Strain analysis in Jurassic argillites of the Monte Sirino area (Lagonegro Zone,southern Apennines,Italy) and implications for deformation paths in pelitic rocks

Analysis of strain in Jurassic argillites forming part of the folded and thrusted sedimentary succession of the Lagonegro basin (southern Italian Apennines) has been carried out using ellipsoid-shaped reduction spots as strain markers. Most of the determined finite strain ellipsoids are of oblate type and show a peculiar distribution of the maximum extension direction (X), with maxima either subparallel or subperpendicular to the local fold axes. Using the strain matrix method, two different deformation histories have been considered to assist the interpretation of the observed finite strain pattern. A first deformation history involved vertical compaction followed by horizontal shortening (occurring by a combination of true tectonic strain and volume loss), whereby all strain is coaxial and there is no change in the intermediate axis of the strain ellipsoid. By this type of deformation sequence, which produces a deformation path where total strain moves from the oblate to the prolate strain field and back to the oblate field, prolate strain ellipsoids can be generated and may be recorded where tectonic deformation has not been large enough to reverse pretectonic compaction. This type of deformation history may be of local importance within the study area (i.e. it may characterize some fold hinge regions) and, more generally, is probably of limited occurrence in deformed pelitic rocks. A second deformation sequence considered the superposition of pre-tectonic compaction and tectonic strain consisting of initial layer-parallel shortening followed by layer-parallel shear (related to flexural folding). Also in this instance, volume change during tectonic deformation and tectonic plane strain have been assumed. For geologically reasonable amounts of volume loss due to compaction and of initial layer-parallel shortening, this type of deformation history is capable of producing a deformation path entirely lying within the oblate strain field, but still characterized by a changeover, during deformation, of the maximum extension axis (X) from a position parallel to the fold axis to one perpendicular to it. This type of deformation sequence may explain the main strain features observed in the study area, where most of the measured finite strain ellipsoids, determined from the limb regions of flexural folds, display an oblate shape, irrespective of the orientation of their maximum extension direction (X) with respect to the local structural trends. More generally, this type of deformation history provides a mechanism to account for the predominance of oblate strains in deformed pelitic rocks.     

Geochemistry of fine-grained clastic sedimentary rocks of the Neoproterozoic Ikorongo Group, NE Tanzania: Implications for provenance and source rock weathering

The Neoproterozoic Ikorongo Group, which lies unconformably on the late Archaean Nyanzian Supergroup of the Tanzania Craton, is comprised of conglomerates, quartzites, shales, siltstones, red sandstones with rare flagstones and gritstones and is regionally subdivided into four litho-stratigraphic units namely the Makobo, Kinenge, Sumuji and Masati Formations.We report geochemical data for the mudrocks (i.e., shales and siltstones) from the Ikorongo basin in an attempt to constrain their provenance and source rock weathering. These mudrocks are compositionally similar to PAAS and PS indicating derivation from mixed mafic–felsic sources. However, the siltstones show depletion in the transition elements (Cr, Ni, Cu, Sc and V) and attest to a more felsic protolith than those for PAAS and PS. The Chemical Index of Alteration (CIA: 52–82) reveal a moderately weathered protolith for the mudrocks. The consistent REE patterns with LREE-enriched and HREE-depleted patterns ((La/Yb)_CN = 7.3–38.3) coupled with negative Eu anomalies (Eu/Eu* = 0.71 on average), which characteristics are similar to the average PAAS and PS, illustrate cratonic sources that formed by intra-crustal differentiation.Geochemical considerations and palaeocurrent indications suggest that the provenance of the Ikorongo Group include high-Mg basaltic-andesites, dacites, rhyolites and granitoids from the Neoarchaean Musoma-Mara Greenstone Belt to the north of the Ikorongo basin. Mass balance calculations suggest relative contributions of 47%, 42% and 11% from granitoids, high-magnesium basaltic-andesites and dacites, respectively to the detritus that formed the shales. Corresponding contributions to the siltstones detritus are 53%, 43% and 4%.     

