The Temtebasement in North Cameroon is crosscut by dyke swarms with N 20°–40°trending,including dykes 15–30 m wide,up to 3 km-long.Representative rocks exhibit intersertal to sub-ophitic textures.Electron ...The Temtebasement in North Cameroon is crosscut by dyke swarms with N 20°–40°trending,including dykes 15–30 m wide,up to 3 km-long.Representative rocks exhibit intersertal to sub-ophitic textures.Electron microprobe analyses identified diopside,augite,pargasite,biotite,Ti-magnetite,plagioclase,and sanidine.Whole-rock ICP-MS and ICP-AES chemical analyses showed compositions of basaltic andesite,basaltic trachyandesite and trachyandesite in composition.Igneous differentiation was likely governed by fractional crystallization associated with limited fluid metasomatism.Some lavas could have been moderately contaminated by crustal materials during feeding of local cracks through turbulent magma flows.Discrimination geochemical diagrams and immobile trace and REE element ratios show that the mantle source of Temtedolerites was a deep phlogopitebearing EMII mantle component and has undergone moderate to high partial melting rate.Correlations of fieldwork and analytical data with previous results evidence the Temtedolerite dyke swarms as fingerprints of crustal extension accompanying regional uplift in an active continental margin when early rifting led to the formation of the Poli marginal basin.展开更多
<sup>40</sup>Ar/<sup>39</sup>Ar dating of plagioclases,pyroxene or/and amphibole of separated grains from Avanavero Dolerite located in the Guiana Shield,in Roraima State,Brazil yield plateau a...<sup>40</sup>Ar/<sup>39</sup>Ar dating of plagioclases,pyroxene or/and amphibole of separated grains from Avanavero Dolerite located in the Guiana Shield,in Roraima State,Brazil yield plateau ages ranging from 1840±50 Ma to 1179±10 Ma.The highest ages are interpreted to represent the emplacement of dolerites within the sandstones ofRoraima Supergroup,while the younger results reveal partial argon loss during the cooling history of the samples related to a large展开更多
This article examined the use of the <sup>40</sup>Ar/<sup>39</sup>Ar incremental heating and K/Ar analyses to date dolerites located in the extreme north region in Brazil. The material used for...This article examined the use of the <sup>40</sup>Ar/<sup>39</sup>Ar incremental heating and K/Ar analyses to date dolerites located in the extreme north region in Brazil. The material used for dating was whole rock and separated minerals（plagioclases,pyroxene or/and amphibole）in 6 samples of dykes and sills from Roraima.The dolerites yield plateau ages ranging from 1955±17 Ma to 1179±10 Ma.The reason展开更多
High-resolution U–Pb(ID-TIMS,baddeleyite)ages are presented for mafic dykes from selected swarms in two important Amazonian regions:the Carajás Province in the east,and the Rio Apa block in the
Dolerite is one of the most widely used road pavement construction materials in South Africa and is classified as a basic crystalline rock. The variable performance of road pavements constructed with such rocks in Sou...Dolerite is one of the most widely used road pavement construction materials in South Africa and is classified as a basic crystalline rock. The variable performance of road pavements constructed with such rocks in South Africa has frequently been linked to the durability of the freshly crushed dolerite which can undergo rapid deterioration in service. This poor durability affects the life cycle of the road as they can fail prematurely. The prevailing conditions under which rapid deterioration of dolerite occurred on the road are somewhat contradictory. The objective of this study was to provide insight into what influences rapid deterioration in dolerite when used in the base course of road pavements. The evaluation was completed by conducting surveys, field investigations and laboratory experiments comprising mineralogical analyses and engineering tests. Surveys were done to identify the investigation sites. Field investigations comprise visual condition assessment of road pavement surface and pavement structure using test pits. Field investigations were followed by a sampling of material from the identified investigation sites for laboratory material testing. Laboratory testing included standard engineering soil tests and specialised techniques for mineralogical analysis. Two approaches for durability investigations were followed. For each approach, two samples were used (control sample and investigated sample), and the results of the sample investigated were compared with the control sample results. Findings from both approaches were compared. An interesting finding was that, contrary to conventional wisdom, not all of the dolerites investigated contained discernible smectite contents.展开更多
