Use of the magnetic,geochemical, and sedimentary records in establishing paleoclimate change in the environment of Sebkha: Case of the Sebkha Mhabeul in southeastern Tunisia

In this study, we explored the evidence of late Holocene climate changes in southern Tunisia, examining extreme events of flood and sandstorm sedimentary succession records, in Sebkha Mhabeul. A sediment-derived climatic proxy was inferred from a 93.5-cm-deep core (Mh1), whose dating by tephrochronology has already been achieved in previous works. Multiple geochemical, sedimentological, magnetic susceptibility, and quartz grain microtexture proxies were used to determine the shifts in regional climatic conditions. The Sebkha core captured sensitive changes in the precipitation/evaporation (P/E) balance by adjustments in salinity, and is especially valuable for reconstructing variability over centennial timescales. The Sebkha Mhabeul area showed higher salinity during the Roman Warm Period (RWP: 2100–1400 cal yr BP), the Medieval Warm Period (MWP: 1000–600 cal yr BP) and the present era, and generally lower salinity during the Dark Ages (DA: 1400-1000 cal yr BP) and the Little Ice Age (LIA: 600-200 cal yr BP).In southeastern Tunisia, the hydrological behavior was sometimes consistent with the flood activities in the eastern Mediterranean, and sometimes contemporaneous with the flood pulsations of the western Mediterranean.     

Late Jurassic to Early Cretaceous volcanism of Hong Kong: Insights from the Ping Chau Formation

The vast expanse of Mesozoic igneous rocks in Hong Kong contain important geological records of late Mesozoic magmatic events and tectonic processes from the coastal region of Southeast China. Of these,the Ping Chau Formation in the northwestern New Territories is the youngest known stratum. We perform a detailed study of the volcanic rocks of the Ping Chau Formation utilizing zircon U-Pb dating,with major and trace elements geochemistry. LA-ICP-MS zircon U-Pb data reveal Early Cretaceous age from two volcanic rock samples, with zircon crystallization from magmas at 140.3 ± 0.8 Ma and 139.3 ± 0.9 Ma,respectively. These rocks have high contents of total alkalis(Na_2O + K_2O = 5.58-9.45 wt.%), high-field-strength elements and light rare earth elements, conspicuous negative Eu anomalies, and depletions in Nb, Ta, Ti, Sr, Ba and P. Using this data, in combination with previous studies on the late Mesozoic volcanic belt in Southeast China, we propose that the volcanic rocks of the Ping Chau Formation probably originated from deep melting of the crust in a back-arc extensional setting induced by the subduction of the paleo-Pacific Plate. This formation represents the final stages of Early Cretaceous volcanic activity in Hong Kong, as associated with large-scale lithospheric extension, thinning and magmatism. Our results provide new information that can be used in evaluating the significance of Early Cretaceous volcanism and tectonics in Southeast China.     

An integrated ore prospecting model for the Nyasirori gold deposit in Tanzania

The Nyasirori gold deposit, located in the middle-western end of the Musoma-Mara Archean greenstone belt in Tanzania, is a tectonic altered rock type gold deposit controlled by shear tectonic zone. This work conducted high-precision ground magnetic measurements to delineate fault structures and favorable prospecting targets, utilized induced polarization (IP) intermediate gradient to roughly determine the distribution and extension of the tectonic altered zone and gold ore (mineralized) bodies, and further carried out IP sounding and magnetotelluric sounding to locate the tectonic altered zone and gold ore (mineralized) bodies. The anomalous gradient belt of the combination of positive and negative micromagnetic measurements reflects the detail of shallow surface tectonic alteration zone and gold mineralization body. Micromagnetic profile anomalies indicate the spatial location and occurrence of concealed tectonic alteration zone and gold (mineralized) ore bodies. Soil geochemical measurements indicate that the ore-forming element Au correlates well with As and Sb, and As and Sb anomalies have a good indication to gold orebodies. Based on the multi-source geological-geophysical-geochemical information of the Nyasirori gold deposit, this work established an integrated prospecting model and proposed a set of geophysical and geochemical methods for optimizing prospecting targets.     

