Geochemistry of meta-volcanic rocks from the Longbohe Cu deposit, Yunnan Province, China： Implications for the genesis and tectonic setting

The Longbohe Cu deposit, which is located in the southern part of the Honghe ore-forming zone, Yunnan Province, China, belongs to a typical ore field where volcanic rocks are of wide distribution and are associated with Cu mineralization in time and space. The volcanic rocks in the ore field, which have experienced varying degree of alteration or regional metamorphism, can be divided into three types, i.e., meta-andesite, meta-subvolcanic rock and meta-basic volcanic rock in accordance with their mineral assemblages. These three types of volcanic rocks in the ore field are relatively rich in Na and the main samples plot in the area of alkali basalts in the geochemical classification diagram. With the exception of very few elements, these three types of volcanic rocks are similar in the content of trace elements. In comparison to the basalts of different tectonic settings, the meta-volcanic rocks in the ore field are rich in high field strength elements (HFSE) such as Th, Nb, etc. and depleted in large ion lithophile elements (LILE) such as Sr, Ba, etc. and their primary mantle-normalized trace element patterns show remarkable negative Th and Nb anomalies and negative Sr and Ba anomalies. These three types of volcanic rocks are similar in REE content range and chondrite-normalized REE patterns with the exception of Eu anomaly. Various lines of evidence show that these three types of volcanic rocks in the ore field have the same source but are the products of different stages of magmatic evolution, their original magma is a product of partial melting of the metasomatically enriched mantle in the tensional tectonic setting within the continent plate, and the crystallization differentiation plays an important role in the process of magmatic evolution.     

Brittle deformation in the Afikpo Basin （Southeast Nigeria）： Evidence for a terminal Cretaceous extensional regime in the Lower Benue Trough, Nigeria

This paper deals with the regional and structural framework of the Cretaceous rocks in the Afikpo Basin located in the southeastern part of the Lower Benue Trough. Results from regional tectonics are presented together with those of the microtectonic analysis of microfaults in the Owutu-Afikpo-Adadama area in the basin. The Owutu-Afikpo-Adadama ridge at the north-central part of the basin marks the boundary between the Late Cenomanian-Turonian-Conianian sediments and the Campanian-Maastrichtian sandstones. This ridge trends N45°E on average and is faulted in three main directions, namely: (1) N-S normal faults; (2) NE-SW strike-slip faults; and (3) NW-SE strike-slip faults. The faulted rocks along these brittle discontinuities are mainly cataclastics with internal fracture cleavage and sigmoidal quartz mosaics that are reminiscent of extensional deformation. The cataclasites often bear slickenside striations.     

Geochemistry and Petrology of Emeishan Basalts and Subcontinental Mantle Evolution in Southwestern China

Three major volcanic rock sequences in the P2β formation(Emeishan basalts)were sampled dur-ing a comprehensive study of the Late Permian volcanics associated with the Panxi paleorift in southwestern China .Two of the three sections-Emei and Tangfang are composed of continental flood basalts(CFB) while the third-Ertan is an alkalic center.Multi-element chemical analyses indi-cate a predominance of low MgO transitional quartz tholeiites at Emei and Tangfang,whereas the Ertan suite ranges from high-MgO alkaline olivine basalts to rhombic porphyry trachytes and quartz-bearing aegerine-augite syenites.Consanguineity of the rocks from the three sections is sug-gested by consistently high TiO2 ,K2O,incompatible trace elements and uniformly fractionated REE patterns typical of alkalic compositions,but antypical of CFB.Sr isotope data for ten Emei basalt samples(^87Sr/^86Sr=0.7066-0.7082)which show no correla-tion with Rb/Sr ratios (0.02-0.12) and Nd isotopes for two of the samples(^143Nd/^144Nd=0.51171-0.51174)are interpreted as being related to the mantle evolution.The primary magmas re-sponsible for all the three sequences have been modeled in terms of a uniformly metasomatized man-tle source.Trace element models support the derivation of the Emei and Tangfang primary magmas from 10-15 percent partial melting of spinel lherzolite,followed by fractional crystallization of olivive and clinopyroxene.The primary alkaline olivine basalts at Ertan are generated by 7-10 percent par-tial melting of a chemically equivalent source in the garnet-peridodite stability region.The assumed mantle composition is characterixzed by Rb=3.8-5.5 ppm,Sr=62-83ppm,Ba=45-64 ppm,La=3.8-5.6ppm,and Yb=0.46-0.57ppm.The proposed mechanism of regional mantle enrichment requires metasomatic stabilization of phlogopite which becomes depleted later during par-tial melting.Such enrichment is consistent with the models proposed for alkalic systems in which a large mantle diaper acts as the agent for upward enrichment as well as uplift and extension of the crust.     