鄂西南利川三叠纪须家河组地球化学特征及其对风化、物源与构造背景的指示

鄂西南利川地区三叠纪须家河组砂岩碎屑颗粒富石英(Q),贫岩屑(L)与长石(F),平均值分别为:76.23%、7.08%与4.88%,Q/(Q+F+L)平均值为0.86,具有锆石-板钛矿-磁铁矿-电气石重矿物组合,指示源岩以酸性岩或低级变质岩为主。砂岩主量元素Si O2含量高(77.14%~92.79%,平均84.14%),Al2O3次之(3.86%~14.15%,平均9.69%),(Fe2O3T+Mg O)*(0.98%~3.20%,平均1.50%)、Ti O2*(0.09%~1.09%,平均0.39%)含量低,Al2O3/Si O2比值低(0.04~0.18,平均0.12),K2O/Na2O比值高(4.90~82.41,平均40.01),最接近被动大陆边缘特征值。样品具有与上地壳相似的高场强元素与大离子亲石元素组成,ΣREE分布于62×10-6~495×10-6之间,平均181×10-6,球粒陨石标准化配分型式与上地壳极为相似,呈现轻稀土富集、重稀土平坦、中度Eu、Ce负异常特征,特征微量、稀土元素含量及比值,如:Th、La、Ce、Rb/Sr、Th/U、La/Sc,指示了晚三叠世构造背景为被动与活动大陆边缘。样品成分变异指数ICV均1(0.18~0.68,平均0.45),指示物源主要为再循环的沉积物,而沉积物再循环会导致粘土矿物比例增加,从而使化学风化指标CIA值得到累积。较高的CIA值(72.10~96.28,平均81.18)表明沉积物累积经历的化学风化作用强烈,而CIA与ICV强烈负相关,则表明CIA值的变化主要是由不同时期输入碎屑物成分不同引起的。结合物源、构造背景判别图解,上述特征综合表明研究区须家河组形成于被动大陆边缘(为主)与活动大陆边缘环境,其物源来自东南的雪峰隆起区(为主)与北侧的秦岭造山带。     

Geology and geochemistry of Jurassic pelagic sediments, Scisti silicei Formation, southern Apennines, Italy

A better understanding of genesis and palaeoenvironmental setting of the Scisti silicei Formation (Lagonegro units, southern Italy) was achieved by means of geochemical analysis integrated with new stratigraphic information. Data show that major and trace element geochemistry of ancient clay-rich beds and banded cherts add new insights into the Mesozoic evolution of the Lagonegro basin. Sedimentary contributions to Jurassic shales sampled during this study were mainly derived from two major sources: (i) a dominant terrigenous fine-grained component, having affinity with average upper continental crust that had not undergone intense weathering and (ii) biogenic siliceous material. The latter component occurs in clay-rich layers from the “basal member” of the Scisti silicei Formation.

Composition varies up section and accounts for changes in the detrital supply due to bathymetric oscillations. The compositional variations from the basal to the overlying member are consistent with a distal source passing in time to a more “proximal” source, as indicated by sharp changes in the concentrations of detrital elements (Ti, Zr and Nb). It is likely that increased detrital input occurred through turbidity current deposition. Finally, the chemical features of the clay-rich layers from the upper cherty portion of the studied succession imply a progressive deepening of the basin.

The lack of any mafic and hydrothermal contributions in the Jurassic shales as well as the continental nature of detrital input suggests that the Lagonegro basin was located between two carbonate platforms, in accordance with the classical restoration of the African–Apulian palaeomargin. Thus, the basin acted as a preferential sink connected to the African cratonic areas through a southern entry point.     

Provenance, tectonics and source weathering of modern fluvial sediments of the Brahmaputra–Jamuna River, Bangladesh: Inference from geochemistry

This present study describes the elemental geochemistry of fluvial sediments in the Kurigram (upstream) to Sirajganj–Tangail (downstream) section of the Brahmaputra–Jamuna River, Bangladesh, with the aim of evaluating their provenance, weathering and tectonic setting. Petrographically, the sediments are rich in quartz (68%), followed by feldspars (8.5%) and lithic grains (7%). The bulk sediment chemistry is influenced by grain size. Concentrations of TiO₂, Fe₂O₃, MgO, K₂O, P₂O₅, Rb, Nb, Cr, V, Y, and, Ce, Th and Ga slightly decrease with increasing SiO₂/Al₂O₃ and grain size, suggesting clay matrix control. In contrast, concentrations of CaO, Na₂O, Sr and Pb increase with increasing SiO₂/Al₂O₃ and grain size, suggesting residence of these substances in feldspar. Decrease in Zr as grain size increases is likely controlled both by clay matrix and heavy minerals. In addition, heavy minerals' sorting also influences Ce, Th, Y and Cr abundances in some samples. The sediments are predominantly quartzose in composition with abundant low-grade metamorphic and sedimentary lithics, low feldspars and trace volcanic detritus, indicating a quartzose recycled orogen province as a source of the sediments. Discriminant diagrams together with immobile element ratio plots show that, the Brahmaputra–Jamuna River sediments are mostly derived from rocks formed in an active continental margin. Moreover, the rare earth element ratios as well as chondrite-normalized REE patterns with flat HREE, LREE enrichment, and negative Eu anomalies indicate derivation of the sediments of Brahmaputra–Jamuna River from felsic rock sources of upper continental crust (UCC). The chemical indices of alteration suggest that Brahmaputra–Jamuna River sediments are chemically immature and experienced low chemical weathering effects. In the A–CN–K ternary diagram, most of the samples close to the plagioclase–K-feldspar join line and to the UCC plot, and in the field of various lithologies of Higher Himalayan Crystalline Series, suggesting that rocks in these series are likely source rocks. Therefore, the elemental geochemistry of the Brahmaputra–Jamuna River sediments is controlled mostly by mechanical breakdown of lithic fragments and subsequent preferential attrition of muscovite > albite > quartz.     

