2005 Vol. 16, No. 2
The Altay orogenic belt of Xinjiang in NW China represents one of the important sites of juvenile crustal growth during the Phanerozoic. However, some important issues, e.g., tectonic evolution and petrogenesis, still remain controversial. The picrites in the south margin of the Altay orogenic belt were discovered in the lower part of marine volcanic~sedimentary sequences of the northwest~striking Middle Devonian Beitashan Formation (Fm.), which consists chiefly of intermediate~basic volcanic rocks intercalated minor carbonate, siltstone and siliceous rocks. The picrites are usually highly porphyritic, and contain abundant forsteritic olivine phenocrysts with minor clinopyroxene distributed in the groundmass, which consist of olivine, clinopyroxene and plagioclase with minor Fe~Ti oxides. The MgO contents of the picrites range from 14 wt% to 22 wt% with Mg# (atomic Mg/(Mg+Fe) ratio) of 0.75~0.80. They are characterized by slightly negative Ti anomalies, remarkably negative Nb and Ta anomalies and slightly positive P and Sm anomalies with the similar abundances of HFSE as MORB on the MORB~normalized trace element patterns, all of which characterize typical island arc magmas. In combing with the southwestward migration of the magmas of the Beitashan Fm., we propose that the magmas may result from the southwestward subduction of Junggar ocean plate. The Zr/Nb ratios (23~66) of both picrites and basalts resemble the MORB (10~66), suggesting that they were derived from the MORB~like sources. However, the basalts and picrites display some distinguishable element ratios and REE patterns, e.g., Ti/V (23~35) and Zr/Sm (18~23) ratios of basalts are higher than those of picrites (14~17 and 14~15 respectively), and the basalts display flat~type REE~chondrite patterns whereas the picrites are characterized by lower total REE concentrations ((26-34)×10~6) and slight enrichment of light REE. These distinguished geochemical characteristics could be interpreted by different partial melting degrees and mantle sources, i.e., the basalts were generated by lower partial melting of amphibole~bearing spinel peridotite which was metasomatized by fluids released from subducted oceanic crust, and the picrites were resulted from the higher degree of partial melting of metasomatized garnet peridotite under high temperature. In contrast, the andesite with significant LREE and LILE enrichment may be resulted from the partial melting of eclogites.
In Lingyuan region of West Liaoning Province, the Zhangjiakou Formation (J 3z) and the Yixian Formation (K 1 y ) display an angular unconformity. That is, the Lower Mesozoic strata of the Zhangjiakou Formation are ENE (near E~W) oriented, while the overlying strata of the Yixian Formation exhibit an NNE orientation. The results of LA~ICP~MS zircon U~Pb ages show the Zhangjiakou Formation formed from about 130 Ma to 132 Ma in Lingyuan and 135 Ma to 136 Ma in Luanping (North Hebei Province), respectively. Three conclusions can be drawn: (1) The Zhangjiakou Formation in Lingyuan is comparable to that in Luanping, with the volcanic rocks of the Zhangjiakou Formation from Lingyuan being younger than those from Luanping. (2) 5~6 Ma difference between the top of the Zhangjiakou Formation and the bottom of the Yixian Formation in Lingyuan proves the angular unconformity between the two formations; and it reflects that the 5-6 Ma interval period is the main period of the transition of tectoni
Neodymium (Nd) depleted mantle model ages indicate that the Kangding complexes in the northern segment of the western Yangtze craton, argued as representative of the craton’s basement, actually formed after 1.6 Ga. An SmNd isochron of 706 Ma constrains the upper limit formation of the complex from Shaba, Sichuan Province. Lead (Pb) isotopic compositions imply that the southern and northern basements of the western Yangtze craton can not be correlated. The southern basement is characterized by higher 206 Pb/ 204 Pb ratios (≥17.69) than the northern ones (<17.69). However, Hercynian granitoids, gabbros as well as the Permian basalts in the northern segment show no difference from the southern segment in Pb isotopic compositions. This suggests that the southern and northern segments were controlled by one common mantle domain during the Late Paleozoic. Based on Nd model ages of sediments and metasediments, as well as the ages of their relict zircons, we infer that the Yangtze craton is