Rare-earth element geochemistry as a tracer for clarifying the provenance of the stream sediments in Namhan River, Korea

In the last forty years, the rare-earth elements （REEs） have been used as a powerful tool for solving various geological and geochemical problems due to their unique and chemically coherent behavior. The river sediments are produced by weathering, transportation and deposition. Nesbitt suggested that the rare-earth elements had been mobilized and fractionated during supracrustal alteration of the Torrongo granite, southeastern Australia. Nevertheless, recently, our replicate estimation for REE patterns in sediments revealed that there is nearly no variation in REE patterns. This suggests that invariability of REE patterns in weathering and solidification can be used as a tracer for constraining the provenance of stream sediments. In order to trace the pathway of the river sediments geochemically, based on the REE geochemistry from the river sediments and rocks, we have monitored the REE abundance of stream sediments at branch rivers of the Namhan River in South Korea for three years. The branches studied are Bokhacheon, Shinduncheon and Yanghwacheon. The sediments were divided into coarse （〉300 μm） and fine fractions （〈300 μm）. As a result, we could observe that major element compositions were similar to each other regardless of particle size and sampling date. This suggests that it is difficult to deduce a geochemical difference between river sediments based on major element composition and particle size. The geochemical characteristics of surrounding soils were similar to those of fine river sediments. And the chondrite-normalized REE patterns of most of the fine and coarse sediments in combination with grain sizes and drainage area showed very close relationship with the surrounding rocks. However, some sites showed that there were large variations in REE patterns including total REE abundance and Eu anomaly due to feldspars. This variation trend of REE patterns suggests that changes might have happened in sediment supply for the drainage system in the study area due to floods or large-scale construction.     

Petrography and geochemistry of the Proterozoic sandstones of Somanpalli Group from Pomburna area,Eastern Belt of Pranhita–Godavari Valley,central India: Implications for provenance,weathering and tectonic setting

In this paper, we, for the first time, report geochemistry of sandstone from Somanpalli Group from Pomburna area in the Eastern Belt of Pranhita–Godavari (PG) Valley, central India and studied to infer their provenance, intensity of paleo-weathering and depositional tectonic setting. Petrographic study of sandstones show QFL modal composition of arenite. Chemical results show high \(\hbox {SiO}_{2}\) and CIA but lower \(\hbox {Al}_{2}\hbox {O}_{3}, \hbox {TiO}_{2}\), Rb, Sr, \(\hbox {K}_{2}\hbox {O}\) indicating mixed sources. Major elements chemistry parameters such as, \(\hbox {K}_{2}\hbox {O/Al}_{2}\hbox {O}_{3}\) ratio and positive correlation of Rb with \(\hbox {K}_{2}\hbox {O}\), reflects a warm and humid climate for study area. The tectonic discrimination plots (\(\hbox {SiO}_{2}/20\)–\(\hbox {K}_{2}\hbox {O} + \hbox {Na}_{2}\hbox {O}\)–\(\hbox {TiO}_{2} + \hbox {Fe}_{2}\hbox {O}_{3} + \hbox {MgO};\,\hbox {K}_{2}\hbox {O}/\hbox {Na}_{2}\hbox {O}\) vs. \(\hbox {SiO}_{2}\); Th–Sc–Zr/20) indicate dominantly passive margin and slight active tectonic setting. Concentrations of Zr, Nb, Y, and Th are higher compared to the UCC values and the trends in Th/Cr, Th/Co, La/Sc and Cr/Zr ratios support a felsic and mafic source for these sandstones and deposition in passive margin basin. Chondrite normalized REE pattern reflects LREE depletion, negative Eu anomaly and flat HREE similar to UCC, felsic components. ICV value (0.95) also supports tectonically quiescent passive margin settings. CIA values (74) indicate high degree of chemical weathering and warm and humid paleoclimatic condition.     