Two petrochemical types of doleritic dykes have been distinguished in Bafoussam area: 1) the olivine-bearing dolerites which are the most representative are dark green, showing characteristics of alkaline basalts and,...Two petrochemical types of doleritic dykes have been distinguished in Bafoussam area: 1) the olivine-bearing dolerites which are the most representative are dark green, showing characteristics of alkaline basalts and, 2) the calcite-bearing dolerites are grayish green, showing characteristics of tholeiitic basalts. The studied dolerites are less differentiated (1 FeOt/MgO 2);they present the evolution of a subalkaline magma following a calc-alkaline series (alkaline dolerites) and the tholeiitic series (tholeitiic dolerite). The chondrite normalized multielement spider diagrams of all the dolerites show a pronounced depletion in Ba, Th, and enrichment in Rb. Their geotectonic context is inferred to be intraplate and the contrast between the two types is probably due to a diverse source: crustal source for calcite-bearing dolerites and mantle source for olivine-bearing dolerites.展开更多
This work presents the petrographic and geochemical data of the dolerite <span>dykes crosscutting the Pan-African basement of Figuil (North-Cameroon) and </span>Léré (South-West Chad) in order to...This work presents the petrographic and geochemical data of the dolerite <span>dykes crosscutting the Pan-African basement of Figuil (North-Cameroon) and </span>Léré (South-West Chad) in order to approach their petrogenesis and their emplacement context. Two groups of dolerites have been highlighted by petrograph<span>ic and geochemical studies. These groups were discrimin</span>ated by their TiO<sub>2</sub>, Fe<sub>2</sub>O<sub>3</sub>, REE, Ba, Nb, Zr, La and Hf contents which are relatively higher in group I;group II, on the other hand, has higher MgO, Mg#, Sc, Ni and Cr contents. The mineralogical assemblage of these dolerites is made up by plagioclases, pyroxenes, olivine, oxides, amphibole, biotite and sometimes pyrite, calcite, apatite, epidote and chlorite. The behaviour of the major and trace elements suggest that studied dolerites have an evolution dominated by fractional crystallization. Most dolerite samples show higher REE concentrations and (La/Yb)<sub>N</sub> > 8.7, (Tb/Yb)<sub>N</sub> > 1.9 and Dy/Yb > 2 ratios characterizing a garnet-bearing mantle. The difference in incompatible elements between the two groups is explained by the degree of partial melting of the same source which becomes more important over time. Low (Ce/Yb)<sub>N</sub> values (3.3 - 11.58) <span>also suggest relatively low partial melting degree of the source. Fractional crystalli</span>zation process was possibly combined with minor crustal contamination as shown by enrichment of Th/Yb from group II to Group I that might be due to turbulent magma emplacement. The chemical compositions of these dolerites are similar to that of continental tholeiites with slightly moderate nega<span>tive Nb-Ta anomalies which are attributed to crustal contamination of magma</span>s. As other dolerites of Cameroon, continental tholeiitic signature of the studied dolerites is evidenced in geotectonic discrimination diagrams with Group II dolerite compositions falling within the field of tholeiitic basalts and group I within the field of alkali basalts.展开更多
The increasing demand for Aluminium pushes the miners to multiply the research in order to answer this demand. The objective of this work is to characterize the bauxitic deposit of Limbiko. The methodology consists in...The increasing demand for Aluminium pushes the miners to multiply the research in order to answer this demand. The objective of this work is to characterize the bauxitic deposit of Limbiko. The methodology consists in collecting samples, and establishing stratigraphic logs, sections and geological maps. The analyses were carried out in the laboratory of the Company des Bauxites de Guinea. Statistical methods were used to process the geochemical data. The study area is characterized by sedimentary formations of the Devonian, in which the dolerites of the Mesozoic were injected. It is on these Devonian and Mesozoic formations that the Limbiko bauxite deposit was developed. The stratigraphic logs show the succession of formations. The parent rock is surmounted by an alteration crust. The petrographic study is based on the bauxitic and transitional zones characterized by ferruginous laterites. Both zones contain some ferriplantite. Mineralogical analysis shows the presence of gibbsite, goethite, alumogoethite, rutile and clay minerals. The geochemical study of the major elements shows that the more the contents of SiO<sub>2</sub>, Fe<sub>2</sub>O<sub>3</sub> decrease, the more the content of Al<sub>2</sub>O<sub>3</sub> increases and those of TiO<sub>2</sub>, Al<sub>2</sub>O<sub>3</sub> increase in the same direction. The PCA confirms the mineralogical results by classifying the samples into clay, bauxite and ferrite.展开更多
