Trace Element Distribution Patterns Research Articles

Lithospheric transformation of the northern North China Craton by changing subduction style of the Paleo-Asian oceanic plate: Constraints from peridotite and pyroxenite xenoliths in the Yangyuan basalts

Mechanisms of lithospheric transformation beneath the North China Craton (NCC) were widely discussed, but the northern NCC was less studied compare to the eastern part. In this research, In-situ major and trace element and Sr isotopic compositions of spinel peridotite and pyroxenite xenoliths carried by the Yangyuan Tertiary basalt were analyzed to constrain subduction style of the Paleo-Asian oceanic plate (PAOP) and its influence on the northern NCC. These xenoliths were classified into dunite, pyroxenite, and two groups of lherzolites according to their petrography and chemical features. The petrography and mineral chemistry suggest that the dunite, pyroxenite, and Group 1 lherzolites are residues of partial melting which experienced subsequent hydrous metasomatism. Correlations between 87Sr/86Sr with the Zr/La and Zr/Nb ratios in clinopyroxenes from these xenoliths suggest that the metasomatic agents should be fluids derived from the subducted PAOP. Mineral RbSr isochron of the pyroxenite show that the minimum age of the metasomatism event should be 295 ± 38 Ma. Major element compositions of minerals in the Group 2 lherzolites are more fertile compare to the dunite and Group 1 lherzolites. Clinopyroxenes in these xenoliths exhibit LREE-depleted REE patterns with relatively high HREE contents and 87Sr/86Sr ratios (0.70403–0.70482). Chemical zonations are preserved in several clinopyroxenes with incompatible elements depleted from core to rim, which should be the results of melt depletion after metasomatism. Patches of silica-rich glasses were present along the grain boundaries. These glasses have sediment-like Pb isotopic compositions and trace elements distribution patterns and thus are supposed to represent the melt from recycled sediments. Combined with geophysical and tectonic evidences, we propose that roll-back of the southward low-angle subducted PAOP in the late stage of subduction contributed to remelting of the PAOP subduction-modified mantle wedge beneath the northern NCC.

Multi-stage precipitation and redistribution of gold, and its collection by lead-bismuth and lead immiscible liquids in a reduced-intrusion related gold system (RIRGS); Dublin Gulch, western Canada

Dublin Gulch reduced intrusion-related gold system (RIRGS), located in the Selwyn Basin area of western Canada, represents one of the best examples of RIRGS mineralization globally and hence can be studied to unravel genesis and evolution of these types of deposits. Based on textural relationships, mineralogy, and trace element mineral chemistry, three auriferous vein stages were identified. The paragenetic sequence for the auriferous vein stages are: 1) Eagle Style (ES), quartz-albite, low sulfide and sulfosalt content (<5% vol.), As-Fe-Mo-W-Pb-Bi-Au-Ag veins; 2) Potato Hills Style-1 (PHS-1), quartz, high sulfide and sulfosalt content (>30% vol.), As-Fe-W-Pb-Bi-Au-Ag veins; and, 3) Potato Hills Style-2 (PHS-2), high sulfide and sulfosalt content (>30% vol.), Fe-Pb-Sb-Zn-Cu-Au-Ag veins. In the ES and PHS-1 veins, Au is present both as native gold (Au, Ag) and as invisible gold in arsenopyrite, whilst in the PHS-2 veins, Au is present as invisible gold in pyrite. Native gold micrograins (individual grains, 1–100’s µm in size) are observed associated with Pb minerals [in anhedral-to-globular cosalite (Pb2Bi2S5) in ES veins, or galena (PbS) in the PHS-1 veins]. Native gold is also observed as micrograins along arsenopyrite margins and in quartz fractures. We suggest a variation on the hydrothermal Bi melt collector model to explain the Au-Pb ± Bi association. The Au, Ag, Pb, and Bi are interpreted to have been locally remobilized from arsenopyrite, which shows textures and trace element distribution patterns consistent with fluid-and-deformation assisted recrystallization. We suggest Pb and Bi were mobilized either as immiscible nanodroplets that coalesced to form larger Pb ± Bi liquid accumulations or into the hydrothermal fluid and subsequently exsolved to form immiscible Pb ± Bi liquids. We propose that remobilized Au and Ag were collected by these Pb+/-Bi immiscible liquids. Subsequent retrograde alteration (sulfidation) is interpreted to have converted the Au-Ag-Pb ± Bi alloys to native gold and cosalite (ES), and native gold and galena mineral assemblages (PHS-1). The similarity of the Au/Ag ratios in native gold and arsenopyrite supports a local source for the native goldmicrograins. Temperatures required to attain liquid Pb0.5Bi0.5 (>145.2 °C) and Pb (>327.5 °C), are consistent with arsenopyrite geothermometry (ES 345–405 °C; PHS-1 ≈ 380 °C). These suggested new variations (Pb-Bi and Pb) on the hydrothermal melt (Bi) collector model are important, given the common association of native gold with Pb ± Bi sulfosalts in many gold deposits, and the relatively low temperatures required to maintain these liquids and collect Au from the hydrothermal fluid.

