weathered source rocks. Discriminant function diagrams, immobile trace element ratios (Th/Sc, Zr/Sc, Ce/Sc, and Ti/Zr), and REE parameters (∑LREE/HREE and GdN/YbN) in the Gondwana shales further support felsic detritus input, with compositions close to the average rhyolites, UCC, and I- and S-type granites.

Major, trace, and REE geochemistry of the Meghna River sediments, Bangladesh: Constraints on weathering and provenance

The major, trace, and rare earth element (REE) compositions of sediments from the Meghna River in Bangladesh have been examined to infer their sediment type, compositional maturity, chemical weathering intensity, provenance, and tectonic setting. Geochemically, the sediments are classified as litharenites, shale, and wacke. The ICV (Index of Compositional Variability) values (0.92 to 1.10) indicate that the sediments have low compositional and mineralogical maturity. Major and trace element abundances display marked depletion of Na2O, CaO, MgO, K2O, Fe2O3T, Sr, and Ba relative to UCC (upper continental crust), suggesting loss of feldspars during chemical weathering in the source. Zirconium, Th, Ce, and Y are enriched relative to UCC, indicating that these elements are primarily controlled by resistant heavy minerals. The CIA (Chemical Index of Alteration), PIA (Plagioclase Index of Alteration), and Rb/Sr and K2O/Rb ratios of the river sediments suggest low to moderate intensity of chemical weathering in the source area. Chondrite‐normalized REE patterns show that high LREE enrichment, nearly flat HREE fractionation (LaN/YbN = 7.62 to 8.73) and marked negative Eu anomalies (Eu/Eu* = 0.49 to 0.69) suggesting a felsic source provenance. Several discriminant function diagrams and immobile trace element ratios (Th/Sc, Zr/Sc, Ce/Sc, and Ti/Zr) and REEs (∑LREE/HREE, Eu/Eu*, and GdN/YbN) parameters indicate that the river sediments were primarily derived from a common felsic source rock, with composition close to the average rhyolite, granodiorite, granite, and UCC. Tectonic setting signatures of the Meghna River sediments are both active continental margin and passive margin environments.

The Record of the Transition From an Oceanic Arc to a Young Continent in the Talamanca Cordillera

AbstractThe Talamanca Cordillera in the Central America Arc (Costa Rica‐Panama) preserves the record of the geochemical evolution from an intraoceanic arc to a juvenile continental arc in an active subduction zone, making it a testbed to explore processes that resulted in juvenile continental crust formation and explore potential mechanisms of early continental crust generation. Here we present a comprehensive set of geochronological, geochemical, and petrological data from the Talamanca Cordillera that tracks the key turning point (12–8 Ma) from the evolution of an oceanic arc depleted in incompatible elements to a juvenile continent. Most plutonic rocks from this transition and postintrusive rocks share striking similarities with average upper continental crust and Archean tonalite, trondhjemite, and granodiorite. We complement these data with seismic studies across the arc. Seismic velocities within the Caribbean Plate (basement of the arc) show a relatively uniform lateral structure consistent with a thick mafic large igneous province. Comparisons of seismic velocity profiles in the middle and lower crust beneath the active arc and remnant Miocene arc suggest a transition toward more felsic compositions as the volcanic center migrated toward the location of the modern arc. Seismic velocities along the modern arc in Costa Rica compared with other active arcs and average continental crust suggest an intermediate composition beneath the active arc in Costa Rica closer to average crust. Our geochemical modeling and radiogenic isotopes systematics suggest that input components from melting of the subducting Galapagos hotspot tracks are required for this compositional change.

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.

U–Pb Geochronology of Hydrothermal Monazite from Uraniferous Greisen Veins Associated with the High Heat Production Mount Douglas Granite, New Brunswick, Canada

