柴北缘斜长角闪岩的地球化学特征及其构造背景

分布在柴北缘超高压变质带中的斜长角闪岩主要有两种类型,一种由榴辉岩退变而成,一种只经历角闪岩相变质作用。它们的原岩属于拉斑玄武质岩石,轻稀土富集,Nd同位素组成亏损,这些玄武岩浆分别来自不同的地幔源区,地壳混染不明显,形成的环境可能为大陆裂谷或初始洋盆,这可能是柴北缘早古生代洋盆打开的前兆。随着早古生代洋盆的关闭,这些基性火成岩部分经历了超高压变质作用,即发生了深俯冲,部分只经历角闪岩相变质。无论哪种情况,它们在遭受变质作用之前,就与陆壳岩石共生在一起,支持柴北缘榴辉岩、斜长角闪岩与片麻岩的关系为原地关系(in situ)?     

Vertical geochemical variations and arsenic mobilization in the shallow alluvial aquifers of the Chapai-Nawabganj District, northwestern Bangladesh: implication of siderite precipitation

Core sediments from three disturbed boreholes (JOR, GHAT, and RAJ) and two undisturbed boreholes (DW1 and DW2) were collected in the study area of the Chapai-Nawabganj district of northwestern Bangladesh for geochemical analyses. In the study area, groundwater samples from fourteen As-contained private wells and five nested piezometers at both the DW1 and DW2 boreholes were also collected and analyzed. The groundwater arsenic concentrations in the uppermost aquifer (10–40 m of depth) range from 3 to 315 μg/L (mean 47.73 ± 73.41 μg/L), while the arsenic content in sediments range from 2 to 14 mg/kg (mean 4.36 ± 3.34 mg/kg). An environmental scanning electron microscope (ESEM) with an energy dispersive X-ray spectrometer was used to investigate the presence of major and trace elements in the sediments. Groundwaters in the study area are generally the Ca–HCO₃ type with high concentrations of As, but low levels of Fe, Mn, NO₃ ^? and SO ₄ ^?2 . The concentrations of As, Fe, Mn decrease with depth in the groundwater, showing vertical geochemical variations in the study area. Statistical analysis clearly shows that As is closely associated with Fe and Mn in the sediments of the JOR core (r = 0.87, p < 0.05 for Fe and r = 0.78, p < 0.05 for Mn) and GHAT core (r = 0.95, p < 0.05 for Fe and r = 0.93, p < 0.05 for Mn), while As is not correlated with Fe and Mn in groundwater. The comparatively low Fe and Mn concentrations in some groundwater and the ESEM image revealed that siderite precipitated as a secondary mineral on the surface of the sediment particles. The correlations along with results of sequential extraction experiments indicated that reductive dissolution of FeOOH and MnOOH represents a mechanism for releasing arsenic into the groundwater.     

Arsenic-enriched aquifers: Occurrences and mobilization of arsenic in groundwater of Ganges Delta Plain, Barasat, West Bengal, India

Hydrogeochemical characteristics and elemental features of groundwater and core sediments have been studied to better understand the sources and mobilization process responsible for As-enrichment in part of the Gangetic plain (Barasat, West Bengal, India). Analysis of water samples from shallow tubewells (depth 24.3–48.5 m) and piezometer wells (depth 12.2–79.2 m) demonstrate that the groundwater is mostly the Ca-HCO₃ type and anoxic in nature (mean Eh_SHE = 34 mV). Arsenic concentrations ranged from <10–538 μg/L, with high concentrations only present in the shallow to medium depth (30–50 m) of the aquifer along with high Fe (0.07–9.8 mg/L) and relatively low Mn (0.15–3.38 mg/L) as also evidenced in core sediments. Most groundwater samples contained both As(III) and As(V) species in which the concentration of As(III) was generally higher than that of As(V), exhibiting the reducing condition. Results show lower concentrations of NO₃, SO₄ and NO₂ along with higher values of DOC and HCO₃, indicating the reducing nature of the aquifer with abundant organic matter that can promote the release of As from sediments into groundwater. Positive correlations of As with Fe and DOC were also observed. The presence of DOC may actively drive the redox processes. This study revealed that reduction processes of FeOOH was the dominant mechanism for the release of As into the groundwater in this part of the Ganges Delta plain.     

