Jiama is a giant,high-grade porphyry copper system in the Gangdese metallogenic belt,Tibet.Multistage intermediate-felsic porphyries intruded in this deposit,some of which are strongly associated with copper-polymetal...Jiama is a giant,high-grade porphyry copper system in the Gangdese metallogenic belt,Tibet.Multistage intermediate-felsic porphyries intruded in this deposit,some of which are strongly associated with copper-polymetallic mineralization.These ore-bearing porphyries include monzogranite,granodiorite,and quartz diorite porphyries.A new granite aplite dyke was found in the south of Jiama.Its age,genesis,and relationship with ore-related magmatism are obscure.Here,its emplacement age and petrogenesis were determined using mineralogy,zircon U-Pb dating,geochemistry,and Sr-Nd-Pb isotope studies.The zircon LA-ICP-MS U-Pb age of the aplite dyke is 16.66±0.21 Ma(n=14,MSWD=0.66),earlier than that of the ore-bearing porphyries(~15 Ma)in Jiama.Furthermore,the aplite exhibits high amounts of silicon(SiO_(2)=73.39%-74.74%),potassium(K_(2)O=5.12%-6.61%),aluminum(Al_(2)O_(3)=14.25%-14.69%),and light/heavy rare earth elements(LREE/HREE=12.12-16.19)as well as negative europium(δEu=0.47-0.72)and weak negative cerium anomalies(δCe=0.84-0.93).The aplite dyke is characteristic of metaluminous-peraluminous I-type granite,which is rich in large-ion lithophile elements(Rb,Ba,Th and U)and depleted in high-field-strength elements(Nb,P and Ti).The aplite dyke and ore-bearing porphyries in the Jiama deposit are the results of a partial melting of the juvenile lower crust,according to whole-rock geochemistry and Sr-Nd-Pb isotope data,but the dyke and ore-bearing porphyries were emplaced from the same magma chamber at different times.Thus,the aplite dyke shows the composition of the early evolution stage of shallow magma in the Jiama deposit and is the product of rapid condensation and crystallization.展开更多
This paper report paleomagnetic data from late Cretaceous diorite dykes that sub-vertically intrude granodiorites in the eastern Gangdese belt near the city of Lhasa.Our research goals are to provide further constrain...This paper report paleomagnetic data from late Cretaceous diorite dykes that sub-vertically intrude granodiorites in the eastern Gangdese belt near the city of Lhasa.Our research goals are to provide further constraints on pre-collisional structure of the southern margin of Asia and the onset of the India-Asia collision.Magnetite is identified as the main magnetic carrier in our study.The magnetite shows no evidence of metamorphism or alteration as determined from optical and scanning electron microscope observations.A strong mineral orientation is revealed by anisotropy of magnetic susceptibility analysis both for the intruded dykes and the country rocks.The authors interpret this AMS fabric to have formed during intrusion rather than deformation.Fifteen of 23 sites yield acceptable site mean characteristic remanences with dual polarities.A scatter analysis of the virtual geomagnetic poles suggests that the mean result adequately averaged paleosecular variation.The paleomagnetic pole from the Gangdese dykes yields a paleolatitude of 14.3°N±5.8°N for the southern margin of Asia near Lhasa.The paleolatitude corresponds to an in-between position of the Lhasa terrane during about 130‒60 Ma.Furthermore,the mean declination of the characteristic remanent magnetization reveals a significant counterclockwise rotation of 18°±9°for the sampling location since about 83 Ma.In the light of tectonic setting of the dykes,the strike of the southern margin of Asia near Lhasa is restored to trend approximately about 310°,which is compatible with the hypothesis that the southern margin of Eurasia had a quasi-linear structure prior to its collision with India.展开更多
We present new 40Ar-39 Ar plagioclase crystallization ages from the dykes exposed at the northern slope of the Satpura Mountain range near Betul-Jabalpur-Pachmarhi area,~800 km NE of the Western Ghats escarpment.Among...We present new 40Ar-39 Ar plagioclase crystallization ages from the dykes exposed at the northern slope of the Satpura Mountain range near Betul-Jabalpur-Pachmarhi area,~800 km NE of the Western Ghats escarpment.Among the two plateau ages,the first age of 66.56±0.42 Ma from a dyke near Mohpani village represents its crystallization age which is either slightly older or contemporaneous with the nearby Mandla lava flows(63-65 Ma).We suggest that the Mohpani dyke might be one of the feeders for the surrounding lava flows as these lavas are significantly younger than the majority of the main Deccan lavas of the Western Ghats(66.38-65.54 Ma).The second age of 56.95±1.08 Ma comes from a younger dyke near Olini village which cuts across the lava flows of the area.The age correlates well with the Mandla lavas which are chemically similar to the uppermost Poladpur,Ambenali and Mahabaleshwar Formation lavas of SW Deccan.Our study shows that the dyke activities occurred in two phases,with the second one representing the terminal stage.展开更多
A precise dating of a mafic dyke of a swarm in shield areas has great advantage to identify Large Igneous Provinces(LIPs;short-lived,mantle-generated magmatic event)(Bryan and Ernst,2008;Ernst et al.,2010).