A combination of in situ laser ablation inductively coupled plasma–mass spectrometry (LA ICP–MS) analyses guided by Scanning Electron Microscope–Back-Scattered Electron imaging (SEM–BSE) was applied to hydrothermal monazite from greisen veins of the Late Devonian, highly evolved, uraniferous Mount Douglas Granite, New Brunswick, Canada. Understanding the uraniferous nature of the suite and characterizing the hydrothermal system that produced the associated mineralized greisen veins were the main goals of this study. The uraniferous nature of the Mount Douglas Granite is evident from previous airborne radiometric surveys, whole-rock geochemical data indicating high U and Th (2–22 ppm U; 19–71 ppm Th), the presence of monazite, zircon, xenotime, thorite, bastnaesite, and uraninite within the pluton and the associated hydrothermal greisen veins, as well as anomalous levels of U and Th in wolframite, hematite, and martite within greisen veins. New U–Pb geochronology of hydrothermal monazite coexisting with sulfide and oxide minerals yielded mineralization ages ranging from 344 to 368 Ma, with most of them (90%) younger than the crystallization age of the pluton (368 ± 3 Ma). The younger mineralization age indicates post-magmatic hydrothermal activities within the Mount Douglas system that was responsible for the mineralization. The production of uraniferous greisen veins by this process is probably associated with the High Heat Production (HHP) nature of this pluton, resulting from the radioactive decay of U, Th, and K. This heat prolongs post-crystallization hydrothermal fluid circulation and promotes the generation of hydrothermal ore deposits that are younger than the pluton. Assuming a density of 2.61 g/cm3, the average weighted mean radiogenic heat production of the Mount Douglas granites is 5.9 µW/m3 (14.1 HGU; Heat Generation Unit), in which it ranges from 2.2 µW/m3 in the least evolved unit, Dmd1, up to 10.1 µW/m3 in the most fractionated unit, Dmd3. They are all significantly higher than the average upper continental crust (1.65 µW/m3). The high radiogenic heat production of the Mount Douglas Granite, accompanied by a high estimated heat flow of 70 mW/m2, supports the assignment of the granite to a ‘hot crust’ (&gt;7 HGU) HHP granite and highlights its potential for geothermal energy exploration.

The quest for an extraterrestrial component in Muong Nong-type and splash-form Australasian tektites from Laos using highly siderophile elements and Re-Os isotope systematics

Abstract Extremely low and variable concentrations of osmium (Os) and other highly siderophile elements (HSE) in most tektites make it challenging to establish direct links between these impact-related materials and their possible extraterrestrial contribution. New Os concentrations (2–43 ppt) and 187Os/188Os ratios (0.131–0.68) in a suite of fifteen well-characterized Australasian tektites from Laos (Muong Nong and splash-form types) with variable Ni enrichment indicate a maximum of ∼0.005% addition of a chondritic impactor. This is similar to some Australasian tektites from Vietnam with similarly low siderophile contents, but significantly lower than found in previous studies of more Ni-rich Australasian splash-form tektites and microtektites from different parts of the Australasian strewn field (e.g., Indonesia, South China Sea). The contents of HSE and Re–Os isotopic compositions of layered Muong Nong-type Australasian tektites are highly variable, suggesting mingling of crustal-derived (siderophile element-poor) and extraterrestrial (siderophile element-rich) materials. The absence of a direct correlation between HSE and Ni contents is interpreted to result from a fractionation process related to their different vaporization/condensation temperatures. The low Os abundance in most of the analyzed Australasian tektites, combined with non-radiogenic 187Os/188Os far below average upper continental crust, may provide a direct test to distinguish continental versus seawater impact scenario. In the absence of any specific low-Os target, a particular process of Os loss following impact is required. We envisage a scenario where evaporative loss of >>90% Os in the form of Os oxides from the overheated tektite melt is aided by volatile species derived from dissociated seawater and/or saline pore water embedded in sediments off-shore Indochina, consistent with elevated contents of halogens in Australasian tektites. This water-assisted Os loss could also play significant role for Central European tektites, while the continental surface with limited amount of water would prevent from more efficient HSE loss as could be the case for Ivory Coast tektites.

Compositional variations, chemical weathering, and provenance of sands from the Cox’s Bazar and Kuakata beach areas, Bangladesh

The modal and chemical composition of sands from Cox’s Bazar beach (CBB) and Kuakata beach (KB) areas of Bangladesh has been investigated to infer their maturity, chemical weathering, and provenance signatures. The CBB and KB sands are typically high quartz, low feldspar, and lithic fragments, representing a recycled orogen source. Major element compositions of CBB sands are characterized by high SiO2 (83.52–89.84 wt%) and low Al2O3 (4.39–6.39 wt%), whereas KB sands contained relatively low SiO2 (63.28–79.14 wt%) and high Al2O3 (9.00–11.33 wt%) contents. The major, trace and rare earth element (REE) compositions of beach sands display comparable distribution patterns with enriched Th and SiO2 for both sands relative to upper continental crust (UCC). Pb, Rb, Y, and Fe for KB sands are little higher than UCC and the rest of the elements are marked depleted for both suites reflecting destruction of plagioclase and K-feldspar during fluvial transportation. The CBB and KB sands are compositionally low mature to immature in nature subsequently classified as subarkose and litharenite, respectively. Chondrite-normalized REE patterns for CBB and KB sands show LREE enrichment and nearly flat HREE (LaN/YbN, 7.64–9.38 and 5.48–8.82, respectively) coupled with prominent Eu anomalies (Eu/Eu*, 0.51–0.72 and 0.52–0.76, respectively), suggesting felsic source provenance. The provenance discrimination diagrams, immobile trace element ratios (Th/Sc, Zr/Sc, Ce/Sc, and Ti/Zr), and REE (∑LREE/HREE, Eu/Eu* and GdN/YbN) parameters indicate that CBB and KB sands were largely derived from felsic source rocks, with compositions close to average rhyolite, granodiorite, granite, and UCC.