Interrelationship of TOC,As, Fe,Mn, Al and Si in shallow alluvial aquifers in Chapai-Nawabganj,Northwestern Bangladesh: implication for potential source of organic carbon

Two boreholes and ten piezometers in the Ganges flood plain were drilled and installed for collecting As-rich sediments and groundwater. Groundwater samples from the Ganges flood plain were collected for the analysis of cations (Ca²⁺, Mg²⁺, K⁺, Na⁺), anions (Cl⁻, NO₃ ⁻, SO₄ ²⁻), total organic carbon (TOC), and trace elements (As, Mn, Fe, Sr, Se, Ni, Co, Cu, Mo, Sb, Pb). X-ray powder diffraction was performed to characterize the major mineral contents of aquifer sediments and X-ray fluorescence (XRF) to analyze the major chemical composition of alluvial sediments. Results of XRF analysis clearly show that fine-grained sediments contain higher amounts of trace element because of their high surface area for adsorption. Relative fluorescence index (15–38 QSU) of humic substance in groundwater was measured using spectrofluorometer, the results revealed that groundwater in the Ganges flood plain contains less organic matter (OM). Arsenic concentration in water ranges from 2.8 to 170 μg/L (mean 50 μg/L) in the Ganges flood plain. Arsenic content in sediments ranges from 2.1 to 14 mg/kg (mean 4.58 mg/kg) in the flood plains. TOC ranges from 0.49 to 3.53 g/kg (mean 1.64 g/kg) in the Ganges flood plain. Arsenic is positively correlated with TOC (R ² = 0.55) in sediments of this plain. Humic substances were extracted from the sediments from the Ganges flood plain. Fourier transform infrared analysis of the sediments revealed that the plain contains less humic substances. The source of organic carbon was assigned from δ¹³C values obtained using elemental analysis-isotope ratio mass spectrometry (EA-IRMS); the values (−10 to −29.44‰) strongly support the hypothesis that the OM of the Ganges flood plain is of terrestrial origin.     

Sonophotocatalysis of endocrine-disrupting chemicals

Sonolysis and photolysis often exhibit synergistic effects in the degradation of organic molecules. An assay of fish oocyte maturation provides an appropriate experimental system to investigate the hormonal activities of chemical agents. Oocyte maturation in fish is triggered by maturation-inducing hormone (MIH), which acts on receptors on the oocyte surface. A synthetic estrogen, diethylstilbestrol (DES), possesses inducing activity of fish oocyte maturation, and a widely used biocide, pentachlorophenol (PCP), exhibits a potent inhibitory effect on fish oocyte maturation. In this study, the effects of the combined treatment by sonolysis with photolysis (sonophotocatalysis) to diminish the hormonal activity of DES and the maturation preventing activity of PCP was examined. By sonophotocatalysis, hormonal activity of DES was completely lost within 30min and the inhibiting activity of PCP was lost within 120min. These results demonstrated that sonophotocatalysis is effective for diminishing the endocrine-disrupting activity of chemical agents.     

Mineral chemistry of submarine lavas from Hilo Ridge, Hawaii: implications for magmatic processes within Hawaiian rift zones