Geochemistry of late quaternary sediments from Tecocomulco lake, central Mexico: Implication to chemical weathering and provenance

This paper presents the first detailed multi-element geochemical data from the late Quaternary sediments of the Tecocomulco lake basin (central Mexico) and rocks exposed in the basin catchments to understand the extents of chemical weathering and provenance of the siliciclastic fractions. Ternary diagrams of A-CN-K, A-C-N and A-CNK-FM and elemental ratios suggest that most of the lacustrine sediments were derived from mafic volcanic deposits comprising the Chichicuatla and the Apan-Peñon andesites and the Apan-Tezontepec basaltic-andesites. The felsic tephra layers have chemical compositions comparable to the Acoculco volcanic sequences. The calculated indices of chemical weathering such as chemical index of alteration (CIA), plagioclase index of alteration (PIA) and chemical index of weathering (CIW) indicate low to extreme chemical weathering for the lacustrine sediments and low chemical weathering for tephra layers. The varying degree of chemical weathering in lacustrine sediments is related to the fluctuating average annual precipitation during the late Quaternary. However, the low weathering of tephra layers are due to their higher rate of deposition. The dacite-rhyolitic tephra layers of ca. 31,000 ¹⁴C yr BP are relatively more weathered compared to the unweathered rhyolitic tephra of ca. 50,000 ¹⁴C yr BP. This could be due to the rapid deposition of ca. 200 cm of tephra layers during the ca. 50,000 ¹⁴C yr BP volcanic eruption that might have prevented the interaction between tephra layers and weathering agents.     

Geochemistry of Renuka Lake and wetland sediments,Lesser Himalaya (India): implications for source-area weathering,provenance, and tectonic setting

The geochemical investigation of sediments deposited in the Renuka Lake basin and its adjoining wetland has shown variation in the distribution and concentration of major, trace and REEs. The major elements are depleted in the lake in relation to wetland and that of Post Archaean Australian, Shale (PAAS), except for CaO which is strikingly in excess and has a dilution effect on SiO₂ and other oxides and trace elements. The Wetland sediments, on the other hand, are enriched in Al₂O₃, Fe₂O₃, K₂O and TiO₂ and the latter three show a positive correlation with Al₂O₃ in both wetland and lake sediments suggesting their association with phyllosilicates and similar source rocks. The enrichment of Y, Zr, Ni, Th, U and Nb in wetland compared to lake and their similarity with PAAS in the former, suggests more clay fractions in the wetland. A high Zr/Hf ratio in wetland and lake sediments and a positive correlation of Zr with Y and HREE indicate Zr control on HREEs. However, higher Zr/Yb and Zr/Th ratios in wetland compared to lake indicate mineral sorting during the process of lighter particles (clays) being trapped in wetland soil. This is also reflected from negative correlation of Gd_N/Yb_N with Al₂O₃ and a strong positive correlation with SiO₂ in wetland sediments. The wetland in this context has a control on lake sediment chemistry. The chondrite normalized REE patterns are essentially the same for lake as well as wetland sediments but abundance decreases in the former. The similarity of pattern with that of PAAS and negative Eu anomaly indicates a cratonic source of sediments. In a plot of the individual samples, wetland samples cluster while lake samples are separated indicating fractionation of lake sediments. A strong positive correlation of La_N/Yb_N with Al₂O₃ and a positive correlation of Zr-∑LREE and Zr-La_N/Yb_N suggest that LREEs are controlled by both phyllosilicates and zircon. The chemical index of alteration (CIA) indices in lake sediments and in wetland are higher than PAAS indicating moderate chemical weathering in the source area. The petrography, lack of felsic magmatic rock fragments, and negative correlation between Zr-(Gd/Yb)_C indicate sedimentary source rocks for the detritus. This is in conformity with the Lesser Himalayan sedimentary sequence belonging to neo-Proterozoic–Proterozoic age and constituting lake catchment of Renuka. The tectonic delineation and discriminant function plots of lake and wetland sediments indicate their cratonic and/or quartzose sedimentary orogenic terrain source that has been deposited in a passive margin setting.     