Geological and Geochemical Characteristics of Early Cretaceous Mafic Dikes in Northern Jiangxi Province, SE China and Their Geodynamic Implications

The development of Early Cretaceous mafic dikes in northern and southern Jiangxi allows an understanding of the geodynamic setting and characteristics of the mantle in southeast China in the Cretaceous. Geological and geochemical characteristics for the mafic dikes from the Wushan copper deposit and No. 640 uranium deposit are given in order to constrain the nature of source mantle, genesis and tectonic implications. According to the mineral composition,the mafic dikes in northern Jiangxi can be divided into spessartite and olive odinite types, which belong to slightly potassium-rich calc-alkaline lamprophyre characterized by enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE), large depletion in high strength field elements (HSFE) and with negative Nb, Ta and Ti anomalies, as well as 87Sr/86Sr ratios varying from 0.7055 to 0.7095 and 143Nd/r44Nd ratios varying from 0.5119 to 0.5122.All features indicate that the magma responsible for the mafic dikes was derived mainly from metasomatic lithosphere mantle related to dehydration and/or upper crust melting during subduction. Differences in geochemical characteristics between the mafic dikes in northern Jiangxi and the Dajishan area, southern Jiangxi were also studied and they are attributed to differences in regional lithospheric mantle components and/or magma emplacement depth. Combining geological and geochemical characteristics with regional geological history, we argue that southeast China was dominated by an extensional tectonic setting in the Early Cretaceous, and the nature of the mantle source area was related to enrichment induced by asthenosphere upwelling and infiltration of upper crust-derived fluids responding to Pacific Plate subduction.     

Trace and rare-earth element geochemistry： A tool for petrogenetic and geotectonic modeling of ensimatic ortho-amphibolites from Pan-African belt of Obudu Plateau, Southeastern Nigeria

A model for the petrogenetic affinity and original geotectonic setting of ortho-amphibolites from the Obudu Plateau was tested using the distribution patterns of trace and rare-earth elements from the geochemical analyses of twelve representative amphibolite samples. Discrimination plots, normalized patterns of the incompatible trace elements against average MORB, low ratios of Ba/Nb (9–23) and Ba/Ta (130–327) and other geochemical characteristics suggest that the protoliths were dominantly of tholeiitic MO...     

Did marine transgression happen in Late Cretaceous period in the southeast of Songliao Basin ？——Discussion from evidence of water chemistry

Cretaceous sediment is considered to be the important oil reservoir in the Songliao Basin, which is dominated by continental fluvial and lacustrine strata. In the past decades, research on whether the marine transgression happened in the Late Cretaceous period in the southeast of the Songliao Basin is always a hotspot in China. Previous studies analyzed the marine biology and geochemistry of sediments to argue if the marine transgression had occurred. However, the geochemistry of formation water which could indicate the paleo-environment has not attracted attention. In this paper, the geochemical variations of formation water between the Chaoyanggou anticline, Changchun anticline and Binxian depression in the southeast of the Songliao Basin are analyzed, then the results by comparisons with chemical properties of sea water indicated： （1） the extensive range of pH values from 6 to 10 concerning the formation water. The ranges of TDS of the formation water are from 1 g/L to 20 g/L, which is less than the value in the seatwater; （2） the level of the element Br, which is an important trace element to indicate the marine sedimentary environment, is lower than that from the sea water. The concentrations of the trace element B fluctuate intensively, probably due to the enhancement of water-rock interaction and the enrichment of organic matter; and （3） ionic ratio of Na/Cl and Br＊ 1000/Cl in the formation water are higher than the value of 0.8575 and lower than 1.5363 in the sea water, respectively, which might be attributed to halite dissolution.     