Changes of provenance of Permian and Triassic sedimentary rocks from the Ailaoshan suture zone (SW China) with implications for the closure of the eastern Paleotethys

Tectonic evolution of the Ailaoshan Ocean (eastern Paleotethys), especially on the timing of opening and closure, has long been debated. Published works on the Paleotethyan evolution were mainly dedicated to structural tectonics and magmatic and metamorphic rocks, whereas studies on sediment chemistry are rare. In this study, we present new chemical data on the Permian and Middle-Upper Triassic clastic sedimentary rocks from the Ailaoshan suture zone in southwestern Yunnan (SW China). Compared with the Post-Archean Australian Shale (PAAS) and the average upper continental crust, these Ailaoshan clastic rocks are generally depleted in LILEs (e.g. K and Rb) and HFSEs (e.g. Nb and Zr), and show varying LREE/HREE enrichments ((La/Yb)N = 1.32–11.86), flat HREE pattern and negative Eu anomalies (Eu/Eu* = 0.3–0.9). In spite of these similarities, the Permian samples have relatively low SiO2 (avg. 71.4 wt%) contents, K2O/Na2O (avg. 0.65) and Eu/Eu* (avg. 0.65) ratios, but high Al2O3 (avg. 14.1 wt%), TiO2 (avg. 0.60 wt%), (Fe2OT 3 + MgO) (avg. 6.96 wt%) contents, Al2O3/SiO2 (avg. 0.20) and (La/Yb)N (avg. 6.9) ratios, similar to greywackes from typical continental arcs. In contrast, the Upper Triassic samples have relatively high SiO2 (avg. 81.9 wt%) contents, K2O/Na2O (avg. 8.48) and LaN/YbN (avg. 9.74) ratios, but low Al2O3 (avg. 9.98 wt%), TiO2 (avg. 0.54 wt%), (Fe2OT 3 + MgO) (avg. 5.26 wt%) contents, Al2O3/SiO2 (avg. 0.12) and Eu/Eu* (avg. 0.64) ratios, similar to greywackes from typical passive continental margin. Moreover, the Permian samples have lower CIA values (Chemical Index of Alteration: 47–74, avg. 59) but higher ICV values (Index of Compositional Variability: 0.70–1.29, avg. 1.03) than those of PAAS, indicating relatively weak chemical weathering of the source rocks with low sediment maturity. In contrast, the Upper Triassic samples have relatively high CIA (77–84, avg. 80) but low ICV values (0.52–1.12, avg. 0.79), suggesting intense chemical weathering of the source rocks with high sediment maturity. The wide range of CIA (52–84, avg. 69) and ICV (0.50–1.14, avg. 0.88) values for the Middle Triassic samples suggest multiple detrital sources with varying degree of chemical weathering and diverse sediment maturity, as also supported by their wide range of detrital contents and chemical compositions. Based on the distinct chemical contrast between the Permian and Triassic sedimentary rocks, we infer that a significant change in detrital provenance and depositional setting may have occurred across the Middle and Late Triassic boundary, which was likely caused by the closure of the Ailaoshan Ocean.

New research on the origin of mottled clay in Quaternary basins in the coastal area of south China

Abstract Last Glacial Maximum (LGM) mottled clay occurs widely in Late Quaternary basins in south China coastal areas. Current research attributes its origin to exposure weathering of Late Pleistocene marine/fluvial deposits during the LGM. However, field data suggest that this is not the case as there is no gradual transition in lithology, grain size, structure and material composition among these layers. Instead, the mottled clay possesses sedimentary characteristics of exotic dust. In this study, three typical drill cores in the Pearl River Delta were studied using grain size analysis, diffuse reflection spectroscopy (DRS) and geochemical analysis to ascertain the clay’s sedimentary characteristics and origin. Grain size distribution patterns and parameters of the mottled clay were similar to those of a typical loess, indicating aeolian origin. In DRS curves, the peak height of hematite > goethite, indicating that the mottled clay had not experienced strong hydration and constitutes a continental product. This conforms to a typical loess but differs from the underlying marine/fluvial deposits. The chemical composition of the mottled clay was homogeneous in the vertical and planar directions. Upper continental crust (UCC) normalized curves of major and trace elements of the mottled clay were close to the average UCC and were consistent with typical aeolian deposits. The spatial and temporal distribution characteristics and relationship with the underlying layer suggest that the mottled clay was a loess-like deposit during the LGM and its mottled structure originated from strong modification of oxidation during the postglacial period after homogeneous dust had accumulated.