Geochemical characteristics of modern river sediments in Myanmar and Thailand: Implications for provenance and weathering

The elemental composition of organic matter and the major and trace element compositions of stream sediments from Myanmar (Ayeyarwady and Sittaung rivers) and Thailand (Mekong and Chao Phraya rivers, and their tributaries) were determined to examine their distributions, provenance, and chemical weathering processes. Higher total organic carbon (TOC) and total nitrogen (TN) contents in the finer grained sediments indicate hydrodynamic energy may control their distributions. TOC/TN ratios indicate inputs of both aquatic macrophyte and higher vascular plant material to the river sediments. The major element abundances of the sediments are characterized by predominance of SiO₂ in coarser fractions and a marked negative correlation with Al₂O₃, representing primary grain size primarily control on SiO₂ content. Marked depletion of most labile elements (Na₂O, CaO, K₂O, Ba and Sr) relative to UCC (upper continental crust), indicate destruction of feldspar during chemical weathering in the source area or during transport. However, enrichment of some high field strength elements (Zr, Th, Ce and Y) relative to UCC and higher Zr/Sc ratios indicate moderate concentration of resistant heavy minerals in finer-grained samples. Discriminant diagrams and immobile trace element characteristics indicate that the Mekong, and Chao Phraya river sediments were largely derived from felsic sources with compositions close to typical rhyolite, dacite/granodiorite, UCC, I- and S-type granites. Relative enrichment of ferromagnesian elements (e.g. MgO, Cr, Ni) and high Cr/V and low Y/Ni ratios in Ayeyarwady and Sittaung sediments indicate the presence of a mafic or ultramafic component in their sources. The ICV (Index of Compositional Variability), CIA (Chemical Index of Alteration), PIA (Plagioclase Index of Alteration), α^Al, Rb/Sr and K₂O/Rb ratios indicate that the Ayeyarwady and Sittaung sediments record low to moderate degrees of chemical weathering in their source, compared to moderate to intense chemical weathering in the Mekong and Chao Phraya river basins. These results are compatible with existing major ion data for river waters collected at the same locations.     

Geochemistry of Mansar Lake sediments, Jammu, India: Implication for source-area weathering, provenance, and tectonic setting

The sediment geochemistry, including REE, of surface and core samples from Mansar Lake, along with mineralogical investigations, have been carried out in order to understand the provenance, source area weathering, hydrolic sorting and tectonic setting of the basin. The geochemical signatures preserved in these sediments have been exploited as proxies in order to delineate these different parameters.The major element log values (Fe₂O₃/K₂O) vs (SiO₂/Al₂O₃) and (Na₂O/K₂O) vs (SiO₂/Al₂O₃) demarcate a lithology remarkably similar to that exposed in the catchment area. The chondrite normalized REE patterns of lake samples are similar to Post Archaean Australian Shale (PAAS) with LREE enrichment, a negative Eu anomaly and almost flat HREE pattern similar to a felsic and/or cratonic sedimentary source. However, the La–Th–Sc plot of samples fall in a mixed sedimentary domain, close to Upper Continental Crust (UCC) and PAAS, suggesting sedimentary source rocks for the Mansar detritus. It also indicates that these elements remained immobile during weathering and transportation. The mineralogical characteristic, REEs, and high field strength elements (HFSE), together with the high percentage of metamorphic rock fragments in the Siwalik sandstone, support a metamorphic source for lower Siwalik sediments. A very weak positive correlation between Zr and SiO₂, poor negative correlation with Al₂O₃, negative correlation of (La/Yb)_N and (Gd/Yb)_N ratios with SiO₂ and positive correlation with Al₂O₃, suggest that Zr does not dominantly control the REE distribution in Mansar sediments. The petrographic character and textural immaturity indicate a short distance transport for the detritus. The distribution of elements in core samples reflect fractionation. The higher Zr/Th and Zr/Yb ratios in coarse sediments and PAAS compared to finer grained detritus indicate sedimentary sorting. Plots of the geochemical data on tectonic discrimination diagrams suggest that the sediments derived from the lower Siwalik were originated within a cratonic interior and later deposited along a passive margin basinal setting. It therefore reveals lower Siwalik depositional history.     