The crustal history of volcanic rocks can be inferred from the mineralogy and compositions of their phenocrysts which record episodes of magma mixing as well as the pressures and temperatures when magmas cooled. Submarine lavas erupted on the Hilo Ridge, a rift zone directly east of Mauna Kea volcano, contain olivine, plagioclase, augite ±orthopyroxene phenocrysts. The compositions of these phenocryst phases provide constraints on the magmatic processes beneath Hawaiian rift zones. In these samples, olivine phenocrysts are normally zoned with homogeneous cores ranging from ∼ Fo₈₁ to Fo₉₁. In contrast, plagioclase, augite and orthopyroxene phenocrysts display more than one episode of reverse zoning. Within each sample, plagioclase, augite and orthopyroxene phenocrysts have similar zoning profiles. However, there are significant differences between samples. In three samples these phases exhibit large compositional contrasts, e.g., Mg# [100 × Mg/(Mg+Fe⁺²)] of augite varies from 71 in cores to 82 in rims. Some submarine lavas from the Puna Ridge (Kilauea volcano) contain phenocrysts with similar reverse zonation. The compositional variations of these phenocrysts can be explained by mixing of a multiphase (plagioclase, augite and orthopyroxene) saturated, evolved magma with more mafic magma saturated only with olivine. The differences in the compositional ranges of plagioclase, augite and orthopyroxene crystals between samples indicate that these samples were derived from isolated magma chambers which had undergone distinct fractionation and mixing histories. The samples containing plagioclase and pyroxene with small compositional variations reflect magmas that were buffered near the olivine + melt ⇒Low-Ca pyroxene + augite + plagioclase reaction point by frequent intrusions of mafic olivine-bearing magmas. Samples containing plagioclase and pyroxene phenocrysts with large compositional ranges reflect magmas that evolved beyond this reaction point when there was no replenishment with olivine-saturated magma. Two of these samples contain augite cores with Mg# of ∼71, corresponding to Mg# of 36–40 in equilibrium melts, and augite in another sample has Mg# of 63–65 which is in equilibrium with a very evolved melt with a Mg# of ∼30. Such highly evolved magmas also exist beneath the Puna Ridge of Kilauea volcano. They are rarely erupted during the shield building stage, but may commonly form in ephemeral magma pockets in the rift zones. The compositions of clinopyroxene phenocryst rims and associated glass rinds indicate that most of the samples were last equilibrated at 2–3 kbar and 1130–1160 °C. However, in one sample, augite and glass rind compositions reflect crystallization at higher pressures (4–5 kbar). This sample provides evidence for magma mixing at relatively high pressures and perhaps transport of magma from the summit conduits to the rift zone along the oceanic crust-mantle boundary. Received: 8 July 1998 / Accepted: 2 January 1999     

Petrogenesis of the Nanling Mountains granites from South China: Constraints from systematic apatite geochemistry and whole-rock geochemical and Sr–Nd isotope compositions

The widespread Mesozoic granitoids in South China (135,300 km²) were emplaced in three main periods: Triassic (16% of the total surface area of Mesozoic granitoids), Jurassic (47%), and Cretaceous (37%). Though much study has been conducted on the most abundant Jurassic Nanling Mountains (NLM) granites, their rock affinities relative to the Triassic Darongshan (DRS) and Cretaceous Fuzhou–Zhangzhou Complex (FZC) granites which are typical S- and I-type, respectively, and the issue of their petrogenetic evolution is still the subject of much debate. In this study, we discuss the petrogenesis of NLM granites using apatite geochemistry combined with whole-rock geochemical and Sr–Nd isotope compositions. Sixteen apatite samples from six granite batholiths, one gabbro, and three syenite bodies in the NLM area were analyzed for their major and trace element abundances and compared with those collected from DRS (n = 7) and FZC (n = 6) granites. The apatite geochemistry reveals that Na, Si, S, Mn, Sr, U, Th concentrations and REE distribution patterns for apatites from DRS and FZC granites basically are similar to the S and I granite types of the Lachlan Fold Belt (Australia), whereas those from NLM granites have intermediate properties and cannot be correlated directly with these granite types. According to some indications set by the apatite geochemistry (e.g., lower U and higher Eu abundances), NLM apatites appear to have formed under oxidizing conditions. In addition, we further found that their REE distribution patterns are closely related to aluminum saturation index (ASI) and Nd isotope composition, rather than SiO₂ content or degree of differentiation, of the host rock. The majority of apatites from NLM granites (ASI = 0.97–1.08 and εNd(T) = −8.8 to −11.6) display slightly right-inclined apatite REE patterns distinguishable from the typical S- and I-type. However, those from few granites with ASI > 1.1 and εNd(T) < −11.6 have REE distribution patterns (near-flat) similar to DRS apatites whereas those from granites with ASI < 1.0 and εNd(T) > −6.6 and gabbro and syenite are similar to FZC apatites (strongly right-inclined). In light of Sr and Nd isotope compositions, magmas of NLM intrusives, except gabbro and syenite, and few granites with εNd(T) > −8, generally do not involve a mantle component. Instead, they fit with a melt derived largely from in situ melting or anatexis of the pre-Mesozoic (mainly Caledonian) granitic crust with subordinate pre-Yanshanian (mainly Indosinian) granitic crust. We suggest that an application, using combined whole-rock ASI and εNd(T) values, is as useful as the apatite geochemistry for recognizing possible sources for the NLM granites.     