Recycling of Paleo-oceanic crust: Geochemical evidence from Early Paleozoic mafic igneous rocks in the Tongbai orogen, Central China

Crustal subduction generally develops from oceanic to continental subduction in collisional orogens, with the disappearance of previously subducted paleo-oceanic crust. However, it is not straightforward to trace the fate of subducted paleo-oceanic crust due to modification by subsequent continental collision. In this study, we present a combined study of secondary ion mass spectrometry (SIMS) zircon UPb ages and O isotopes, laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) zircon LuHf isotopes, whole-rock major-trace elements and SrNd isotopes of Early Paleozoic mafic igneous rocks in the Tongbai orogen, central China. These geochemical data are interpreted and modelled in terms of crust-mantle interaction in a subduction channel, in order to account for geochemical transfer from the subducting crust to the mantle wedge. The results provide insight not only into paleo-oceanic crust recycling but also into the nature of orogenic lithospheric mantle. SIMS zircon UPb dating of the mafic igneous rocks yields Early Paleozoic ages of 436 ± 4 to 449 ± 4 Ma for magma crystallization. These mafic rocks have arc-type trace element distribution patterns, with enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE) but depletion in high field strength elements (HFSE) relative to rare earth elements (REE) and LILE. They have depleted whole-rock SrNd isotopic compositions, with initial 87Sr/86Sr ratios of 0.7037 to 0.7047 and positive εNd(t) values of 2.7 to 5.0. The mafic igneous rocks also display positive zircon εHf(t) values of 8.9 to 14.8 and variable δ18O values of 4.4 ± 0.3 to 5.9 ± 0.3‰. These geochemical features suggest derivation from fertile mantle sources enriched in LILE and LREE but depleted in radiogenic isotopes. The mantle sources are suggested to be part of the orogenic lithospheric mantle, which was generated by metasomatic reaction of the overlying mantle wedge with previously subducted Erlangping oceanic crust-derived fluids. Some samples have high Ba/Th, Th/Nb and (La/Yb)N ratios, suggesting that the metasomatic agents include both basaltic oceanic crust-derived aqueous solutions and sediment-derived hydrous felsic melts. Model calculations confirm that the geochemical composition of the Tongbai mafic igneous rocks can be explained by reaction of depleted mid-oceanic ridge basalt (MORB) mantle peridotite with 1–10% of subducted Erlangping oceanic crust-derived aqueous solutions plus 0.05–0.1% of sediment-derived hydrous melts. This melt-peridotite reaction would generate a pyroxenite-rich lithology in the mantle source. During the tectonic transition in the Ordovician to Silurian, partial melting of such mantle sources gave rise to mafic magmatism due to back-arc extension. Thus, the Early Paleozoic mafic igneous rocks in the Tongbai orogen record the recycling of earlier subducted paleo-oceanic crust and the nature of the orogenic lithospheric mantle in the collisional orogen.

Laser Ablation-Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS) elemental mapping and its applications in ore geology

With the development of two-dimensional trace element mapping by laser-ablation inductively-coupled-plasma mass spectrometry (LA-ICP-MS), it is now possible to reveal in much greater detail any compositional interior textures of the pyrites, thus permitting the investigation of the history of pyrite crystals and providing new insight into the genesis and evolution of mineralisation. Exploiting the advantages of high spatial resolution, low detection limits, and capability of full quantification, LA-ICP-MS elemental mapping is being widely applied to imaging sulphides from various types of deposits. In this study, we applied the LA-ICP-MS elemental mapping technique on pyrites from different types of deposits from China and Canada, for the purpose of monitoring the changes of physio-chemical condition, defining ore genesis, and revealing the distribution patterns of precious metals. The interior compositional textures of pyrites show that the mineralization system of the Bayinwula roll-front U deposit in Inner Mongolia was open and continuously affected by the re-charge of oxidized groundwater, along with a periodic change of pH and Eh that affected the activities of sulphate bacteria and the associated U mineralization. The trace element maps of pyrites from the Zhenzigou Pb-Zn deposit in Liaoning Province indicate that several episodes of fluids were involved and responsible for the mineralisation, namely, an early stage of syn-sedimentary origin and a late hydrothermal mineralisation related to the Yanshanian magmatic activity. The trace element distribution patterns of pyrites from the Palaeoproterozoic placer Au deposit in Canada showed that the subeuhedral-euhedral pyrite, which is a dominant type of pyrite in the conglomerates, is detrital origin and later modified by hydrothermal (or metamorphic) fluids, and Au is mostly associated with the detrital pyrite grains/cores, and disseminated in the porosity on the surface.