Geologic, petrographic and petrochemical studies of the lateMesozoic K-rich melanocratic dykes, including lamprophyres, andesite porphyrites and dacite-porphyry in the gold field system in the Jiaodong Peninsula, Chin...Geologic, petrographic and petrochemical studies of the lateMesozoic K-rich melanocratic dykes, including lamprophyres, andesite porphyrites and dacite-porphyry in the gold field system in the Jiaodong Peninsula, China, have shown that these dykes are characterized by rich potassium and alkali but poor titanium. They belong to an ultra-high potassic, shoshonitic and high potassic calc-alkaline rock series. The parental magma has relatively high initial strontium ratios ((87Sr/86Sr)i-0.70895-0.71140) and low (143Nd/144Nd)i ratios (varying from 0.51135 to 0.51231); and its δ18OsMow. whole rock values vary from +5.8‰ to +10.6‰ with a mean of +7.1‰. These features suggest that the source region of the magma is an enriched mantle wedge transformed from a continental lithosphere mantle which has experienced me tasomatism by mantle-derived fluids with H2O-dominated fluids that were provided during the underthrusting of an ocean crust. The initial magma was generated by low-degree partial melting of the enriched mantle in its mature stage in the back-arc spreading environment. The evolution of magmas is associated with two trends, i.e., fractional crystallization and mixing with or intensive contamination by palaeo-crust materials or metamorphic rocks. The former process is evident in the gold field system of quartz-vein type, whereas the latter is dominated in the gold field system of the altered-rock type. This conclusion is very important for more detailed study of petrogenesis and mineralization through the crust-mantle interaction (exchange) in the Mesozoic in this region.展开更多
The Helanshan tectonic belt(HTB) is a major tectonic divide between the Alxa and Ordos blocks in the North China Craton. The geochronology and petrogenesis of the mafic dykes in the northern HTB are keys to understand...The Helanshan tectonic belt(HTB) is a major tectonic divide between the Alxa and Ordos blocks in the North China Craton. The geochronology and petrogenesis of the mafic dykes in the northern HTB are keys to understanding the tectonic evolution of this belt. The mafic dykes, intruded into the Neoarchean-Paleoproterozoic metamorphic basement, are mainly composed of diabase with a mineral assemblage of plagioclase(45%-60%), pyroxene(25%-35%), minor quartz and Fe-Ti oxides. The LA-ICPMS U-Pb analysis of zircon grains from representative dykes yield a weighted mean age of 206 ± 1.9 Ma, which represents the crystallization age of the dyke. The diabases show high contents of Fe_2 O_3~T(11.88-17.55 wt.%), low contents of SiO_2(45.65-50.95 wt.%) and MgO(3.31-5.50 wt.%) with low Mg#(=100×MgO/(MgO + FeO) atomic ration) of 33-44. They are characterized by enrichment of light rare earth elements(LREEs) and large ion lithophile elements(LILEs)(e.g., Rb, Ba and Pb), and slight depletion of high field strength elements(HFSEs). These features suggest that the magma has undergone extensive fractionation of olivine and pyroxene but only minor crustal contamination during its evolution. Their high Sm contents and La/Sm ratios, and low Sm/Yb ratios indicate that magma from which the dykes formed was derived from low degree(about 5%) partial melting of an enriched garnet + spinel lherzolite mantle source. Together with regional geology, these geochemical and geochronological data suggest that the mafic dykes in the HTB were formed in an intracontinental extensional setting during the late Triassic.展开更多
The mafic dykes are composed of a series of northwest-trending diabase in Kuokesu of the Kuruktag region, Xinjiang. Four whole-rock samples of the mafic dykes in the region give an 40Ar / 36Ar-40K/ 36Ar isochron age o...The mafic dykes are composed of a series of northwest-trending diabase in Kuokesu of the Kuruktag region, Xinjiang. Four whole-rock samples of the mafic dykes in the region give an 40Ar / 36Ar-40K/ 36Ar isochron age of 282 Ma with the initial 40Ar/ 36Ar = 507. The characteristics of the major, trace and rare earth elements of the dyke rocks indicate that they belong to the calc-alkaline series and were emplaced in extensional structures at an intermediate depth of the crust. This shows that there was a large-scale extension in the Permian Period, which might be associated with the post-collisional extension of the ancient Tianshan Mountains.展开更多