Geochemical normalization of magnetic susceptibility for investigation of floodplain sediments

Magnetic susceptibility (MS) is commonly used as a proxy for industrial pollution in natural sediments or as a proxy for the percentage of detrital components in peat or carbonates. The MS may also reflect the geology of the sediment source and post-depositional processes in sediments, such as soil development. The aim of our research was to test the usefulness of Fe-normalized mass-specific MS (χ) and Ti-normalized Fe in a study of floodplain sediments. We sampled 27 floodplain sediment cores from several geologically different catchments throughout the Czech Republic, analysed their Fe and Ti concentrations using X-ray fluorescence spectroscopy, and determined their χ. To decipher sediment grain-size dependence and possible magnetic enrichment, background functions for χ were constructed using similar approach as that used for geochemical background functions of the risk element concentrations with Fe concentrations as an independent variable. It provides a mechanism to calculate χ of sediments as it would be “pristine”, i.e. without post-depositional changes and pollution. Sediments derived from “mafic” source rocks had χ/Fe larger by two orders of magnitude than sediments derived from “felsic” rocks. Sediments derived from “mafic” source rocks also exhibit lower mean Fe/Ti ratio in pristine sediment strata than the average upper continental crust. The magnetic carriers inherited from mafic rocks are stepwise destroyed by pedogenesis in the floodplains and thus slowly approach χ of sediments derived from felsic rocks. Gleying processes may change χ/Fe, Fe/Ti ratio allows identifying a past action of those post-depositional processes.

Jurassic high heat production granites associated with the Weddell Sea rift system, Antarctica

The distribution of heat flow in Antarctic continental crust is critical to understanding continental tectonics, ice sheet growth and subglacial hydrology. We identify a group of High Heat Production granites, intruded into upper crustal Palaeozoic metasedimentary sequences, which may contribute to locally high heat flow beneath the West Antarctic Ice Sheet. Four of the granite plutons are exposed above ice sheet level at Pagano Nunatak, Pirrit Hills, Nash Hills and Whitmore Mountains. A new UPb zircon age from Pirrit Hills of 178.0±3.5Ma confirms earlier RbSr and UPb dating and that the granites were emplaced approximately coincident with the first stage of Gondwana break-up and the developing Weddell rift, and ~5m.y. after eruption of the Karoo-Ferrar large igneous province. Aerogeophysical data indicate that the plutons are distributed unevenly over 40,000km2 with one intruded into the transtensional Pagano Shear Zone, while the others were emplaced within the more stable Ellsworth-Whitmore mountains continental block. The granites are weakly peraluminous A-types and have Th and U abundances up to 60.7 and 28.6ppm respectively. Measured heat production of the granite samples is 2.96–9.06μW/m3 (mean 5.35W/m3), significantly higher than average upper continental crust and contemporaneous silicic rocks in the Antarctic Peninsula. Heat flow associated with the granite intrusions is predicted to be in the range 70–95mW/m2 depending on the thickness of the high heat production granite layer and the regional heat flow value. Analysis of detrital zircon compositions and ages indicates that the high Th and U abundances are related to enrichment of the lower-mid crust that dates back to 200–299Ma at the time of the formation of the Gondwanide fold belt and its post-orogenic collapse and extension.