Trace element characterization of bitumen constraints on the hydrocarbon source of the giant gas field in Sichuan Basin, South China

The exploration of high production, natural gas wells (&gt;106 m3/d) has been extremely successful in the Gaoshiti and Moxi areas of the Leshan–Longnvsi paleo‐uplift, in the west‐central Sichuan Basin. This paper focuses on the Sinian and Cambrian reservoirs from the Gaoshiti and Moxi areas, where a dataset of new trace elements in bitumen and potential source rock is reported and where hydrocarbon‐source correlation is attempted based on trace elements characteristics of the 12 potential source rocks and 22 bitumen samples. Results show that the bitumen from the three reservoirs can be distinguished from each other using trace element distribution patterns and redox‐ and provenance‐sensitive elemental ratios. Bitumen in the second and fourth members of the Sinian Dengying Formation were derived primarily from the third member of the source rocks of the Dengying Formation, which represents a self‐sourced paleo‐petroleum system. In contrast, bitumen in the reservoirs of the Cambrian Longwangmiao Formation is derived from the Cambrian Qiongzhusi Formation and third member of Dengying Formation. The trace element method used in this study improves our understanding of the complex hydrocarbon–source relation and confirm the potential target for petroleum exploration in the study area.

U-Pb (LA-ICP-MS) age of detrital zircons and the sources of terrigenous sediments of the Ipsit suite, Karagass series (Sayan segment of the Sayan-Baikal-Patom belt)

The petrographic, lithogeochemical and U-Pb (LA-ICP-MS) geochronological studies were carried out to investigate the terrigenous rocks sampled from the lower part of the Ipsit suite of the Karagass series (Sayan segment of the Sayan-Baikal-Patom belt). These rocks include sandstones, aleurite sandstones and aleurolites, and their mineral compositions are close to that of arkose. Most of the studied rock samples show petrographic features typical of the epigenetic changes at the stage of catagenesis: regeneration of quartz clastic grains, pelitization of potassium-feldspar clastic grains, occurrence of clay-hydromica aggregate, sericitization of plagioclase, chloritization of biotite, and silicification of dolomite pieces, and occurrence of authigenous tourmaline. The above was confirmed by the analysis of the concentrations of petrogenic elements in the studied rocks from the lower part of the Ipsit suite. The analysis results show that the concentrations of K2O are elevated, while the concentrations of Na2O are relatively very low, which may be due to the redistribution of these elements during epigenetic transformations. According to the classification by genetic types on the basis of the system of petrochemical modules, the rocks of the lower part of the Ipsa suite are of the petrogenic nature. The acidic igneous rocks are dominant in the source area, as evidenced by the presence of granitoid and quartzite fragments in the clastogenic component, as well as the set of accessory minerals typical of the igneous rocks of the acidic composition, and the distribution pattern of rare and trace elements. According to the U-Pb (LA-ICP-MS) dating of detrital zircons from the aleurite sandstone sampled from the lower part of the Ipsit suite, the zircons are exclusively of the Archean-Early Proterozoic ages. Such ages correlate with the age of the granitoids of the Sayan complex and the felsic volcanites from the Maltsev layer of the Elash series (Biryusa block). Furthermore, the detrital-zircon age spectra of the aleurite sandstone of the lower part of the Ipsit suite are identical to the detrital-zircon age spectra of the terrigenous rocks from the underlying strata of the Shangulezh and Tagul suites of the Karagass series. This study suggests that sedimentation of the Ipsit suite of the Karagass series took place due to the influx of detrital material from the southern part of the Siberian craton into the sedimentation basin, and the acidic igneous rocks of the Biryusa block were one of the main sources of detrital material.

Geochronology and petrogenesis of Jurassic intraplate alkali basalts in the Junggar terrane, NW China: Implication for low-volume basaltic volcanism

High-volume volcanic fields which are observed at continental intraplate setting are generally known as the large igneous provinces (LIPs), fed by hot and active plumes upwelling from the deep mantle. However, the source and mechanism of low-volume effusive volcanism within continental interiors remains poorly unknown. Here, we present a combined study of 40Ar/39Ar geochronology, mineral chemistry, whole-rock major and trace elements as well as Sr-Nd isotopes of Jurassic basalts from the Karamay area, Junggar terrane (NW China), aiming to determine their formation ages, constrain the petrogenesis and reveal their tectonic implications.New whole-rock 40Ar/39Ar dating yields consistent ages of 189.4–193.3 Ma for magma emplacement. The basalts exhibit a columnar joint structure and porphyritic texture with phenocryst minerals of olivine (Fo63–79), clinopyroxene (Wo39–46En38–47Fs12–22) and plagioclase (An25–54). They have SiO2 contents ranging from 45.0–51.8 wt% and belong to alkaline series (δ2.7–13.3, average of 5.0). The basalts are characterized by oceanic island basalt (OIB)-like trace element distribution patterns with enrichment in L-MREE, HFSE (e.g., Nb and Ta) and LILE (e.g., K, Sr and Ba), and slight depletion in HREE, relative to normal mid-ocean ridge basalt (N-MORB). Positive εNd(t) (+3.0), low to moderate 87Sr/86Sri (0.7048–0.7049) isotopic compositions, and trace element ratios (e.g., high Nb/U > 49.0 and Ce/Pb > 13.9) suggest that crust contamination was insignificant in the formation process. P–T estimates of major phenocryst minerals in these basalts reflect high crystallization temperatures (>1100 °C) but low pressures (<9.8 kbar). Geochemical features and REE modeling indicate that the basaltic magmas were likely derived by low degrees of partial melting (~5% – 10%) of an asthenospheric mantle source in the garnet stability field. Calculated lithospheric thickness and initial melting depths for the studied basalts are about 120 km and 180–200 km, respectively. Based on the available data and regional secular geological evolution, we suggest that decompression melting of upwelling asthenosphere triggered by small-scale removal of thickened lithospheric root could be a possible mechanism to explain Jurassic low-volume intraplate volcanism in the Junggar terrane.