A kimberlite field, represented by fertile and sterile kimberlite pipes (chimneys) is located in the region of Kenieba (West Mali, Kédougou-Kenieba inlier, West African Craton). Thirty pipes and kimberlite dykes ...A kimberlite field, represented by fertile and sterile kimberlite pipes (chimneys) is located in the region of Kenieba (West Mali, Kédougou-Kenieba inlier, West African Craton). Thirty pipes and kimberlite dykes have been identified in the birimian formations, composed mainly of metasediments and granitoids, covered by sedimentary formations (sandstones and conglomerates) of Neoproterozoic age. All these formations are injected with dykes and doleritic sills of Jurassic age. The study of kimberlite pipes is still stammering in Mali, and thus no previous study has allowed to characterize the structures controlling their implementation. The reinterpretation of aeromagnetic data validated by field work indicates that the major structures of the Kenieba region are oriented NNE-SSW, NE-SW, E-W and NW-SE. These structures (faults and kimberlite pipes) are often associated with dolerite dykes, which would imply an injection of dolerite magma into the other formations. The location of the known kimberlite pipes makes it possible to say that the direction NW-SE is the most favorable for the exploration of kimberlites in the region of Kenieba.展开更多
This work reports an important episode of extensional,mafic magmatism that impacted the North China Craton(NCC)during the Permo-Triassic and influenced the evolution of this Craton.We sampled 30 representative mafic d...This work reports an important episode of extensional,mafic magmatism that impacted the North China Craton(NCC)during the Permo-Triassic and influenced the evolution of this Craton.We sampled 30 representative mafic dykes cropping out in eastern NCC,within the Hebei Province,China.New zircon LA-ICP-MS U-Pb ages,geochemical,and Sr-Nd-Pb isotopic data for these mafic dykes identified them as typical dolerites formed between 211.9±1.3 and 263.9±2.6 Ma.The mafic rocks fall into the alkaline and calc-alkaline series.They are enriched in light rare earth elements,some large ion lithophile elements(e.g.,Rb.Ba,and Sr),K,and Pb,and depleted in Nb,Ta,and Ti.The dykes have high initial 87Sr/86Sr ratios(0.7034-0.7178),negative eNd(t)values(—2.2 to—5.9),and relatively constant initial Pb isotopic ratios,that are EMI-like:(206Pb/204Pb)i=16.39-16.74,(207pb/204pb)j=15.22-15.24,and(208Pb/204Pb)i_=36.66-36.86,respectively.Our results indicate that the Hebei Province dolerites were likely derived from magma gen erated through low-medium degree partial melting(3.0-20%)of an EMI-like garnet-lherzolite mantle source.The parental magmas fractionated olivine,clinopyroxene,and Ti-bearing phases with obvious crustal contamination during rapid ascent and dyke emplacement.Since the possible influence of subduction of the Yangtze Plate has been excluded,we propose a model for the origin of the investigated mafic rocks,involving the Pennian collision between the Siberian Block and the NCC.The mafic dykes of eastern NCC within Hebei Province formed during a period of crustal thinning in response to extension after the early Permian collision.展开更多
The mafic dykes(dolerites)during the Early Paleozoic are widely spread in Langao-Ziyang,southern Qiling Block,and the investigation on these dykes are very important.Previous studies have mainly focused on the Siluria...The mafic dykes(dolerites)during the Early Paleozoic are widely spread in Langao-Ziyang,southern Qiling Block,and the investigation on these dykes are very important.Previous studies have mainly focused on the Silurian mafic dykes;however,research on the Earlier Paleozoic mafic dykes is relatively weak at present.Therefore,the overall understanding of the mantle source and genetic dynamic setting during the Early Paleozoic in this area is lacking.To study the accurate age and origin of the Early Paleozoic mafic dykes in Ziyang,southern Shaanxi Province,the mafic dykes from dabacunand Qinmingzhai were selected and the petrology,zircon U-Pb chronology,geochemistry,and Sr-Nd-Hf isotopes were studied.Analysis indicates that the mafic dykes studied are mainly composed of dolerite,and they are the products of the Early Ordovician(475.8-480.7 Ma).Furthermore,the dolerites belong to alkaline rock series,and they are characterized by enrichment in LREE,Rb,Ba,Sr,Nb,(87Sr/86Sr)i=0.7020-0.7050,εNd(t)=3.0-4.0),εHf(t)=4.5-12.1,176Hf/177Hf=0.282681-0.282844.This suggests that the mafic dyke were derived from the partial melting of a depleted lithospheric mantle,and the genetic process is mainly controlled by the mantle plume based on the discussion of the genetic model.Furthermore,the genetic process experienced the separation and crystallization of olivine and clinopyroxene at the same time,with little crustal contamination.展开更多