变质流体作用的元素地球化学研究

变质流体作用是变质岩－流体体系的重要地质作用过程，可以通过有效的地质地球化学方法揭示，综述了变质流体作用的地球化学研究进展，主要包括：流体包裹体，同位素和元素地球化学等方面，强调了元素地球化学研究对于示踪变质流体作用过程的重要性。     

Geochemistry of Flooded Underground Mine Workings Influenced by Bacterial Sulfate Reduction

Unlike the majority of the water in the flooded mine complex of Butte Montana, which includes the highly acidic Berkeley pit lake, groundwater in the flooded West Camp underground mine workings has a circum-neutral pH and contains at least 8 μM aqueous sulfide. This article examines the geochemistry and stable isotope composition of this unusual H₂S-rich mine water, and also discusses problems related to the colorimetric analysis of sulfide in waters that contain FeS_(aq) cluster compounds. The West Camp mine pool is maintained at a constant elevation by continuous pumping, with discharge water that contains elevated Mn (90 μM), Fe (16 μM), and As (1.3 μM) but otherwise low metal concentrations. Dissolved inorganic carbon in the mine water is in chemical and isotopic equilibrium with rhodochrosite in the mineralized veins. The mine water is under-saturated with mackinawite and amorphous FeS, but is supersaturated with Cu- and Zn-sulfides. However, voltammetry studies show that much of the dissolved sulfide and ferrous iron are present as FeS_(aq) cluster molecules: as a result, the free concentration of the West Camp water is poorly constrained. Concentrations of dissolved sulfide determined by colorimetry were lower than gravimetric assays obtained by AgNO₃ addition, implying that the FeS_(aq) clusters are not completely extracted by the Methylene Blue reagent. In contrast, the clusters are quantitatively extracted as Ag₂S after addition of AgNO₃. Isotopic analysis of co-existing aqueous sulfide and sulfate confirms that the sulfide was produced by sulfate-reducing bacteria (SRB). The H₂S-rich mine water is not confined to the immediate vicinity of the extraction well, but is also present in flooded mine shafts up to 3 km away, and in samples bailed from mine shafts at depths up to 300 m below static water level. This illustrates that SRB are well established throughout the southwestern portion of the extensive (>15 km³) Butte flooded mine complex.     

Coeval potassic and sodic calc-alkaline series in the post-collisional Hercynian Tanncherfi intrusive complex, northeastern Morocco: geochemical, isotopic and geochronological evidence

The post-collisional late Hercynian Tanncherfi intrusive complex (TIC) is part of a widespread intrusive episode in the Moroccan Meseta. The complex contains a wide range of rock types, from monzogabbros to monzogranites. Two distinct magmatic series are recognized: (1) a potassic (shoshonitic) series consisting of monzogabbros, quartz monzonites and monzogranites; and (2) a sodic (granodioritic) series represented by quartz monzodiorites and granodiorites. All the Tanncherfi plutonic rocks display similar spider-diagram profiles, with LILE and LREE enrichment and Nb, Ta, Ti depletion, which are typical of subduction-related magmas. Combined major, trace element compositions and Sr, Nd isotopic results indicate that the two series have been derived from a LILE- and LREE-enriched continental lithospheric mantle source, under different partial melting and/or depth conditions. Intrusion of the Tanncherfi rocks was not temporally related to subduction and the enrichment of their source is likely to be linked to preceding subduction events. The two series evolved by fractional crystallization, of clinopyroxene, plagioclase, hornblende, biotite, K-feldspar and accessories (Fe–Ti oxide minerals, titanite, apatite and zircon) for the potassic series while the sodic series combined fractional crystallization with assimilation of felsic magmas with lower Sr isotopic ratio than the more mafic term of the series, the quartz monzodiorite. The intrusion of the potassic magmas (344±6 Ma) marks a major change in the tectonic regime of eastern Meseta. These magmas intruded during post-thickening uplift and extension, both probably favored by convective thinning of the lithosphere. This model provides a reasonable mechanism for the genesis of other Hercynian intrusive complexes in Morocco.     