REE geochemistry of the recent playa sediments from the Thar Desert, India: An implication to playa sediment provenance

The playas (saline lakes) situated in the Thar Desert, north-west India, provide prominent examples of alkaline brine and varying assemblages of detrital and evaporite mineralogy. The eastern margin of the desert is relatively semi-arid, whereas the central to western region is arid to hyper-arid in nature. Rare earth elements (REEs) systematics in the sediments of nine different playas of the Thar Desert were studied to understand the provenance of the sediments and the intensity of chemical weathering in the region. Based on the REE patterns, fractionation of light REE (LREE) (La/Sm)_N and heavy REE (HREE) (Gd/Yb)_N, and Eu anomaly (Eu/Eu*), the upper continental crust normalised playa sediments are divided into two different groups. The eastern margin playa sediments show homogeneous REE contents, relatively positive Eu anomaly and depleted HREE values, whereas the western arid core playa sediments have highly variable REE contents, relatively negative Eu anomaly and similarly fractioned LREE and HREE patterns. The dissimilarity in the degree of HREE fractionations both in the eastern and western playa sediments is attributed to the differential distribution of minerals, depending upon their resistance to chemical weathering. It is believed that the relatively higher abundance of REE bearing heavy minerals and the presence of higher amounts of evaporites influence the large variation of REE distribution and enriched HREE in the western playa sediments. Apart from the relatively higher abundance of heavy minerals, the presence of rock fragments of variable petrographic character and roundness mirror the lower rock–water interaction in the arid western region. The presence of well-rounded metamorphic rock fragments and minerals, sourced from the eastern margin Aravalli mountains, indicates that the playas of the entire desert get the detrital and dissolved material mainly from the Aravalli mountains. Additionally, the western playas receive sediments from their surrounding Proterozoic and Mesozoic formations. This interpretation is supported by the presence of angular rock fragments of basalt, rhyolite and limestone in the western playas.     

Textural and geochemical characteristics of the Ajali Sandstone,Anambra Basin,SE Nigeria: Implication for its provenance

This study presents the mineralogical, textural and geochemical characteristics of the regional Maastrichtian Ajali Sandstone in Anambra Basin, SE Nigeria. The intent is to highlight possible constraints on the chemical weathering conditions of the source materials on one hand, and to infer the provenance on the other hand. The investigation approach involved field studies and collection of samples from 12 different outcrop locations, followed by laboratory studies involving grain-size analysis (GSA), major and trace elements analyses using the X-ray fluorescence (XRF) method as well as thin section petrography. Field studies show that the sandstones are friable at all locations and range in color from white in freshly cut stone, to reddish brown on weathering. In addition, the sandstone units are cross-bedded and show graded bedding exemplified by fining upward sequence. Textural examination indicates that the sandstones range from fine to medium sands, constituting about 76 to 99% sand fraction, with graphic mean grain size of 0.23 to 0.53 mm. Standard deviation (sorting) ranges from 0.56 to 1.24 Ø and implies moderately well sorted sediments. Inferred from the textural indices, the depo-environmental discrimination of the Ajali Sandstone revealed a fluvial/river system-dominated sedimentary process. The sandstones are quartz arenite with quartz greater than 90% and less than 5% K-feldspar which indicate a predominant basement source as also revealed by the heavy mineral assemblages. In addition, major elemental oxides shows SiO₂ content greater than 96% for the fresh Ajali Sandstone samples with extreme depletion of mobile oxides such as Na₂O, CaO and the ferromagnesian minerals through weathering and sedimentary processes. Provenance and tectonic setting discrimination using geochemical data and compositional maturity revealed typical felsic igneous-dominated cratonic environment while inter-elemental ratios (such as Zr/Cr, Y/Ni, Th/Sc, La/Sc and La/Co) and ternary plots (e.g. Th–Sc–Zr; La-Th–Sc and Th-Co-Zr) reflect passive continental margin setting for the Ajali Sandstone. Consequently, the source area is constrained to the Precambrian basement rock units of Adamawa-Oban massif areas to the east of the Anambra Basin and the adjacent Abakaliki Anticlinorium.     