The present work is an attempt to assess the effect of crustal contamination through the Granitoids host rock, within the southern Bastar mafic dykes of Chhattisgarh, India, in the light of geochemical characteristics...The present work is an attempt to assess the effect of crustal contamination through the Granitoids host rock, within the southern Bastar mafic dykes of Chhattisgarh, India, in the light of geochemical characteristics. Petrographically, these dykes are classified as Amphibolite, Dolerite/Meta-Dolerite and Diorite. Geochemically, all dyke samples have been classified as “high iron sub-alkaline Tholeiites”. On account of overlapping magnesium and iron concentration in Amphibolite and Dolerite dykes, distinctly higher High-Field Strength Element (HFSE), higher Rare-Earth Element (REE) concentrations in the Dolerite dykes than in the Amphibolite dykes, it is inferred that both dyke swarms are fed from two different Tholeiitic magmas. Conclusively, these dykes are recognized as belonging to two different swarms, BD1 and BD2 respectively. This is also corroborated by differences in the LREE patterns i.e. BD2 dykes have relatively enriched LREE pattern than that of BD1 dykes. It is evident from higher LaN/LuN ratio in the BD2 swarm, than in BD1 dyke swarm. These dykes intrude in the coarse-grained leucocratic Bastar Granitoids/Granite Gneisses, which are the host for these dykes. The comparative study of the Primordial mantle-normalized multi-element spider grams, and Chondrite-normalized rare-earth element patterns for the average of BD1 and BD2 dykes and the average of Bastar Granitoids, clearly reflect that the great degree of variation in LIL elements, observed in the Bastar dykes, is either due to metamorphism or due to secondary alteration, and not due to crustal contamination. This is further supported by much higher average “Nb”/“La” ratios in Bastar Mafic Dykes, than in the Bastar Granitoids. Crustal assimilation plays almost no role in the petrogenesis of the Bastar mafic dykes. The Bastar mafic dykes owe their incompatible element characteristics, certainly mantle derived.展开更多
Magmatism in the Cretaceous sedimentary basins of the Figuil and Léré regions constitutes one of the fundamental parameters in the reconstruction of the history of the Cretaceous sedimentary basins. The main...Magmatism in the Cretaceous sedimentary basins of the Figuil and Léré regions constitutes one of the fundamental parameters in the reconstruction of the history of the Cretaceous sedimentary basins. The main objective of this paper is to constrain the petrogenetic processes of hypovolcanic rocks and to determine their geodynamic context of emplacement. The petrographic study of mafic hypovolcanic and trachytic rocks was carried out under a polarizing microscope on thin sections. For the geochemical study, the major oxides and some trace elements were analyzed by ICP-AES. Trace and rare earth elements were analyzed by ICP-MS. The dolerites of the Cretaceous sedimentary basins are composed of dykes of amphibole bearing dolerites, biotite and pyroxene bearing dolerite, pyroxene bearing dolerites and trachytes. The dykes are in the order of 20 to 100 m wide by several kilometers long and oriented from N23<span style="white-space:normal;"><span style="white-space:normal;"><span style="color:#4F4F4F;font-family:-apple-system, "font-size:16px;white-space:normal;background-color:#FFFFFF;">°</span></span></span>E to N90<span style="color:#4F4F4F;font-family:-apple-system, "font-size:16px;white-space:normal;background-color:#FFFFFF;">°</span>E. The textures of these rocks are sub-ophitic to intergranular for dolerites and trachytic for evolved rocks (trachytes). The geochemical study shows that the dolerites are basaltic in composition with alkaline to subalkaline character. The sampled dykes have an evolution dominated by fractional with the minor impact of the crustal assimilation characterized by low Rb/Y ratios for dolerites (0.36 - 0.97) and high values of Rb/Y for the Pan-African granitoid<span style="font-size:10.0pt;font-family:;" "="">s’</span><span style="font-size:10.0pt;font-family:;" "=""> samples (1.95 - 4.01).</span><span style="font-size:10.0pt;font-family:;" "=""> </span><span style="font-size:10.0pt;font-family:;" "="">The nature of doleritic and trachytic magma sources is supported by their (Tb/Yb)N > 1.9 (1.91 - 3.79) and Dy/Yb > 2 (2.32 - 3.50) ratios of most samples, which suggests melting in a garnet-bearing mantle. Concerning the geodynamic context of the studied rocks, doleritic samples are classified as within-plate tholeiite and volcanic arc basalt, and within-plate alkali basalts.</span>展开更多