Geochronology and geochemistry of late Jurassic adakitic intrusions and associated porphyry Mo–Cu deposit in the Tongcun area,east China: Implications for metallogenesis and tectonic setting

The genesis of adakites and associated Mo–Cu mineralization in non–arc settings in China is poorly constrained. Here, we present geochronology, geochemistry and Sr–Nd–Pb isotopes for the Tongcun intrusive complex, and report Pb isotopes and ⁴⁰Ar–³⁹Ar age for the Tongcun Mo–Cu deposit. The Tongcun intrusive complex is composed mainly by granodiorite and monzogranite (phase 1 and phase 2), with emplacement age of 160 Ma to 148 Ma. The Tongcun complex can be classified as typical high–K calc–alkaline I–type granitoid and also shows adakitic geochemical features. Moreover, the negative Nb, Ta, Ti, and P anomalies and enriched initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7083–0.7092 of the Tongcun intrusive complex are consistent with those of the subduction–related magmatism. The ⁴⁰Ar–³⁹Ar dating of sericite, which is intergrown with chalcopyrite, indicates that the late Cu mineralization event occurred at ~ 155.5 Ma. The early Mo (Cu) and the late Cu mineralization events in this deposit were temporally, spatially and genetically associated with the emplacement of monzogranite (phase 1). There are no obvious linear correlation between SiO₂ and most of the major and trace elements, and all rock samples fall within the fields of unfractional crystallization felsic granites in Zr + Nb + Ce + Y versus FeO^T/MgO and (K₂O + Na₂O)/CaO diagrams, indicating that partial melting rather than fractional crystallization has played an important role for the formation of the Tongcun intrusive complex. Magmatic inherited zircons from the Tongcun granitoids with the age peaked at 780–812 Ma, imply that the Neoproterozoic igneous rocks in the lower crust have been incorporated into the magma source. The uniform ε_Nd(t) (− 6.3 to − 7.3), initial ⁸⁷Sr/⁸⁶Sr, ²⁰⁷Pb/²⁰⁴Pb (15.596–15.621), and ²⁰⁸Pb/²⁰⁴Pb (38.374–38.650), as well as high K₂O contents (3.36–4.10 wt.%) and relatively high Mg# values (35.40 to 40.30) suggest the Tongcun intrusive complex was derived from partial melting of the thickened lower continental crust triggered by basaltic magma underplating plus additional input from the EM II mantle-derived basaltic melts. The Tongcun area was controlled by a compression setting related to the subduction of the Paleo–Pacific Plate in Mesozoic period.     

Geochemical characterization of Cretaceous sandstones from the Southern Benue Trough,Nigeria

Geochemical studies of sandstones from the three lithostratigraphic successions in the southern Benue Trough of Nigeria were undertaken for a geochemical characterization of the sandstones, and to assess their strati-graphic and source evolution.Major and trace elements data were obtained from outcrop sandstone samples. The SIO2/Al2O2, Fe2O3/K2O ra-tios and CaO contents have been used to characterize the Cretaceous sandstones into Al-rich and high and low Fe2O3/K2O ratio sandstones. Results indicate that there are geochemical features that display stratigraphic trends across the succession from the Asu River Group, Eze-Aku Group to the proto-Niger Delta succession which may imply a discontinuous evolution from different source terrains of Precambrian and Mesozoic ages that supplied the sediments. The Asu River Group sandstones have lower SiO2, Al2O3, Fe2O3 and higher MgO; the Eze-Aku sand-stones have higher TiO2, CaO, alkalis and lower MgO while the proto-Niger Delta sandstones have higher SiO2, Al2O3, Fe2O3 and lower alkalis and CaO. These discontinuities signify the influence of tectonic impulses that af-fected the southern Benue Trough during the Cretaceous time. Changes in ratios of TiO2/Al2O3, Fe2O3, Cr and Zr suggest an increasingly mafic contribution to the depositional basin with time. The chemical index of alteration in-creases with time, possibly suggesting that a more intense weathering regime in the hinterland developed with time.     