Rare-earth elements as source indicators of Pan-African granites from Obudu Plateau,Southeastern Nigeria

The rare-earth element （REE） concentrations of representative granite samples from the southeast of the Obudu Plateau, Nigeria, were analyzed with an attempt to determine the signatures of their source, evolutionary history and tectonic setting. Results indicated that the granites have high absolute REE concentrations （190×10^-6-1191×10^-6; av.=549×10^-6） with the chondrite-normalized REE patterns characterized by steep negative slopes and prominent to slight or no negative Eu anomalies. All the samples are also characterized by high and variable concentrations of the LREE （151×10^-6-1169×10^-6; av.= 466×10^-6）, while the HREE show low abundance （4×10^-6-107×10^-6; av.=28×10^-6）. These are consistent with the variable levels of REE fractionation, and differentiation of the granites. This is further supported by the range of REE contents, the chondrite-normalized patterns and the ratios of LaN/YbN （2.30-343.37）, CeN/YbN （5.94-716.87）, LaN/SmN （3.14-11.68） and TbN/YbN （0.58-1.65）. The general parallelism of the REE patterns, suggest that all the granites were comagmatic in origin, while the high Eu/Eu＊ ratios （0.085-2.807; av.=0.9398） indicate high fo2 at the source. Similarly, irregular variations in LaN/YbN, CeN/YbN and Eu/Eu＊ ratios and REE abundances among the samples suggest behaviors that are related to mantle and crustal sources.     

The rare earth element compositions of sediments from the loess tableland in the Liyang Plain,southern China: implications for provenance and weathering intensity

To identify the weathering intensity and to deduce the provenance of sediments (black-brown soil and loess) from the loess tableland in the Liyang Plain is of great importance for understanding the development and origin of civilization of this critical region in China. The geochemical results show similar REE distribution patterns among sediments sampled from the YC profile in the Liyang Plain, reticulated red soils from the Dongting lake area, Xiashu loess in Zhenjiang, and loess from the Loess Plateau in China. These similarities imply a single provenance, from dust storms. REEs, especially the LREE/HREE ratio and Eu anomaly, can trace weathering intensity. Higher LREE/HREE ratios, and remarkably Eu anomalies, are present in the top loess (L₀), Lower LREE/HREE ratios and insignificant Eu anomalies are present in black-brown soil (S₀*) from the Liyang Plain. These demonstrate that the black-brown soils have undergone a lower intensity of weathering than the loess of YC profile. Moreover, CIA values for sediments from the YC profile provide powerful evidence to support the above result. Comparisons of analyses of the REE contents of sediments from the YC profile, of reticulated red soils from the Dongting Lake area, Xiashu loess from the Yangtze River, and loess from the Loess Plateau, clearly show the weathering intensity decreases according to the following sequence: reticulated red soil from the Dongting Lake area > sediments of YC profile from the Liyang Plain, which formed from dust storms ≈ Xiashu loess from lower reach of the Yangtze River > loess from the Loess Plateau.     

Geochemistry of lower Jurassic shales of the Shemshak Formation, Kerman Province, Central Iran: Provenance, source weathering and tectonic setting

Lower Jurassic shales of the Shemshak Formation of Kerman Province, Central Iran, were analyzed for major and selected trace elements to infer their provenance, intensity of palaeoweathering of the source rocks and tectonic setting. Plots of shales on Al₂O₃ wt.% versus TiO₂ wt.% diagram and Cr (ppm) versus Ni (ppm) diagram indicate that acidic (granitic) rocks constitute the source rocks in the provenance. Average CIA, PIA and CIW values (84%, 92%, 93%, respectively) imply intense weathering of the source material. Plots of shales on bivariate discriminant function diagram reveal an active continental margin setting for the provenance. The inferred tectonic setting for the Lower Jurassic shales of the Shemshak Formation of Kerman Province is in agreement with the tectonic evolutionary history of the Central Iran during the Jurassic period.     