Anisotropy of magnetic susceptibility (AMS) studies were carried out on a precisely dated (2216.0 ± 0.9 Ma),450 km long NeS striking dyke in the Dharwar Craton,to determine the magma flow direction along the dyke...Anisotropy of magnetic susceptibility (AMS) studies were carried out on a precisely dated (2216.0 ± 0.9 Ma),450 km long NeS striking dyke in the Dharwar Craton,to determine the magma flow direction along the dyke length.In order to use the imbrication of the magnetic foliation,forty eight samples were collected from 13 locations along the length of the dyke.Magnetogranulometry studies show that AMS fabric is dominated by medium grained interstitial Ti-poor multidomain magnetite.The corrected anisotropy degree (Pj) of the samples was found to be low to moderate,between 1.007 and 1.072,which indicates primary magnetic fabric.The magnetic ellipsoid is either triaxial,prolate or oblate and clearly defines normal,intermediate and inverse magnetic fabrics related to magma flow during the dyke emplacement.The maximum susceptibility axes (Kmax) of the AMS tensor of the dyke is predominantly inclined at low angles (<30°),with no systematic variation in depth along the NeS profile,indicating sub-horizontal flow even at mid crustal levels which could probably be governed by location of the focal region of the magma source (mantle plume?),flow dynamics together with the compressive stresses exerted by the overlying crust.展开更多
The work is a vivid description of the structural relationship between brittle deformation of the Precambrian basement in the southern continental part of the Cameroon Line and intrusive Paleozoic and Mesozoic basalt ...The work is a vivid description of the structural relationship between brittle deformation of the Precambrian basement in the southern continental part of the Cameroon Line and intrusive Paleozoic and Mesozoic basalt dykes swarms. A multidisciplinary approach that involves a combination of remote sensing techniques and field studies show that the major trend of brittle structures correspond to well-known regional structures: N70°E (Adamawa Shear Zone), N135°E (upper Benue trend) and N30°E (Cameroon Volcanic Line) corresponding to E-W and N-S directions respectively. Basalt dykes are associated to NE-SW, E-W and NW-SE oriented fractures. An integration of the available information on brittle structures and basalt dykes directions suggest an emplacement of the Mesozoic and Paleozoic basalt dykes structurally controlled by Precambrian structures that were originated through Riedel’s fracture kinematic model with dextral strike-slip Adamawa Shear Zone as the main shear zone during late stage of the Pan-African collision. Spatially, the restriction of the basalt dykes to the corridor of the Adamawa Shear Zone indicate that a rejuvenation of Precambrian faults may very well be the origin of the dykes with possibility that they may have been reworked several times during the Phanerozoic eon.展开更多
Main factors, which should be considered in the classification of dyke foundation, are discussed in this paper. Engineering conditions should be taken into account when the levee safety is appraised based on engineeri...Main factors, which should be considered in the classification of dyke foundation, are discussed in this paper. Engineering conditions should be taken into account when the levee safety is appraised based on engineering geologic appraisement and classification. On the basis of safety appraisement, dyke foundation may be classified with regard of suitable reinforcement measures. Examples are presented to illustrate the instructive significance of dyke foundation classification to dyke reinforcement design.展开更多