Tectonic Setting and Nature of the Provenance of Sedimentary Rocks in Lanping Mesozoic- Cenozoic Basin： Evidence from Geochemistry of Sandstones

The geochemical composition of sandstones in the sedimentary basin is controlled mainly by the tectonic setting of the provenance, and it is therefore possible to reveal the tectonic setting of the provenance and the nature of source rocks in terms of the geochemical composition of sandstones. The major elements, rare-earth dements and trace elements of the Mesozoic-Cenozoic sandstones in the Lanping Basin are studied in this paper, revealing that the tectonic settings of the provenance for Mesozoic-Cenozoic sedimentary rocks in the Lanping Basin belong to a passive continental margin and a continental island arc. Combined with the data on sedimentary facies and palaeogeography, it is referred that the eastern part of the basin is located mainly at the tectonic setting of the passive continental margin before Mesozoic, whereas the western part may be represented by a continental island arc. This is compatible with the regional geology data. The protoliths of sedimentary rocks should be derived from the upper continental crust, and are composed mainly of felsic rocks, mixed with some andesitic rocks and old sediment components. Therefore, the Lanping Mesozoic-Cenozoic Basin is a typical continental-type basin. This provides strong geochemical evidence for the evolution of the paleo-Tethys and theb asin-range transition.     

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.     

冈底斯带门巴区晚白垩世花岗岩年代学、地球化学及构造背景

门巴区晚白垩世花岗岩主要分布于冈底斯中段的扎雪、金达和桑巴附近,由黑云母钾长花岗岩、钾长花岗岩和二云母钾长花岗岩及花岗闪长岩等组成。其中花岗闪长岩SHRIMP 锆石U--Pb 年龄为68. 8 ± 1. 6 Ma; 钾长花岗岩黑云母K--Ar 年龄为78. 2 ± 1. 2 Ma 和81. 5 ± 1. 4 Ma; 黑云母钾长花岗岩和二云母钾长花岗岩的黑云母K--Ar 分别为90. 8 ± 1. 81 Ma 和91. 2 ± 1. 8 Ma。岩石学和地球化学分析结果表明,S 型和低Sr 低Yb 特征的黑云母钾长花岗岩和二云母钾长花岗岩形成于拉萨地块与羌塘地块碰撞造山过程中的地壳加厚背景; S 型和总体低Sr、高Yb 特征的钾长花岗岩形成于造山带晚期阶段的伸展背景; I 型花岗闪长岩应是新特提斯洋俯冲作用的结果,形成于岛弧构造环境。     

鄂尔多斯镇原地区下白垩统砂岩地球化学特征及其对物源区、构造背景的指示

本文通过对鄂尔多斯盆地镇原地区下白垩统砂岩进行主量元素、微量元素及稀土元素REE等地球化学分析, 就下白垩统砂岩的沉积再循坏特征、化学风化及其构造背景和物质来源等展开讨论。研究表明: ① 下白垩统洛河组、环河组和罗汉洞组的物源主体上具有相似性, 且在构造背景、物源区及物源特征上有良好的继承性。物源区总体具有富石英、富方解石胶结物、钾长石或富钾矿物较斜长石多的特点, 罗汉洞组较环河组和洛河组硅质更高、方解石胶结物更少、质更纯。稀土元素REE配分模式曲线一致, 轻重稀土分馏明显, 富集大离子亲石元素Ba、Pb、Sr和轻稀土元素, 亏损高场强元素Nb、Ta、Th、Y等,自下而上随着地层变新, 沉积岩中总的稀土含量有变小的趋势, 重稀土元素分馏程度减小; ② 研究区下白垩统砂岩原始物质主要为长英质与基性岩混合岩 (如中酸性侵入岩、长英质变质岩)及部分沉积岩。研究区大部分沉积物是鄂尔多斯盆地西南缘逆冲推覆带母岩风化剥蚀的产物搬运至此沉积形成, 系首次沉积的物质, 少数沉积物 (即风成砂)为就地再次搬运堆积形成, 系再循环沉积物质; ③ 在构造背景上具有以被动大陆边缘构造环境占主要优势的特点。碎屑物主要来源于陆块源区, 其次来源于再旋回造山带和混合源区。     