The chemical and dynamic distribution characteristics of iron, cobalt and nickel in three different anaerobic digestates: Effect of pH and trace elements dosage

The enhancement of the bioavailability and process controllability of trace elements (TEs) addition is of significance to improve the anaerobic digestion (AD) performance. In order to understand the bioavailability of TEs, the chemical form distribution patterns of endogenous/exogenous Fe, Ni and Co with different dosage (Fe: 1, 10 and 100 mg/kg; Ni and Co: 0.1, 1 and 10 mg/kg) and different pH condition (6.5; 7.5 and 8.5) were investigated in three different anaerobic digestates. The results showed that the exogenous TEs dosage exerted no obvious effects on the TEs distribution patterns. The chemical fractionation of TEs was more affected by pH, redox potential, AD process and digestate resource. Under pH 6.5 and with redox potential rising, the acid-extractable and reducible fractions and of Fe, Ni and Co were increased while the oxidizable fractions were decreased, which enhanced the bioavailability of TEs.

Hybridization Melting Between Continent‐Derived Sediment and Depleted Peridotite in Subduction Zones

AbstractTo better constrain the processes by which subducted continent‐derived sediment and the overlying mantle wedge react to produce hybrid magmas, we have performed a series of high‐pressure interaction experiments involving a phyllitic metasediment and a depleted peridotite (dunite) from the Mediterranean section of the Alpine‐Himalayan orogenic belt at 2–3 GPa. Two different experimental methods were compared, namely, the reaction of separate, juxtaposed peridotite and metasediment blocks (reaction experiments) and capsules in which the two rocks were included as an intimately mixed powder before the experiments (mixed experiments). In reaction experiments, only the metasediment partially melted, and a marked, thin reaction zone dominated by orthopyroxene formed between the dunite and phyllite blocks. In contrast, a hybrid melt phase was found in all mixed experiments. Trace element analyses indicated that Cs, Rb, Ba, Th, U, Nb, Ta, Zr, and Hf were strongly incompatible, together with marked light rare earth element/heavy rare earth element fractionation. The reaction with peridotite can significantly affect the mobility of medium rare earth element, heavy rare earth element, and transition elements during sediment‐mantle interaction, enriching them in hybrid melts. In addition, the nature of the sedimentary starting materials can considerably affect trace element behavior in resulting hybrid melts. There is a significant resemblance between the trace element distribution patterns of hybrid melts produced in our experiments and those of natural postcollisional potassium‐rich volcanic rocks from the eastern Mediterranean region. The experiments show that phlogopite is not necessarily required in the source to produce the SiO2‐richer end of the range of K‐rich magmas.

Comprehensive large-scale investigation and assessment of trace metal in the coastal sediments of Bohai Sea

The Bohai Sea is characterized as a semi-closed sea with limited water exchange ability, which has been regarded as one of the most contaminated regions in China and has attracted public attention over the past decades. In recent years, the rapid industrialization and urbanization around the coastal region has resulted in a severe pollution pressure in the Bohai Sea. Although efforts from official government and scientific experts have been made to protect and restore the marine ecosystem, satisfactory achievements were not gained. Moreover, partial coastal areas in the Bohai Sea seemingly remain heavily polluted. In this study, we focused on five coastal regions around the Bohai Sea to study the spatial distribution pattern of trace elements in the sediments and their ecological risk. A total of 108 sediment samples were analyzed to determine the contamination degree of trace elements (Cu, Cd, As, Pb, Zn, Cr, and Hg). Contamination factor (CF), pollution load index (PLI), geoaccumulation index (Igeo), and potential ecological risk index (PERI) were utilized to assess the pollution extent of these metals. Spatial distribution patterns revealed that the sedimentary environments of coastal Bohai were in good condition, except Jinzhou Bay, according to the Marine Sediment Quality of China. The concentrations of Hg and Cd were considerably higher than the average upper crust value and presented high potential ecological risk and considerable potential ecological risk, respectively. The overall environment quality of the coastal Bohai Sea does not seem to pose an extremely serious threat in terms of metal pollution. Thus, the government should continue implementing pollution control programs in the Bohai Sea.