Constraints from strontium and neodymium isotopic ratios and trace elements on the sources of the sediments in Lake Huguang Maar

The sediments in Lake Huguang Maar in coastal South China were previously thought to originate mainly from wind-blown dust transported from North China, such that the lake sediments recorded the varying strength of the Asian winter monsoon. An alternative explanation was that the local pyroclastic rocks supplied the lake sediments, but the actual contributions from the different sources remained unclear. Geochemical analyses including ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd and trace elements support the local pyroclastic rock as the dominant source: < 22% of the total Sr in the lake sediments and  17% of the Nd arises from the distant source. Nb/Ta and Zr/Hf for the lake sediments are identical to those for the local rock but differ from the ratios for the wind-blown dust, and chondrite-normalized rare earth element patterns for the lake sediments are similar to those for the local rock and soil, but differ from those for the distant source. The sediments in Lake Huguang Maar are probably input into the lake through runoff and thus controlled by the hydrology of the lake. Wind-blown dust transported by the Asian winter monsoon from arid North China is only a minor contribution to the sediments.     

Continuity of the North Qilian and North Qinling orogenic belts, Central Orogenic System of China: Evidence from newly discovered Paleozoic adakitic rocks

Adakitic intrusive rocks of  430–450 Ma were discovered in the North Qilian orogenic belt, the western section of the Central Orogenic System (COS) in China. These adakitic rocks were lower crust melts rather than slab melts as indicated by their crustal Ce/Pb, Nb/U, Ti/Eu, and Nd/Sm ratios and radiogenically enriched (⁸⁷Sr/⁸⁶Sr)_i of 0.7053–0.7066 and ε_Nd(t) of − 0.9 to − 1.7. While they are all characterized by low Yb (< 1.1 ppm) and Y (< 11.5 ppm) abundances with high Sr/Y (> 65) and (La/Yb)_N (> 13.7) ratios, these adakitic rocks are classified into the low-MgO–Ni–Cr and high-MgO–Ni–Cr groups. The low-MgO samples were derived from partial melting of thickened lower crust, whereas the high-MgO samples were melts from delaminated lower crust, which subsequently interacted with mantle peridotite upon ascent. Adakitic rocks from the adjacent North Qinling orogenic belt also originated from thickened lower crust at  430 Ma. In addition, the North Qilian and North Qinling orogenic belts both consist of lithological assemblages varying from subduction-accretionary complexes at south to central arc assemblages, which include adakitic rocks, then to backarc phases at north. Such a sequence reflects northward subduction of the Qilian and Qinling oceans. In these two orogenic belts, the occurrence of adakitic rocks of common origin and ages together with the similarities in tectonic configurations and lithological assemblages are considered to be the evidence for the continuity between eastern Qilian and western Qinling, forming a > 1000 km Early Paleozoic orogenic belt. In such a tectonic configuration, the Qilian and Qinling oceans that subducted from south possibly represent parts of the large “Proto-Tethyan Ocean”. This inference is supported by the coexistence of Early Paleozoic coral and trilobite specimens from Asia, America and Australia in the North Qilian orogenic belt. Post-400 Ma volcanic rocks occur in the North Qinling orogenic belt but are absent in the North Qilian orogenic belt, indicating that these two orogenic belts underwent distinct evolution history after the closure of the Proto-Tethyan Ocean ( 420 Ma).