鞍山-本溪地区条带状铁建造的铁同位素与稀土元素特征及其对成矿物质来源的指示

通过Fe同位素、稀土元素与主量元素相结合的方法,对辽宁省鞍山-本溪地区新太古代条带状铁建造(BIF)的成矿物质来源提出了有效制约.BIF的化学成分主要由TFe2O3和SiO2组成,并且具有较低的Al2O3和TiO2含量,表明该地区BIF型贫铁矿是由极少碎屑物质加入的化学沉积岩.稀土元素的总量较低,经页岩标准化后的稀土元素配分模式呈现轻稀土亏损、重稀土富集的特征,具有明显的Eu、Y、La正异常,这些特征表明该地区BIF是古海洋的化学沉积岩,同时具有明显的火山热液贡献特征.用多接收电感耦合等离子体质谱仪(MCICP-MS)测定Fe同位素的结果表明,相对于标准IRMM-014,所测样品均显示Fe的重同位素富集,且Fe同位素组成与Eu异常存在明显的正相关关系,表明该地区BIF中铁的来源与海底火山热液活动密切相关,首次从成矿元素Fe本身为条带状铁建造的成矿物质来源提供了直接的证据.     

准噶尔盆地表层沉积物的稀土元素特征与物源指示

准噶尔盆地的新生代沉积的物质来源对认识中亚地区乃至全球气候环境演变、亚洲内陆干旱化以及粉尘输送路径等关键科学问题都具有重要的意义,但目前对盆地内沉积物的物质来源的研究还比较粗略。文章以准噶尔盆地内不同类型的表层沉积物为研究对象,通过对稀土元素特征的研究进一步探讨盆地内不同沉积的物质来源与相互联系。研究结果表明：准噶尔盆地不同区域表层沉积物的稀土元素含量（不含Y元素）变化较大,变化范围为55.54~194.44 μg/g;大部分样品的δCe值在0.95~1.05之间,未见明显的Ce异常,指示了盆地内整体干旱环境下较弱的风化成壤作用,而Eu异常和UCC标准化配分模式表现出的复杂变化,可能指示了准噶尔盆地表层沉积物物质来源的多源性。根据盆地内表层沉积物的稀土元素总量、配分模式以及δEu值表现出的不同特征,结合了盆地内的区域地形和盛行风向等要素,认为盆地东部的戈壁砂和周围山地的基岩碎屑物之间存在着一定的物源继承,同时强劲的西北风也为该地带来了来自盆地北部和中部的物质来源;北部的阿尔泰山碎屑物质是盆地北部戈壁砂和沙漠砂的主要物质来源,但戈壁砂表现出的Eu中度亏损和较高的ΣREE值可能代表着该区域还存在更为广阔的物源;盆地西部各类沉积物之间表现出的稀土元素特征的相似性表明该地缺乏外界物质的参与,物源具有明显的局域性;盆地南缘的沙漠砂和河流表层沉积物主要来源于天山碎屑物质经冰川磨蚀及寒冻风化等作用形成的粉砂粒级物质,但是该地的黄土在地形和风向的影响下表现出了复杂的物质来源,盆地中的沙漠、天山碎屑物质、盆地东部的戈壁砂,甚至于中亚广大荒漠区的粉尘都为黄土提供了部分物源。

     

Mineralogical and geochemical characteristics of Miocene pelitic sedimentary rocks from the south-western part of the Pannonian Basin System (Croatia): Implications for provenance studies

Fifty-two samples of Miocene pelitic sedimentary rock from outcrops on Medvednica,Moslavacka Gora and Psunj Mts.,and boreholes in the Sava Depression and the Pozega Sub-depression were investigated.These sediments formed in different marine(with normal and reduced salinity),brackish,and freshwater environments,depending on the development stage of the Pannonian Basin System.Carbonate minerals,clay minerals and quartz are the main constituents of all pelitic sedimentary rocks,except in those from Moslavacka Gora Mt in which carbonate minerals are not present.Feldspars,pyrite,opal-CT,and hematite are present as minor constituents in some rocks.Besides calcite,dependent on the sedimentary environment and diagenetic changes,high-magnesium calcite,aragonite,dolomite and ankerite/Cadolomite are also present.Smectite or illite-smectite is the main clay minerals in the samples.Minor constituents,present in almost all samples,are detrital illite and kaolinite.In some samples chlorite is also present in a low amount.Major elements,trace elements and rare earth elements patterns used in provenance analysis show that all analysed samples have a composition similar to the values of the upper continental crust(UCC).The contents of major and trace elements as well as SiO_2/Al_2O_3,K_2O/Al_2O_3,Na_2O/K_2O,Eu/Eu~*,La/Sc,Th/Sc,La/Co Th/Co,Th/Cr,Ce/Ce~* and LREE/HREE ratios,show that the analysed pelitic sedimentary rocks were formed by weathering of different types of mostly acidic(silicic),i.e.felsic rocks.     