基金"Agence Universitaire de la Francophonie (AUF)" through the BAGL (Bureau Afrique Centrale et des Grands Lacs) for financial support of Le Projet de soutien aux équipes de recherche 2012/2013 (No 51110SU201) for supplement analyses"Laboratoire GEOPS" of the University Paris-Saclay is thanked for financial support of thin sections and electron microprobe analyses。
文摘The Temtebasement in North Cameroon is crosscut by dyke swarms with N 20°–40°trending,including dykes 15–30 m wide,up to 3 km-long.Representative rocks exhibit intersertal to sub-ophitic textures.Electron microprobe analyses identified diopside,augite,pargasite,biotite,Ti-magnetite,plagioclase,and sanidine.Whole-rock ICP-MS and ICP-AES chemical analyses showed compositions of basaltic andesite,basaltic trachyandesite and trachyandesite in composition.Igneous differentiation was likely governed by fractional crystallization associated with limited fluid metasomatism.Some lavas could have been moderately contaminated by crustal materials during feeding of local cracks through turbulent magma flows.Discrimination geochemical diagrams and immobile trace and REE element ratios show that the mantle source of Temtedolerites was a deep phlogopitebearing EMII mantle component and has undergone moderate to high partial melting rate.Correlations of fieldwork and analytical data with previous results evidence the Temtedolerite dyke swarms as fingerprints of crustal extension accompanying regional uplift in an active continental margin when early rifting led to the formation of the Poli marginal basin.
文摘<sup>40</sup>Ar/<sup>39</sup>Ar dating of plagioclases,pyroxene or/and amphibole of separated grains from Avanavero Dolerite located in the Guiana Shield,in Roraima State,Brazil yield plateau ages ranging from 1840±50 Ma to 1179±10 Ma.The highest ages are interpreted to represent the emplacement of dolerites within the sandstones ofRoraima Supergroup,while the younger results reveal partial argon loss during the cooling history of the samples related to a large
文摘This article examined the use of the <sup>40</sup>Ar/<sup>39</sup>Ar incremental heating and K/Ar analyses to date dolerites located in the extreme north region in Brazil. The material used for dating was whole rock and separated minerals（plagioclases,pyroxene or/and amphibole）in 6 samples of dykes and sills from Roraima.The dolerites yield plateau ages ranging from 1955±17 Ma to 1179±10 Ma.The reason
文摘High-resolution U–Pb(ID-TIMS,baddeleyite)ages are presented for mafic dykes from selected swarms in two important Amazonian regions:the Carajás Province in the east,and the Rio Apa block in the
文摘Dolerite is one of the most widely used road pavement construction materials in South Africa and is classified as a basic crystalline rock. The variable performance of road pavements constructed with such rocks in South Africa has frequently been linked to the durability of the freshly crushed dolerite which can undergo rapid deterioration in service. This poor durability affects the life cycle of the road as they can fail prematurely. The prevailing conditions under which rapid deterioration of dolerite occurred on the road are somewhat contradictory. The objective of this study was to provide insight into what influences rapid deterioration in dolerite when used in the base course of road pavements. The evaluation was completed by conducting surveys, field investigations and laboratory experiments comprising mineralogical analyses and engineering tests. Surveys were done to identify the investigation sites. Field investigations comprise visual condition assessment of road pavement surface and pavement structure using test pits. Field investigations were followed by a sampling of material from the identified investigation sites for laboratory material testing. Laboratory testing included standard engineering soil tests and specialised techniques for mineralogical analysis. Two approaches for durability investigations were followed. For each approach, two samples were used (control sample and investigated sample), and the results of the sample investigated were compared with the control sample results. Findings from both approaches were compared. An interesting finding was that, contrary to conventional wisdom, not all of the dolerites investigated contained discernible smectite contents.