Provenances and tectonic implications of Paleozoic siliciclastic rocks from the Baishuijiang Group of the southern Qinling belt, central China

The Paleozoic Baishuijiang Group is exposed in the southern Qinling belt and consists of turbidite sediments. The provenances of the Paleozoic sedimentary rocks is constrained by the integration of major and trace elements and detrital zircon U–Pb dating, which can help to understand the connection between the provenance and the Paleozoic tectonic evolution of the Qinling orogenic belt. The sandstones and mudstones have intermediate SiO2/Al2O3 and K2O/Na2O ratios, and high Fe2O3+MgO contents. In comparison with average upper continental crust, they show strong negative Nb–Ta and Sr anomalies, slight depletion of Zr–Hf and Th, but moderate enrichment of Sc, Ni and Cr. These rocks show LREE enrichment, and pronounced negative Eu anomalies in chondrite-normalized REE patterns, similar to post-Archean shales. The weathering trend of the sandstones and mudstones suggests andesitic and granodioritic provenances. These sediments are geochemically similar to continental island arc sediments, and therefore were probably deposited at an active continental margin. Detrital zircon U–Pb dating of five sandstones from the Baishuijiang Group yielded ages ranging from 407 to 3000Ma, with six peaks at ca. 425, 710, 780, 930, 1800, and 2500Ma. The youngest age peak at 425Ma and two zircon grains with a weighted average age of 414Ma indicate that the maximum depositional age of the Baishuijiang Group is late Paleozoic. Our new age data thus suggest that these turbidite sediments were mainly derived from the northern Qinling belt and the northern margin of the Yangtze Block. We propose that the Paleozoic sediments were deposited in an ocean basin between the northern Qinling belt and the Yangtze Block. Combined with regional geological evidences, our results indicate that there was a paleo-ocean in the Qinling orogenic belt since Neoproterozoic era. The oceanic crust was continually subducted northward, resulting in multiple accretion events from the early Paleozoic to early Triassic, which ultimately led to the amalgamation of the north China and Yangtze plates.

Platinum-group element abundances and Re–Os isotopic systematics of the upper continental crust through time: Evidence from glacial diamictites

The fine-grained matrix of glacial diamictites, deposited periodically by continental ice sheets over much of Earth history, provides insights into the average composition and chemical evolution of the upper continental crust (UCC) (Gaschnig et al., 2016, and references therein). The concentrations of platinum-group elements (PGEs, including Os, Ir, Ru, Pt and Pd) and the geochemically related Re, as well as 187Re/188Os and 187Os/188Os ratios, are reported here for globally-distributed glacial diamictites that were deposited during the Mesoarchean, Paleoproterozoic, Neoproterozoic and Paleozoic eras. The medians and averages of PGE concentrations of these diamictite composites decrease from the Mesoarchean to the Neoproterozoic, mimicking decreases in the concentrations of first-row transition elements (Sc, V, Cr, Co and Ni). By contrast, Re concentrations are highly variable with no discernable trend, owing to its high solubility. Assuming these diamictites are representative of average UCC through time, the new data are fully consistent with the previous inference that the Archean UCC contained a greater proportion of mafic–ultramafic rocks relative to younger UCC. Linear regressions of PGEs versus Cr and Ni concentrations in all the diamictite composites from the four time periods are used to estimate the following concentrations of the PGEs in the present-day UCC: 0.059±0.016ng/g Os, 0.036±0.008ng/g Ir, 0.079±0.026ng/g Ru, 0.80±0.22ng/g Pt and 0.80±0.26ng/g Pd (2σ of 10,000 bootstrapping regression results). These PGE estimates are slightly higher than the estimates obtained from loess samples. We suggest this probably results from loess preferentially sampling younger UCC rocks that have lower PGE concentrations, or PGEs being fractionated during loess formation. A Re concentration of 0.25±0.12ng/g (2σ) is obtained from a regression of Re versus Mo. From this, time-integrated 187Re/188Os and 187Os/188Os ratios for the UCC are calculated, assuming an average UCC residence duration of ∼2.0Ga, yielding ratios of 20±12 and 0.80±0.38 (2σ), respectively.

Compositional evolution of the upper continental crust through time, as constrained by ancient glacial diamictites

The composition of the fine-grained matrix of glacial diamictites from the Mesoarchean, Paleoproterozoic, Neoproterozoic, and Paleozoic, collected from four modern continents, reflects the secular evolution of the average composition of the upper continental crust (UCC). The effects of localized provenance are present in some cases, but distinctive geochemical signatures exist in diamictites of the same age from different localities, suggesting that these are global signatures. Archean UCC, dominated by greenstone basalts and to a lesser extent komatiites, was more mafic, based on major elements and transition metal trace elements. Temporal changes in oxygen isotope ratios, rare earth elements, and high field strength elements indicate that the UCC became more differentiated and that tonalite–trondhjemite–granodiorite suites became less important with time, findings consistent with previous studies. We also document the concentrations of siderophile and chalcophile elements (Ga, Ge, Cd, In, Sn, Sb, W, Tl, Bi) and lithophile Be in the UCC through time, and use the data for the younger diamictites to construct a new estimate of average UCC along with associated uncertainties.