The role of organo-clay associations in limiting organic matter decay: Insights from the Dajiuhu peat soil, central China

The mineral-bound organic matter (OM) in soils is of great importance for the global carbon cycle. Long-standing debate exists regarding the mechanisms of soil organic matter (SOM) preservation and, especially, the role of organo-clay mineral associations. Investigation of such associations is rarely undertaken in peat soils. In this study, we examine organo-clay relationships in the Dajiuhu peat soil in Hubei Province, central China, based on molecular markers, which allows for understanding of rates of OM degradation and the preservation of OM associated with clay minerals. We analyzed the major and trace element chemistry, clay mineralogy, and lipid compositions of both the coarse- and clay-sized fractions of the soil. The uniformity of trace element and REE distribution patterns in the soil (except for Layer 2) suggests that the source materials were well mixed. Chemical index of alteration (CIA) values of 70–85 indicate moderate to strong weathering processes. A significant correlation between Corg:P and TOC may indicate that redox conditions were a significant control on OM preservation. The clay fraction of Dajiuhu peatland soils also has played an important role in preserving OM, especially in anoxic environments. Fourier transform infrared spectrophotometry (FTIR) and X-ray diffraction (XRD) analysis show that OM is absorbed mainly on the edges of illite grains rather than in interlayer sites. The amphoteric margins of illite crystals allow them to more effectively bind organic compounds. The high Rl/h ratio, high carbon preference index (CPI), and high ratio of unsaturated-to-saturated fatty acids (C18:2/C18) in the clay-sized fraction provide further biomarker evidence that illite efficiently protects OM from biodegradation, thus enhancing the OM content of soils.

Geochemistry of the Samarka terrane cherts (Sikhote-Alin) and the size of the accreted paleo-oceanic plate

We have studied the geochemistry of Late Triassic cherts from tectonic sedimentary complexes of different structural levels of the Samarka terrane. The results show that the contents and distribution patterns of major rock-forming oxides, trace elements, and REE in the cherts of the upper and lower structural levels are somewhat different, which is due to different facies environments of chert accumulation. In particular, the contents of Al2O3, TiO2, and K2O gradually decrease and the contents of Fe2O3 and MnO increase downsection. The contents of Zr, Rb, Hf, Th, and Cr, which were supplied into the bottom sediments with a terrigenous suspension of heavy-mineral fragments, are twice to ten-fold lower than their average contents in the upper crust. In contrast, the contents of Pb, Cu, and Ni, which got into the sediments mostly with metalliferous hydrothermal solutions, are rather high and even exceed their average contents in the middle and upper crust. That is, the contents of the first-group trace elements gradually decrease and the contents of the second-group trace elements increase from upper to lower structural level. The calculated negative Ce anomaly gradually decreases from lower (0.75) to upper (0.88) structural level. The geochemical parameters altogether indicate that the cherts accumulated in the same pelagic depositional environment but in its different parts. The Katen complex composing the lower structural level is the most remote from the continental margin, and the Amba-Matay complex forming the upper structural level is the most proximal. Based on the geochemical and biostratigraphic data and on the time of accretion of paleo-oceanic fragments, we have established the location of each complex within the not yet subducted oceanic plate and estimated the extensionof this plate. Throughout the Jurassic, about 6000 km of the oceanic lithosphere was subducted beneath the eastern margin of the Paleo-Asian continent and partly accreted to it.

Petrogenesis of basalt from the middle Okinawa Trough: New insights from olivine‐hosted melt inclusions

Silicate melt inclusions (SMIs) in basalt from the middle Okinawa Trough are investigated in detail to understand the source geochemical characteristics and petrogenesis of the basalt. The middle Okinawa Trough basalts contain abundant SMIs within olivine phenocrysts. The major element compositions of the SMIs are analysed using an electron microprobe. After correcting for olivine postentrapment crystallization, the major element compositions of the SMIs are more variable in terms of SiO2 (49.7–54.8 wt%) and MgO (5.1–7.2 wt%) than those of the host basalt (SiO2, 52.4 wt%; MgO, 5.7 wt%), which could indicate the existence of more primitive melts than erupted bulk rock. The major element compositions of the SMIs indicate that the SMI‐recorded melt evolution process was mainly dominated by the fractional crystallization of olivine. According to laser ablation inductively coupled mass spectrometer analyses, the trace element compositions of the SMIs are similar to those of the host bulk rock and feature obvious enrichment in Pb and large‐ion lithophile elements relative to high field strength elements, rare earth elements, and highly negative Nb and Ta anomalies. The low Ce/Pb ratios, high Pb contents, and trace element distribution patterns in the SMIs are related to contributions from subducted sedimentary components in the magma source. Based on the electron microprobe analyses, the chemical composition of the olivine suggests that the lithology of the basalt source may be pyroxenite. The basalts may have been generated by the partial melting of pyroxenite during interactions between mantle‐wedge peridotite and Si‐rich melts that were released from the subducted slab.