文摘Two petrochemical types of doleritic dykes have been distinguished in Bafoussam area: 1) the olivine-bearing dolerites which are the most representative are dark green, showing characteristics of alkaline basalts and, 2) the calcite-bearing dolerites are grayish green, showing characteristics of tholeiitic basalts. The studied dolerites are less differentiated (1 FeOt/MgO 2);they present the evolution of a subalkaline magma following a calc-alkaline series (alkaline dolerites) and the tholeiitic series (tholeitiic dolerite). The chondrite normalized multielement spider diagrams of all the dolerites show a pronounced depletion in Ba, Th, and enrichment in Rb. Their geotectonic context is inferred to be intraplate and the contrast between the two types is probably due to a diverse source: crustal source for calcite-bearing dolerites and mantle source for olivine-bearing dolerites.
文摘This work presents the petrographic and geochemical data of the dolerite <span>dykes crosscutting the Pan-African basement of Figuil (North-Cameroon) and </span>Léré (South-West Chad) in order to approach their petrogenesis and their emplacement context. Two groups of dolerites have been highlighted by petrograph<span>ic and geochemical studies. These groups were discrimin</span>ated by their TiO<sub>2</sub>, Fe<sub>2</sub>O<sub>3</sub>, REE, Ba, Nb, Zr, La and Hf contents which are relatively higher in group I;group II, on the other hand, has higher MgO, Mg#, Sc, Ni and Cr contents. The mineralogical assemblage of these dolerites is made up by plagioclases, pyroxenes, olivine, oxides, amphibole, biotite and sometimes pyrite, calcite, apatite, epidote and chlorite. The behaviour of the major and trace elements suggest that studied dolerites have an evolution dominated by fractional crystallization. Most dolerite samples show higher REE concentrations and (La/Yb)<sub>N</sub> > 8.7, (Tb/Yb)<sub>N</sub> > 1.9 and Dy/Yb > 2 ratios characterizing a garnet-bearing mantle. The difference in incompatible elements between the two groups is explained by the degree of partial melting of the same source which becomes more important over time. Low (Ce/Yb)<sub>N</sub> values (3.3 - 11.58) <span>also suggest relatively low partial melting degree of the source. Fractional crystalli</span>zation process was possibly combined with minor crustal contamination as shown by enrichment of Th/Yb from group II to Group I that might be due to turbulent magma emplacement. The chemical compositions of these dolerites are similar to that of continental tholeiites with slightly moderate nega<span>tive Nb-Ta anomalies which are attributed to crustal contamination of magma</span>s. As other dolerites of Cameroon, continental tholeiitic signature of the studied dolerites is evidenced in geotectonic discrimination diagrams with Group II dolerite compositions falling within the field of tholeiitic basalts and group I within the field of alkali basalts.
文摘The increasing demand for Aluminium pushes the miners to multiply the research in order to answer this demand. The objective of this work is to characterize the bauxitic deposit of Limbiko. The methodology consists in collecting samples, and establishing stratigraphic logs, sections and geological maps. The analyses were carried out in the laboratory of the Company des Bauxites de Guinea. Statistical methods were used to process the geochemical data. The study area is characterized by sedimentary formations of the Devonian, in which the dolerites of the Mesozoic were injected. It is on these Devonian and Mesozoic formations that the Limbiko bauxite deposit was developed. The stratigraphic logs show the succession of formations. The parent rock is surmounted by an alteration crust. The petrographic study is based on the bauxitic and transitional zones characterized by ferruginous laterites. Both zones contain some ferriplantite. Mineralogical analysis shows the presence of gibbsite, goethite, alumogoethite, rutile and clay minerals. The geochemical study of the major elements shows that the more the contents of SiO<sub>2</sub>, Fe<sub>2</sub>O<sub>3</sub> decrease, the more the content of Al<sub>2</sub>O<sub>3</sub> increases and those of TiO<sub>2</sub>, Al<sub>2</sub>O<sub>3</sub> increase in the same direction. The PCA confirms the mineralogical results by classifying the samples into clay, bauxite and ferrite.