In this study we present new mineral chemistry,whole-rock geochemical and zircon U-Pb geochronological data for 12 metamafic dykes in the mantle sequence of the Sangsang ophiolite in South Tibet(China).Modal analyses ...In this study we present new mineral chemistry,whole-rock geochemical and zircon U-Pb geochronological data for 12 metamafic dykes in the mantle sequence of the Sangsang ophiolite in South Tibet(China).Modal analyses of these dykes gave averages of^40%-65%plagioclase and^35%-60%amphibole and small amounts of(igneous)clinopyroxene,epidote and opaque minerals.This mineral assemblage resembles that of typical orthoamphibolites.Nevertheless,due to the absence of foliation the investigated rocks are described as metamafic lithologies.These rocks have primitive mantle(PM)-normalized multi-element patterns with negative Nb and Ta anomalies as well as weak,negative Ti anomalies.In addition,they have initial 87Sr/86Sr ratios[(87Sr/86Sr)1]of0.702844-0.703581,initial 143Nd/144Nd ratios[(143Nd/144Nd)i]of 0.512891-0.512959 and high εNd(t)values(+7.9 to+9.3).Uranium-Pb ages of magmatic zircons separated from the investigated metamafic dykes indicate that the parental melts of their protoliths intruded the Sangsang mantle at^119.0-118.5 Ma.The metamorphic mineral assemblages recognized in the investigated dykes are suggestive of a retrograde metamorphic process,from(epidote-)amphibolite facies(~470-610℃,-1.9-4.3 kbar)and to prehnitepumpellyite facies(≤280℃,<3 kbar),active within a rift-produced oceanic lithosphere.Microtextural and geochemical data suggest that the protoliths of the dykes were most likely massive gabbros.Compositional data show that the parental magmas of the gabbroic protoliths were generated by melting of a depleted mantle(DM)source that had been weakly modified by fluids emanating from a subducted oceanic lithospheric slab.The age of the gabbroic protoliths is slightly younger than the existing ages for ophiolites from the central Yarlung-Zangbo Suture Zone(YZSZ)in the literature(~129-123 Ma).We,therefore,suggest that the gabbroic protoliths of the Sangsang metamafic dykes were formed in an incipient forearc setting during Neo-Tethyan subduction reinitiation(Aptian).Our tectonomagmatic model provides insights into the igneous accretion and postsolidification evolution of the oceanic lithosphere in South Tibet.展开更多
It is yet unclear whether large-scale segregation of immiscibile liquids and eruption of high-Si lavas exist in nature(Charlier et al.,2013).We present a possible case of segregation of immscible liquids in the 1780 Ma
About 300 mafic dykes are intruded Eocene volcanic and pyroclastic rocks,north east of Meshkinshahr city in the NW of Iran.Most of dyke swarms display NNE-SSE trend.Their composition varies from basalt to tephrite
Komatiites are presented as direct evidence for higher mantle temperatures during the Archean. In the North China Craton, komatiites with spinifex structure have been identified only at one locality, i.e. the Sujiagou...Komatiites are presented as direct evidence for higher mantle temperatures during the Archean. In the North China Craton, komatiites with spinifex structure have been identified only at one locality, i.e. the Sujiagou area, western Shandong. They were considered as formed during the early Neoarchean mainly based on their association with supracrustal rocks considered to be that age. This study carried out SHRIMP U-Pb zircon dating on metamorphosed trondhjemitic and monzogranitic dykes intruding the Sujiagou komatiites, and they have magmatic zircon ages of 2592 ± 12 Ma and 2586 ± 13 Ma respectively. This provides direct evidence that the komatiites formed during the early Neoarchean.展开更多