内蒙古东部半干旱草原残丘景观区铜多金属元素表生地球化学异常特征及其评价指标体系的构建

分析了研究区内钙积层、风成沙等制约因素对铜多金属元素表生地球化学异常发育的影响,总结了该类特殊景观条件下铜多金属元素表生地球化学异常特征及其在垂向和侧向上的迁移规律。在此基础上,构建了该类景观区铜多金属元素的表生地球化学异常评价指标体系     

八庙-青山金红石矿床地球化学特征

金红石矿床含矿岩石常量元素总体上与玄武岩接近,但以低硅,高钛,高ＣＯ２,高碱度为特征。矿床微量元素与地壳的差别显著,稀土元素较高,分布模式呈轻稀土富集型,Ｅｕ一般为正常型与碧玄岩最接近,Ｔｉ与Ｂａ,Ｃｏ,Ｐｂ,Ｓｒ,Ｔｈ,Ｖ,Ｙ及稀土元素呈正相关与Ｃｒ和Ｎｉ呈负相关,δ＾３４Ｓ多数与（超）铁镁质岩石接近,最大值小于且靠近现代海水。     

浙江陈蔡群斜长角闪岩的地球化学特征及其大地构造背景探讨

陈蔡群斜长角闪岩的基本组成矿物是斜长石和角闪石,不同样品中还可出现透辉石、黑云母、石英和碱性长石等。大多数斜长角闪岩显示石英拉斑和橄榄拉斑系列的特征,个别属于钙碱性系列。其稀土元素配分曲线显示不同程度的LREE富集特征,指示斜长角闪岩原岩在成岩过程方面的多样性。根据微量元素蛛网图的斜率和Nb、Ta异常情况以及Zr-TiO₂相关性,斜长角闪岩主要分成岛弧和洋岛两种类型,指示陈蔡群的原岩形成于洋内俯冲或洋陆俯冲的环境。龙游八都群斜长角闪岩呈MORB的地球化学特征,与陈蔡群斜长角闪岩形成鲜明的对比     

Detrital chromites in metasediments of the East-Arabian continental margin in the Saih Hatat area: constraints for the palaeogeographic setting of the Hawasina and Semail basins (Oman Mountains)

 In Oman, the convergence between Arabia and Eurasia resulted in the Late Cretaceous overthrusting of oceanic crust and mantle lithosphere onto the Arabian continental margin. During this compressional event, a part of the continental plate was subducted to a depth of more than 60 km (0.5 GPa, 250–350  °C to more than 2.0 GPa, 550  °C) resulting in progressive metamorphism of the continental margin sediments, well exposed in the Saih Hatat tectonic window, northeastern Oman Mountains. We attempt to constrain the possibility of one continuous history of extension (starting along the east Arabian continental margin in the Permian) that was followed by one continuous history of convergence starting at 90 Ma near a dead oceanic ridge. This compression resulted in the observed progressive metamorphism by ophiolite overthrusting onto the continental margin. Constraining arguments are the palaeogeographic setting before ophiolite obduction of the As Sifah units and the Hawasina Complex near Ghurba. Detrital chromites in the Triassic–Cretaceous metasediments of the Hawasina Complex are compared with magmatic Semail chromites, and the whole-rock chemistry of these metasediments and associated metabasites are investigated. In contrast to former hypotheses, differences in the chemical composition between detrital and magmatic chromites, and the probable origin of all detrital chromites in the Hawasina Basin from Permian age oceanic rocks, suggest that the high-pressure metamorphic sediments of As Sifah can be considered as part of the basal deposits of the Hawasina Basin. Received: 1 September 1998 / Accepted: 18 January 1999