Mesozoic mafic magmatism in North China: Implications for thinning and destruction of cratonic lithosphere

The North China Craton (NCC) has been thinned from >200 km to in its eastern part. The ancient subcontinental lithospheric mantle (SCLM) has been replaced by the juvenile SCLM in the Meoszoic. During this period, the NCC was destructed as indicated by extensive magmatism in the Early Cretaceous. While there is a consensus on the thinning and destruction of cratonic lithosphere in North China, it has been hotly debated about the mechanism of cartonic destruction. This study attempts to provide a resolution to current debates in the view of Mesozoic mafic magmatism in North China. We made a compilation of geochemical data available for Mesozoic mafic igneous rocks in the NCC. The results indicate that these mafic igneous rocks can be categorized into two series, manifesting a dramatic change in the nature of mantle sources at ~121 Ma. Mafic igneous rocks emplaced at this age start to show both oceanic island basalts (OIB)-like trace element distribution patterns and depleted to weakly enriched Sr-Nd isotope compositions. In contrast, mafic igneous rocks emplaced before and after this age exhibit both island arc basalts (IAB)-like trace element distribution patterns and enriched Sr-Nd isotope compositions. This difference indicates a geochemical mutation in the SCLM of North China at ~121 Ma. Although mafic magmatism also took place in the Late Triassic, it was related to exhumation of the deeply subducted South China continental crust because the subduction of Paleo-Pacific slab was not operated at that time. Paleo-Pacific slab started to subduct beneath the eastern margin of Eruasian continent since the Jurrasic. The subducting slab and its overlying SCLM wedge were coupled in the Jurassic, and slab dehydration resulted in hydration and weakening of the cratonic mantle. The mantle sources of ancient IAB-like mafic igneous rocks are a kind of ultramafic metasomatites that were generated by reaction of the cratonic mantle wedge peridotite not only with aqueous solutions derived from dehydration of the subducting Paleo-Pacific oceanic crust in the Jurassic but also with hydrous melts derived from partial melting of the subducting South China continental crust in the Triassic. On the other hand, the mantle sources of juvenile OIB-like mafic igneous rocks are also a kind of ultramafic metasomatites that were generated by reaction of the asthenospheric mantle underneath the North China lithosphere with hydrous felsic melts derived from partial melting of the subducting Paleo-Pacific oceanic crust. The subducting Paleo-Pacific slab became rollback at ~144 Ma. Afterwards the SCLM base was heated by laterally filled asthenospheric mantle, leading to thinning of the hydrated and weakened cratonic mantle. There was extensive bimodal magmatism at 130 to 120 Ma, marking intensive destruction of the cratonic lithosphere. Not only the ultramafic metasomatites in the lower part of the cratonic mantle wedge underwent partial melting to produce mafic igneous rocks showing negative e Nd( t ) values, depletion in Nb and Ta but enrichment in Pb, but also the lower continent crust overlying the cratonic mantle wedge was heated for extensive felsic magmatism. At the same time, the rollback slab surface was heated by the laterally filled asthenospheric mantle, resulting in partial melting of the previously dehydrated rocks beyond rutile stability on the slab surface. This produce still hydrous felsic melts, which metasomatized the overlying asthenospheric mantle peridotite to generate the ultramafic metasomatites that show positive e Nd( t ) values, no depletion or even enrichment in Nb and Ta but depletion in Pb. Partial melting of such metasomatites started at ~121 Ma, giving rise to the mafic igneous rocks with juvenile OIB-like geochemical signatures. In this context, the age of ~121 Ma may terminate replacement of the ancient SCLM by the juvenile SCLM in North China. Paleo-Pacific slab was not subducted to the mantle transition zone in the Mesozoic as revealed by modern seismic tomography, and it was subducted at a low angle since the Jurassic, like the subduction of Nazca Plate beneath American continent. This flat subduction would not only chemically metasomatize the cratonic mantle but also physically erode the cratonic mantle. Therefore, the interaction between Paleo-Pacific slab and the cratonic mantle is the first-order geodynamic mechanism for the thinning and destruction of cratonic lithosphere in North China.

Petrogenesis of the Late Triassic diorites in the Hoh Xil area, northern Tibet: Insights into the origin of the high-Mg# andesitic signature of continental crust

An integrated petrologic, geochronologic, major and trace element geochemical, and Sr-Nd-Hf isotopic study of Late Triassic (~215Ma) diorites from the Hoh Xil area, northern Tibet, provides new constraints on the genesis of intermediate magmas and insights into the origin of the high-Mg# andesitic signature of continental crust. These dioritic rocks are characterized by high MgO contents (3.3–5.0wt%) and Mg# values (50–57) comparable to the estimates for the bulk continental crust at the same level of SiO2 contents (61.1–64.5wt%). They also display continental crust-like trace element distribution patterns and uniformly enriched isotope compositions ([87Sr/86Sr]i=0.7081 to 0.7094, ɛNd[t]=−8.0 to −6.9, and ɛHf[t]zircon=−10.1 to −5.0). Combining our results with published data from crystallization experiments, we propose that they were probably produced by fractional crystallization from a primitive andesite parent, rather than a primitive basalt parent. This parental magma may be geochemically similar to the roughly contemporaneous primitive andesites in the adjacent Malanshan area of northern Tibet. Our compilation of modern arc lavas shows that progressive fractional crystallization of primitive andesites is also required to reproduce the Mg# versus SiO2 array for natural arc magmas, in addition to differentiation of mantle-derived primitive basaltic magmas and/or mixing of basaltic with felsic magmas. Therefore, we emphasize that fractional crystallization of primitive andesitic magmas is potentially a frequent occurrence in arc crust and hence may play an important role in producing the high-Mg# signature of intermediate magmas comprising the continental crust.