华夏陆块早寒武世沉积地层年代学与地球化学组成：对源区和构造背景的指示

华夏陆块早古生代沉积记录对于了解华南陆块在Gondwana超大陆演化过程中所起的作用具有重要意义。本文对广东省中山市神湾地区下寒武统八村群开展了锆石年代学和元素地球化学研究。结果表明,八村群变泥质粉砂岩的碎屑锆石年龄可分为4034~3122 Ma、2765~2250 Ma、1765~1443 Ma、1126~901 Ma、884~773 Ma以及595~547 Ma六组,其中最年轻的一组碎屑锆石加权平均年龄将该地层的最大沉积时间限定在551±4 Ma。锆石的Hf同位素特征则指示八村群沉积物源可能以古老地壳物质再循环为主,伴少量初生地壳物质。八村群变粉砂岩和变泥岩样品的化学风化指数(CIA)和成分成熟度(ICV)分别为79. 8~86. 4和0. 39~0. 87,均表明源区经历了相对强烈的化学风化作用。元素地球化学结果显示,八村群样品强烈富集不相容元素,而亏损Ni、Cr和Co等,指示八村群源区可能以活动大陆边缘构造背景下的长英质岩石为主。综合对比八村群与世界其他陆块同时期沉积地层的碎屑锆石年龄组成,本文认为八村群代表了新元古代晚期至早古生代早期Gondwana超大陆聚合过程中造山作用的沉积响应,支持华夏陆块位于Gondwana超大陆北缘,靠近北印度板块和羌塘地体,是该超大陆的重要组成部分。     

浙江治岭头地区玄武岩U-Pb年龄、地球化学特征及构造背景

为加深对华南地区东部白垩纪构造演化及壳幔作用的认识,对浙江省治岭头地区发育的玄武岩进行了U-Pb年龄、地球化学和Sr-Nd-Hf同位素分析。玄武岩呈块状构造,斑状结构,斑晶以斜长石、角闪石为主。LA-ICP-MS锆石U-Pb定年结果显示年龄为101±1 Ma,为早白垩世岩浆活动的产物。岩石地球化学分析结果表明,玄武岩整体具有富碱、高K2O/Na2O、高K2O+Na2O值和高Al2O3含量的特征,富集大离子亲石元素(Rb、Sr等)及轻稀土元素,亏损高场强元素(Nb、Ta、Ti、Hf等),稀土元素配分曲线整体右倾,具有微弱的正Eu异常,应属于钾玄岩系列。岩浆演化过程中经历了一定程度的分离结晶作用,Th/La、Th/Ce、Lu/Yb等值更接近于幔源岩浆的比值;(87Sr/86Sr)i=0.70729~0.71029, εNd(t)值及εHf(t)值范围分别为-19.7~-11.8和-9.7~-0.8,指示玄武岩源区具有EMⅠ富集地幔的特征,地壳物质亦有少量参与。结合华南地区晚中生代基性岩浆时空分布及地球化学特征,随着古太平洋板块的持续后撤作用,华南地区早白垩世为弧后扩张环境,由俯冲板片脱水产生的流体交代地幔形成的EMⅠ型富集地幔部分熔融,喷出地表形成了治岭头地区的玄武岩。     

A model for the tectonic evolution of the Benue Trough of Nigeria

Based on the results of a detailed magnetic study of the Lower and Middle Benue Trough of Nigeria carried out by the author and results of previous geophysical studies of the Benue Trough and of similar structures elsewhere in the world, a possible model for the tectonic evolution of the Benue Trough has been developed. It is here suggested that the tectonic evolution of the Benue Trough of Nigeria involved the rise of a mantle plume or mantle upwelling, emplacement of intrusive igneous material in the crust, crustal stretching and thinning and consequently rifting. It is thought that this sequence of events may have been repeated in a cyclic manner with intercyclic structural deformations. The basic ideas of the model is shown to be in agreement with acceptable ideas on riftogenesis.