Petrography and geochemistry of Jumara Dome sediments,Kachchh Basin: Implications for provenance,tectonic setting and weathering intensity

In the Kachchh Mainland, the Jumara Dome mixed carbonate-siliciclastic succession is represented by the Jhurio and Patcham formations and siliciclastic-dominating Chari Formation （Bathonian to Oxfordian）. The Ju- mara Dome sediments were deposited during sea-level fluctuating, and were interrupted by storms in the shallow marine environment. The sandstones are generally medium-grained, moderately sorted, subangular to subrounded and of low sphericity. The sandstones are mineralogically mature and mainly composed of quartzarenite and subar- kose. The plots of petrofacies in the Qt-F-L, Qm-F-Lt, Qp-Lv-Ls and Qm-P-K ternary diagrams suggest mainly the basement uplift source （craton interior） in rifted continental margin basin setting. The sandstones were cemented by carbonate, iron oxide and silica overgrowth. The Chemical Index of Alteration values （73% sandstone and 81% shale） indicate high weathering conditions in the source area. Overall study suggests that such strong chemical weathering conditions are of unconformity with worldwide humid and warm climates during the Jurassic period. Positive correlations between A1203 and Fe203, TiO2, Na20, MgO, K20 are evident. A high correlation coefficient between A1203 and K20 in shale samples suggests that clay minerals control the major oxides, The analogous con- tents of Si, A1, Ti, LREE and TTE in the shale to PAAS with slightly depleted values of other elements ascribe a PAAS like source （granitic gneiss and minor mafics） to the present study. The petrographic and geochemical data strongly suggest that the studied sandstones/shales were deposited on a passive margin of the stable intracratonic basin.     

Geochemical evolution of the Tertiary succession of the NW shelf,Bengal basin,Bangladesh: Implications for provenance,paleoweathering and Himalayan erosion

Whole rock major, trace and rare earth element (REE) compositions of Paleogene to Neogene sedimentary rocks of the NW shelf succession (Province 1) of Bangladesh contain a record of interaction of the India and Asian plates, Himalayan tectonism, and climatic development. Analyses of 66 sandstones and mudrocks from the Tertiary succession of Bangladesh were made to examine provenance, source weathering, and the influence of paleoclimate and tectonism. The sediments display linear geochemical trends due to quartz dilution, and varying quartz–clay ratios produced by hydrodynamic sorting. Chondrite-normalized REE patterns for both sandstones and mudrocks from different groups are similar to upper continental crust, with moderate to high LREE enrichment (lithotypes within formations average La_N/Yb_N 5.31–11.41) and marked negative Eu anomalies (Eu/Eu^* 0.51–0.69). Based on geochemical criteria the succession can be divided into three parts (Jaintia; Barail–Surma; and Dupi Tila). Very high silica contents in Jaintia Tura sandstones and high Chemical Index of Alteration (CIA) indices in Kopili mudrocks (Fe-shales) suggest derivation from a deeply weathered and stable cratonic source (India). The Tura sandstones are interpreted as first-cycle quartz arenites, produced while the Indian plate drifted across equatorial regions during the Paleocene–Eocene Thermal maximum (PETM). The Barail–Surma and Dupi Tila sediments were derived from a felsic orogen (the Himalaya). The Barail–Surma sediments were mainly derived from the Trans Himalayan Batholith and associated granitoids, with significant contribution from the Lesser Himalaya. Mafic input is also evident, probably from intraoceanic arc material within the Himalaya. Barail mudrocks have uniformly high CIA values (92–95), suggesting intense steady-state weathering of their proto-Himalayan source, and warm and humid climate. In contrast, CIA values of Surma mudrocks range from 66 to 93 (average 84), suggesting non-steady state weathering related to active uplift in the Himalaya. The Dupi Tila sediments were derived from a more felsic Lesser Himalaya source, with significant contribution from the Trans-Himalaya and very little or no ophiolitic or arc material. Dupi Tila mudrocks have CIA ratios of 62–99 (average 72), also indicating non-steady state weathering in the rising Himalayan source. Geochemical compositions of the NW shelf sediments are comparable to coeval successions in the Surma basin (Province 2) of Bangladesh and the Siwaliks (India), indicating similar source. Evolution of the Indian monsoon and associated high precipitation caused intense chemical weathering of the Surma and Dupi Tila source, despite rapid uplift. The Surma Group thus bears the signature of evolution of the Asian monsoon in the Bengal basin at 21 Ma, simultaneous with the development of the East Asian monsoon. This supports proposals that both monsoon systems developed at the same time.