A Study on the Environmental Chemistry of Toxic Trace Elements in the Large Area Water Body

This paper adopts the research approach of trace element environmental chemistry, conducts a tracer study on the cyclic process of toxic trace elements in the chemical substances in the watershed from the perspective of time and space, and studies the content, the distribution, the distribution characteristics mechanism in different phases as well as the influencing factors of toxic trace elements and other trace elements. Test results have demonstrated that the water chemistry characteristics of the small watershed in the southern Jiangxi Province affect the content and the distribution pattern of the dissolved trace elements. The pH value of the river water is the most significant factor that controls the behaviors of the dissolved trace elements in the river water. The content of the dissolved trace drops with the increase of pH value. Trace content and its value have showed an inverse relationship.

The Trace Element Distribution Patterns of Ediacaran‐Early Cambrian Black Shales and the Origin of Selenium in the Guangning Area, Western Guangdong Province, South China

AbstractThe Ediacaran and early Cambrian black shales are widespread across the South China Craton (Yangtze and Cathaysia blocks). However, the trace element distribution patterns of the Ediacaran and early Cambrian black shales in the Cathaysia Block are still unclear. In this study, thirty‐four black shale samples in the Lechangxia Group (Ediacaran) and thirteen black shale samples in the lower Bacun Group (early Cambrian) from Guangning area, western Guangdong Province, South China, were analyzed for major and trace elements concentrations. Compared to the upper continental crust, the Ediacaran black shales exhibit strongly enriched Se, Ga, and As with enrichment factor values (EF) higher than 10, significantly enriched Bi and Rb (10&gt;EF&gt;5), weakly enriched Mo, Ba, Cs, V, In, Be, Tl, and Th (5&gt;EF&gt;2), normal U, Cr, Cd, Sc, Pb, Cu, and Li (2&gt;EF&gt;0.5), and depleted Ni, Zn, Sr, and Co. Early Cambrian black shales display strongly enriched Se, Ga, and As, significantly enriched Ba, Bi, and Rb, weakly enriched Mo, Cs, Cd, V, U, Be, In, and Tl, normal Sc, Th, Cr, Li, Cu, Ni, and Pb and depleted Co, Zn, and Sr. Moreover, Se is the most enriched trace element in the Ediacaran and early Cambrian black shales: concentrations vary from 0.25 to 30.09 ppm and 0.54 to 5.01 ppm, and averaging 4.84 and 1.72 ppm, with average EF values of 96.87 and 34.32, for the Ediacaran and early Cambrian shales respectively. The average concentration of Se in the Ediacaran black shales is 2.8 times higher than that of early Cambrian black shales. Se contents in the Ediacaran and early Cambrian black shales exhibit significant variation (P = 0.03). Provenance analysis showed that Se contents of both the Ediacaran and early Cambrian black shales were without detrital provenance and volcanoclastic sources, but of hydrothermal origin. The deep sources of Se and the presence of pyrite may explain the higher Se contents in the Ediacaran black shales. Similar with the Se‐rich characteristics of the contemporaneous black shales in the south Qingling Mountain and Yangtze block, the Ediacaran and early Cambrian black shales in Guangning area, Cathaysia, are also enriched in Se, which may provide a clue for looking for the selenium‐rich resources in western Guangdong Province.

Magmatismo basáltico do Andar Alagoas (Bacia de Campos)

Magmatismo associado com o estágio pré-evaporítico da Bacia de Campos foi estudado a partir de testemunhos de quatro furos de sondagens. Os dados levantados são importantes no contexto da evolução tectônica e estratigráfica da bacia. O objetivo deste trabalho foi a caracterização dessas rochas a partir da petrografia, litoquímica, química mineral e composição isotópica Sr-Nd. A aplicação das metodologias de estudo convencionais foi dificultada devido ao grau de alteração das rochas, marcado por modificações mineralógicas (substituição de minerais por argilominerais, palagonitização do vidro vulcânico, sericitização dos plagioclásios e presença de vesículas), ao elevado valor de perda ao fogo (LOI) e à identificação de sedimentos arenosos englobados pela rocha vulcânica. Para minimizar os efeitos da alteração, empregaram-se metodologias que permitiram avaliar o grau de alteração das amostras e a mobilidade dos elementos químicos na rocha total. Os resultados obtidos estabelecem que as rochas vulcânicas são basaltos originados a partir de magmas com composição subalcalina, toleítica a transicional, baixo TiO2 e alto magnésio. Os elementos terras raras (ETR) e o padrão de distribuição dos elementos traço estão em conformidade com o padrão dos basaltos toleíticos baixo TiO2 da Formação Serra Geral e são diferentes dos enriched-mid ocean ridge basalt (E-MORB) e dos ocean island basalts (OIB) do oceano Atlântico, embora alguns elementos tenham comportamento similar. O piroxênio dominante é a augita e, quimicamente, confirma que os basaltos pertencem à série subalcalina, bem como ao intervalo de transição entre as séries cálcio-alcalina e toleítica. Isotopicamente, as amostras estudadas incidem fora do campo estabelecido como “mantle array”; e seguem o comportamento dos basaltos da Bacia de Campos com alto potássio. Os parâmetros levantados estabelecem a correlação das rochas vulcânicas do Andar Alagoas da Bacia de Campos com os basaltos intracontinentais da Bacia do Paraná.