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Zusammenfassung Es wird eine mögliche Modellvorstellung für die tektonische Entwicklung des Benue-Grabens aufgezeigt, die auf Ergebnissen einer detaillierten magnetischen Untersuchung des Autors im unteren und mittleren Benue-Graben fußt, sowie auf Ergebnissen aus vorangegangenen geophysikalischen Untersuchungen im Benue-Graben und in anderen, vergleichbaren Strukturen. Die tektonische Entwicklung des Benue-Grabens in Nigeria wird mit dem Aufstieg eines mantle plume, einer Mantelaufwölbung oder mit der Platznahme von intrusivem Magma in der Kruste, Krusten-Dehnung und -Ausdünnen in Verbindung gebracht. Es wird angenommen, daß diese Folge von Ereignissen sich zyklisch wiederholte mit strukturellen Deformationen zwischen den Zyklen. Die Grundgedanken dieses Modells stimmen mit den gängigen Vorstellungen über die Entstehung von Riffsystemen überein.

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Résumé L-auteur présente un modèle possible de l'évolution tectonique du graben de Benue (Nigéria). Cette étude se base: d'une part sur une analyse magnétique détaillée, effectuée par l'auteur, des régions inférieure et centrale de la dépression, d'autre part sur des données géophysiques antérieures recueillies dans la même région, ainsi que dans d'autres structures analogues, ailleurs dans le monde. Le modèle propose pour l'évolution tectonique du graben, l'intervention de la montée d'une «mantle plume» ou d'un bombement du manteau, la mise en place de magmas intrusifs dans la croûte, l'étirement et l'amincissement de la croûte, et la fracturation en graben. Cette suite d'événements a pu se répéter de manière cyclique, avec déformations structurales entre les cycles. Les conceptions de base de ce modèle sont en accord avec les idées admises au sujet de la genèse des rifts.

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拉萨南冈底斯花岗岩地球化学特征及构造背景

正拉萨地体记录了多期次碰撞造山和特提斯洋演化的关键信息(许志琴等,2006)。拉萨地体南缘发育大量晚白垩系—古近系岩浆活动且规模较大,冈底斯南部酸性火山岩为典型代表,为印度大陆—亚洲大陆碰撞演化及新特提斯洋向北俯冲消减提供证据。早期学者认为古新世花岗岩类由于新特提斯洋板块俯冲所造成。之后部分研究者通过系统研究林子宗火山岩认为印度—亚洲板块碰撞促     

Petrography and geochemistry of the Middle Miocene Gebel El Rusas sandstones,Eastern Desert,Egypt: Implications for provenance and tectonic setting

Petrography and bulk rock geochemistry of the Middle Miocene sandstones of the lower and upper members of Gebel El Rusas Formation along the Egyptian Red Sea Coastal plain, have been investigated to determine the provenance, tectonic setting, and weathering condition of this formation. The Lower Member is formed mainly of sandstones and conglomerates with clay interbeds. The Upper Member is more calcareous and formed mainly of sandstones and limestones with marls and clays intercalations. Petrographically, the Lower Member sandstones are mostly immature and classified as arkoses with an average framework composition of \(\hbox {Q}_{66}\hbox {F}_{29}\hbox {R}_{5}\), and the Upper Member sandstones are partly submature (more quartzose, less feldspathic) and classified as subarkoses with an average framework composition of \(\hbox {Q}_{80}\hbox {F}_{17}\hbox {R}_{3}\). The Gebel El Rusas sandstones are enriched in Sr, Ba, Zr and Rb and depleted in Co and U, as compared to UCC. The chemical index of alteration (CIA) values suggest moderate weathering conditions. The geochemistry results revealed that the Gebel El Rusas sandstones were derived from felsic-granitic source rocks and deposited in a passive margin of a synrift basin. The inferred tectonic setting for Middle Miocene Gebel El Rusas sandstones in the study area is consistent with the regional geology of the Eastern Desert of Egypt during Middle Miocene.