文摘A kimberlite field, represented by fertile and sterile kimberlite pipes (chimneys) is located in the region of Kenieba (West Mali, Kédougou-Kenieba inlier, West African Craton). Thirty pipes and kimberlite dykes have been identified in the birimian formations, composed mainly of metasediments and granitoids, covered by sedimentary formations (sandstones and conglomerates) of Neoproterozoic age. All these formations are injected with dykes and doleritic sills of Jurassic age. The study of kimberlite pipes is still stammering in Mali, and thus no previous study has allowed to characterize the structures controlling their implementation. The reinterpretation of aeromagnetic data validated by field work indicates that the major structures of the Kenieba region are oriented NNE-SSW, NE-SW, E-W and NW-SE. These structures (faults and kimberlite pipes) are often associated with dolerite dykes, which would imply an injection of dolerite magma into the other formations. The location of the known kimberlite pipes makes it possible to say that the direction NW-SE is the most favorable for the exploration of kimberlites in the region of Kenieba.
文摘This work reports an important episode of extensional,mafic magmatism that impacted the North China Craton(NCC)during the Permo-Triassic and influenced the evolution of this Craton.We sampled 30 representative mafic dykes cropping out in eastern NCC,within the Hebei Province,China.New zircon LA-ICP-MS U-Pb ages,geochemical,and Sr-Nd-Pb isotopic data for these mafic dykes identified them as typical dolerites formed between 211.9±1.3 and 263.9±2.6 Ma.The mafic rocks fall into the alkaline and calc-alkaline series.They are enriched in light rare earth elements,some large ion lithophile elements(e.g.,Rb.Ba,and Sr),K,and Pb,and depleted in Nb,Ta,and Ti.The dykes have high initial 87Sr/86Sr ratios(0.7034-0.7178),negative eNd(t)values(—2.2 to—5.9),and relatively constant initial Pb isotopic ratios,that are EMI-like:(206Pb/204Pb)i=16.39-16.74,(207pb/204pb)j=15.22-15.24,and(208Pb/204Pb)i_=36.66-36.86,respectively.Our results indicate that the Hebei Province dolerites were likely derived from magma gen erated through low-medium degree partial melting(3.0-20%)of an EMI-like garnet-lherzolite mantle source.The parental magmas fractionated olivine,clinopyroxene,and Ti-bearing phases with obvious crustal contamination during rapid ascent and dyke emplacement.Since the possible influence of subduction of the Yangtze Plate has been excluded,we propose a model for the origin of the investigated mafic rocks,involving the Pennian collision between the Siberian Block and the NCC.The mafic dykes of eastern NCC within Hebei Province formed during a period of crustal thinning in response to extension after the early Permian collision.
基金This study was supported by the National Natural Science Foundation of China(Grant:41573022).
文摘The mafic dykes(dolerites)during the Early Paleozoic are widely spread in Langao-Ziyang,southern Qiling Block,and the investigation on these dykes are very important.Previous studies have mainly focused on the Silurian mafic dykes;however,research on the Earlier Paleozoic mafic dykes is relatively weak at present.Therefore,the overall understanding of the mantle source and genetic dynamic setting during the Early Paleozoic in this area is lacking.To study the accurate age and origin of the Early Paleozoic mafic dykes in Ziyang,southern Shaanxi Province,the mafic dykes from dabacunand Qinmingzhai were selected and the petrology,zircon U-Pb chronology,geochemistry,and Sr-Nd-Hf isotopes were studied.Analysis indicates that the mafic dykes studied are mainly composed of dolerite,and they are the products of the Early Ordovician(475.8-480.7 Ma).Furthermore,the dolerites belong to alkaline rock series,and they are characterized by enrichment in LREE,Rb,Ba,Sr,Nb,(87Sr/86Sr)i=0.7020-0.7050,εNd(t)=3.0-4.0),εHf(t)=4.5-12.1,176Hf/177Hf=0.282681-0.282844.This suggests that the mafic dyke were derived from the partial melting of a depleted lithospheric mantle,and the genetic process is mainly controlled by the mantle plume based on the discussion of the genetic model.Furthermore,the genetic process experienced the separation and crystallization of olivine and clinopyroxene at the same time,with little crustal contamination.