Petrogenesis and geodynamic setting of Early Cretaceous felsic rocks in the Gan-Hang Belt, Southeast China: Constraints from geochronology and geochemistry of the tuffs and trachyandesitic rocks in Shengyuan volcanic Basin

The Late Mesozoic geology of the Gan-Hang Belt is characterized by extensive magmatism forming a belt of volcanic-intrusive complex. The geochronology, petrogenesis, and geodynamic setting of the Late Mesozoic magmatic rocks in the Gan-Hang Belt are still controversial. The Shengyuan volcanic Basin is located in the NW region of the belt and mainly contains crystal tuff, welded tuff, and trachyandesitic rocks. We integrate geochronological and geochemical data for these tuffs and trachyandesitic rocks, to explore the origin of these rocks and improve our understanding of the Late Mesozoic magmatic and tectonic evolution of the region. Zircon U-Pb dating shows that these samples were formed in the Early Cretaceous (135–137Ma). All the tuffs have a pronounced A2-type geochemical signature and their chemical compositions are controlled by crystal fractionation. All trachyandesitic rocks exhibit high K2O contents and were attributed to the shoshonite series; they are characterized by arc-like trace element distribution patterns, with significant enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE) but depletion in high field-strength elements (HFSE). Moreover, all the tuffs and trachyandesitic rocks were characterized by negative whole rock εNd(t) and zircon εHf(t). An integrated interpretation of all these geochemical data leads to the conclusion that the Shengyuan trachyandesitic rocks were primarily derived from mantle materials and oceanic crust-derived melt. The Shengyuan tuffs were formed by the partial melting of the Proterozoic orthometamorphic and parametamorphic rocks. Our studies together with previous published data suggest that the Early Cretaceous A-type felsic rocks, with ages between 138Ma and 122Ma, occurred along the Gan-Hang Belt, indicating an important Late Mesozoic extensional event in the Belt. This event represents a back-arc tectonic setting due to the rollback of the Paleo-Pacific plate.

Late Mesozoic high-K calc-alkaline magmatism in Southeast China: the Tongling example

ABSTRACTWe report new zircon U–Pb ages, Hf isotopic and geochemical results for the Tongling granitic plutons of Southeast China. SHRIMP U–Pb ages for the Miaojia quartz monzodiorite porphyrite，the Tianebaodan and Tongguanshan quartz monzodiorites, the Xinqiaotou granodiorite porphyry, and the Shatanjiao and Nanhongchong granodiorite are 143 ± 2, 141 ± 1 and 142 ± 1, 147 ± 1, and 145 ± 1 and 139 ± 1 Ma, respectively. Combined with previous geochronological data, our results indicate that the porphyritic rocks are older than rocks of the same type lacking porphyritic texture. Geochemically, these high-K calc-alkaline intrusive rocks are characterized by arc-like trace element distribution patterns, with significant enrichment in LILE and LREE but depletion in HFSE. Lu–Hf isotopic compositions of zircons from the high-K calc-alkaline (HKCA) rocks have εHf(t) values of magmatic 139–147 Ma zircons from −8.1 to −25.6, with two-stage model ages (tDM2) of 1.71–2.67 Ga, whereas εHf(t) values of inherited 582–844 Ma zircons range from 5.4 to −9.5, with tDM2 of 1.39–2.22 Ma, younger than tDM2 values of igneous zircon, indicating that newly formed mantle material was added to the continental crust of the Yangtze Block. Moreover, εHf(t) values of inherited zircon cores older than 1000 Ma are from −7.8 to −26, similar to magmatic zircons, and the tDM2 values are all greater than 3.0 Ga (3.16–3.75 Ga), reflecting partial melting of ancient sialic material. We conclude that the plutonic melts were derived from both the enriched mantle and the ancient crust. The HKCA Tongling intrusions coincide temporally with the J3–K1 magmatic event that was widespread in Southeast China. This igneous activity may have accompanied sinistral slip along the Tan-Lu fault due to oblique subduction of the Palaeo-Pacific plate.