Drinking water is at risk from aniline pollution and thus aniline degradation and its mechanism have received much attention. In this paper, a soil column, including sediments and aquifer media, was collected from the Weihe riverbed and its bank, and used to research the characteristics of aniline degradation in the riverbank filtration process under denitrification conditions. The results indicate that all aniline could be degraded by the habituated indigenous microbes, and even mostly mineralized under denitrification conditions, but with a long lag phase. Some aniline degradation must involve deamination, while the majority undergoes covalent binding with humic substances to form complexes, and the complexes are easily degraded and even mineralized. During the degradation no intermediates were harmful to denitrifiers. Therefore, under denitrifaction conditions, aniline is degraded in RBF, and up to now aniline has not been monitored in the groundwater along the polluted river. During the 153 d testing pro
The Dabie Mountains are a collisional orogen between the South and North China blocks. The rock assemblages, isotopic dating and tectonic~relationship of the tectonic petrologic units in the eastern Dabie orogen indicate that the orogen is mainly composed of the different~grades metamorphic basement with minor low~grade metamorphic cover. No ophiolitic mélange and the Paleozoic volcanic intrusive rock associated with the southern margin of the North China block were found there, suggesting that they belong to the northern margin of the South China block. The boundary between the tectonic petrologic units is generally an extensional shear zone developed in the exhumation process of the ultrahigh pressure metamorphic rocks. In the northern part of the Dabie Mountains, the extensional~thrust and nappe structure represent the products of extensional tectonism. That is, there is no key tectonic boundary to indicate the occurrence of the suture zone there. Therefore, neither the Shuihou Wuhe shear zone, nor the Mozitan~Xiaotian fault, is the suture zone between the South and North China blocks. The zone is believed to be at the front area of the Xinyang~Shucheng fault, covered by the Mesozoic Cenozoic deposits within the Hefei basin.
Data obtained on conodont distribution in the Permian~Triassic Sovetashen Section of Transcaucasia provide further limitations on the age of the carbon isotopic anomalies discovered by Baud et al. (1989). The significance of Caucasian sections for working out the carbon~isotope standard for the Upper Permian and Lower Triassic (Induan) is shown. Original data on carbon isotope composition of bivalve and brachiopod shells from Permian sediments of North~Eastern Russia (Omolon and Okhotsk areas) have been obtained, which may be used for their correlation.
The Huize Pb~Zn deposits of Yunnan Province, located in the south~central part of the Sichuan~Yunnan~Guizhou (SYG) Pb~Zn multimetal mineralization district (MMD), are strictly controlled by fault zones. The sources of ore~forming fluid in the deposits have been debated for a long time. Calcite, a gangue mineral, has uniform C and O isotopes. The δ13C PDB and δ18O SMOW values vary respectively from -2.1×10-3 to -3.5×10-3(mean -2.8×10-3) and 16.7×10-3~18.6×10-3(mean 17.7×10-3). No obvious difference can be found in C and O isotopes among occurrences and elevations and even ore~bodies. Types of inclusions include those of pure liquid (L),liquid~rich gas~liquid (L+V), and three~phase ones containing a daughter mineral (S+L+V) and immiscible CO2 with three phases (VCO2+LCO2+LH2O).Their homogenization temperatures vary from 110 to 400 ℃, and two peaks are shown. (87Sr/86Sr)0 ratios of calcite in the deposits are higher than those in the mantle and Emeishan basalts, and slightly higher than those in the Baizuo Formation, which the Huize lead-zinc deposits are found in. All of the(87Sr/86Sr)0 are low relative to those in the basement rocks. Fractionation of Sr isotope did not occur in the ore~forming fluid during the precipitation of minerals. The results indicate that the ore~forming fluid is homogeneous and derived from the mixing of different fluids. Gas liquid inclusions can be separated into two groups in 300~400 ℃ with a salinity of 5% ~ 6% and 12% ~ 16% NaCl respectively. However, the salinities of inclusions vary from 7 % to 23 % NaCl in 100~300 ℃, especially in 150~250 ℃. The formation pressures of faulted zones are (50-320)×10 5 Pa. The estimated pressures of the overlying rocks on the ore bodies are 574×105-640×105 Pa. The pressures of ore~forming processes would be 145×105 to 754×105 Pa. Therefore, pressure sharply reduced and boiling occurred when the ore~forming fluid flew into the fault zones. As a result, the ore~forming fluid was highly concentrated, and metallic minerals began to precipitate from the fluid on a great scale. The high~grade lead~zinc deposits were formed when the fluid was under saturation or over saturation conditions