文摘The present work is an attempt to assess the effect of crustal contamination through the Granitoids host rock, within the southern Bastar mafic dykes of Chhattisgarh, India, in the light of geochemical characteristics. Petrographically, these dykes are classified as Amphibolite, Dolerite/Meta-Dolerite and Diorite. Geochemically, all dyke samples have been classified as “high iron sub-alkaline Tholeiites”. On account of overlapping magnesium and iron concentration in Amphibolite and Dolerite dykes, distinctly higher High-Field Strength Element (HFSE), higher Rare-Earth Element (REE) concentrations in the Dolerite dykes than in the Amphibolite dykes, it is inferred that both dyke swarms are fed from two different Tholeiitic magmas. Conclusively, these dykes are recognized as belonging to two different swarms, BD1 and BD2 respectively. This is also corroborated by differences in the LREE patterns i.e. BD2 dykes have relatively enriched LREE pattern than that of BD1 dykes. It is evident from higher LaN/LuN ratio in the BD2 swarm, than in BD1 dyke swarm. These dykes intrude in the coarse-grained leucocratic Bastar Granitoids/Granite Gneisses, which are the host for these dykes. The comparative study of the Primordial mantle-normalized multi-element spider grams, and Chondrite-normalized rare-earth element patterns for the average of BD1 and BD2 dykes and the average of Bastar Granitoids, clearly reflect that the great degree of variation in LIL elements, observed in the Bastar dykes, is either due to metamorphism or due to secondary alteration, and not due to crustal contamination. This is further supported by much higher average “Nb”/“La” ratios in Bastar Mafic Dykes, than in the Bastar Granitoids. Crustal assimilation plays almost no role in the petrogenesis of the Bastar mafic dykes. The Bastar mafic dykes owe their incompatible element characteristics, certainly mantle derived.
文摘Magmatism in the Cretaceous sedimentary basins of the Figuil and Léré regions constitutes one of the fundamental parameters in the reconstruction of the history of the Cretaceous sedimentary basins. The main objective of this paper is to constrain the petrogenetic processes of hypovolcanic rocks and to determine their geodynamic context of emplacement. The petrographic study of mafic hypovolcanic and trachytic rocks was carried out under a polarizing microscope on thin sections. For the geochemical study, the major oxides and some trace elements were analyzed by ICP-AES. Trace and rare earth elements were analyzed by ICP-MS. The dolerites of the Cretaceous sedimentary basins are composed of dykes of amphibole bearing dolerites, biotite and pyroxene bearing dolerite, pyroxene bearing dolerites and trachytes. The dykes are in the order of 20 to 100 m wide by several kilometers long and oriented from N23<span style="white-space:normal;"><span style="white-space:normal;"><span style="color:#4F4F4F;font-family:-apple-system, "font-size:16px;white-space:normal;background-color:#FFFFFF;">°</span></span></span>E to N90<span style="color:#4F4F4F;font-family:-apple-system, "font-size:16px;white-space:normal;background-color:#FFFFFF;">°</span>E. The textures of these rocks are sub-ophitic to intergranular for dolerites and trachytic for evolved rocks (trachytes). The geochemical study shows that the dolerites are basaltic in composition with alkaline to subalkaline character. The sampled dykes have an evolution dominated by fractional with the minor impact of the crustal assimilation characterized by low Rb/Y ratios for dolerites (0.36 - 0.97) and high values of Rb/Y for the Pan-African granitoid<span style="font-size:10.0pt;font-family:;" "="">s’</span><span style="font-size:10.0pt;font-family:;" "=""> samples (1.95 - 4.01).</span><span style="font-size:10.0pt;font-family:;" "=""> </span><span style="font-size:10.0pt;font-family:;" "="">The nature of doleritic and trachytic magma sources is supported by their (Tb/Yb)N > 1.9 (1.91 - 3.79) and Dy/Yb > 2 (2.32 - 3.50) ratios of most samples, which suggests melting in a garnet-bearing mantle. Concerning the geodynamic context of the studied rocks, doleritic samples are classified as within-plate tholeiite and volcanic arc basalt, and within-plate alkali basalts.</span>