Fort Munro Formation represents the products of the Upper Cretaceous (Maastrichtian) in the middle and lower Indus basins. The formation is exposed in the Rakhi Nala (Sulaiman Range), Bara Nala (Lakhi Range) and Naka Pabni (Southern Pab Range) areas. Major and trace elemental geochemistry and petrographic studies of the formation have been carried out to understand the facies trends in the middle and lower Indus basins. A high amount of acid insoluble fraction, Ca/Mg and Mg vs. Ca/Sr ratio reveal that the formation was deposited in a shallow marine regressive environment. High amounts of clastic reflect abundant influx of terrigenous materials from the east (Indian craton) and west (Bibai volcanic). High Sr content indicates that aragonite was the precursor mineral, which was transformed into stable low~Mg calcite during diagenesis. Enrichment of Cu and Zn contents in the samples of the formation implies the influence of volcanic activity and that they were incorporated into the calcite lattice in the late phase.
Phosphorite has become increasingly important as the raw material for phosphatic fertilizer. Phosphorite nodules are widespread in the Kursk complex deposit (Russian platform). Genesis of these nodules has long been a matter of debate, and this has hampered understanding of the mechanism and controls in the formation of the nodules. In this paper, we report the petrographical, mineralogical, and geochemical data of the Lebedinsky phosphorite nodules. Petrographic study reveals complex phosphatic cement resulting in the replacement of apatite minerals around quartz grains. The main mineral composition consists of quartz, phosphate minerals (apatite, francolite mainly), feldspar, iron hydroxides and carbonate minerals. These results, when combined with available data, are used to address the origin of the phosphorite nodules. The nodules are characterized by the universal presence of biogenic and chemical signatures which is phosphorus crown around crystal grains. The structure of the nodules is massive. Their texture is depicted by basaltic cement and concretion, which consists mainly of apatite and its varieties, with general formula: Ca10(PO4, CO3)6(F,OH,Cl). Variation of mineralogy appears dependent on geological setting. Microscopic observations of biogenic fossils in Lebedinsky phosphorite favor a chemical and biogenic origin of phosphorites. Weathering has been suggested to be capable of liberating as much as 20%~35% P2O5 from sedimentary rocks. Previous investigations demonstrate that weathering of the Proterozoic substratum was the main cause in the formation of Fe, Mo, Mn, Pb, Zn, and P in the Russian platform. We therefore suggest that both weathering and biochemical processes have been positive in the formation of the Lebedinsky phosporite nodules. However, whether continental weathering or oceanic bio~chemical processes are more relevant in the phosphorite accumulation remains undetermined.
Determining the upper boundary of the cobalt rich crust distribution of a guyot is important for estimating the mineral resources available, however, it has also long been an unsolved question. Correlations between the sub~bottom structures, revealed by sub~bottom profiling, and crust distribution can be revealed for the first time by the synchronous application of sub~bottom profiling and deep~sea video recording. The lower boundary of the sediment corresponds with the upper boundary of the crust. By analysis of these two kinds of data, the lower boundary of the sediment can be determined; therefore, the upper boundary of the crust distribution can be deduced. According to this method of analysis, the upper boundary of water depth of crust distribution of a seamount in the western Pacific is about 1560 m