Late Silurian-early Devonian mafic--intermediate dykes occur in the Xiemisitai mountain of the northern West Junggar. However, their ages, petrogenesis and geodynamic process remain to be unraveled. We report firstly ...Late Silurian-early Devonian mafic--intermediate dykes occur in the Xiemisitai mountain of the northern West Junggar. However, their ages, petrogenesis and geodynamic process remain to be unraveled. We report firstly in situ zircon U–Pb and hornblende 40Ar-39Ar ages, geochemical and in situ zircon Hf isotopic data for the Xiemisitai dykes. The Xiemisitai dykes give zircon U–Pb ages of 416.7±2.1 Ma and 405.1±2.5 Ma and hornblende 40Ar-39Ar age of 405.9±4.9 Ma, respectively. They are characterized by high εHf(t)values(+11.9 to +15.5), suggesting a depleted mantle features. The Xiemisitai dykes show low Mg#(46–59) and low Cr(11.3–197 ppm) and Ni(19.9–102 ppm) abundances indicating that they have experienced significant fractional crystallization. These dykes contain hornblende and biotite and display negative Nb-Ta-Ti anomalies, enrichment of LREEs, LILEs and depletion of HREEs and HFSEs with high Ba/La ratios, similar with an origin from a depleted lithospheric mantle metasomatized by subducted slabderived fluids. In addition, the Xiemisitai dykes are plotted within melting trends with little to no garnet(Cpx:Grt=6:1) in their source. The La/Yb vs. Tb/Yb plot also indicates the presence of less than 1% residual garnet in the source region for the Xiemisitai dykes. Therefore, it can be inferred that the Xiemisitai dykes were generated at a correspondingly shallow depth, mostly within the spinel stability field. Finally, the Xiemisitai dykes were most probably generated by partial melting of metasomatized lithospheric mantle in relatively shallow level(<80 km). They could be possibly triggered by the asthenospheric upwelling as a result of the rollback of the subducted Irtysh-Zaysan oceanic lithosphere.展开更多
基金supported by the National Key Research and Development Program of China(Grant No.2022YFC2905001)National Natural Science Foundation of China(Grant Nos.42272093 and 42230813)+2 种基金Basic Research Fund of Chinese Academy of Geological Sciences(Grant Nos.JKYZD202316,KJ2102,KK2116,and JKY202208)Geological Survey Project(Grant No.DD20221684)China Scholarship Council(Grant No.CSC202206400059)。
文摘Jiama is a giant,high-grade porphyry copper system in the Gangdese metallogenic belt,Tibet.Multistage intermediate-felsic porphyries intruded in this deposit,some of which are strongly associated with copper-polymetallic mineralization.These ore-bearing porphyries include monzogranite,granodiorite,and quartz diorite porphyries.A new granite aplite dyke was found in the south of Jiama.Its age,genesis,and relationship with ore-related magmatism are obscure.Here,its emplacement age and petrogenesis were determined using mineralogy,zircon U-Pb dating,geochemistry,and Sr-Nd-Pb isotope studies.The zircon LA-ICP-MS U-Pb age of the aplite dyke is 16.66±0.21 Ma(n=14,MSWD=0.66),earlier than that of the ore-bearing porphyries(~15 Ma)in Jiama.Furthermore,the aplite exhibits high amounts of silicon(SiO_(2)=73.39%-74.74%),potassium(K_(2)O=5.12%-6.61%),aluminum(Al_(2)O_(3)=14.25%-14.69%),and light/heavy rare earth elements(LREE/HREE=12.12-16.19)as well as negative europium(δEu=0.47-0.72)and weak negative cerium anomalies(δCe=0.84-0.93).The aplite dyke is characteristic of metaluminous-peraluminous I-type granite,which is rich in large-ion lithophile elements(Rb,Ba,Th and U)and depleted in high-field-strength elements(Nb,P and Ti).The aplite dyke and ore-bearing porphyries in the Jiama deposit are the results of a partial melting of the juvenile lower crust,according to whole-rock geochemistry and Sr-Nd-Pb isotope data,but the dyke and ore-bearing porphyries were emplaced from the same magma chamber at different times.Thus,the aplite dyke shows the composition of the early evolution stage of shallow magma in the Jiama deposit and is the product of rapid condensation and crystallization.
基金financially supported by the National Science Foundation of China(92055205,41672223)the start-up funding from Sun Yat-sen University(74110-18841244).
文摘This paper report paleomagnetic data from late Cretaceous diorite dykes that sub-vertically intrude granodiorites in the eastern Gangdese belt near the city of Lhasa.Our research goals are to provide further constraints on pre-collisional structure of the southern margin of Asia and the onset of the India-Asia collision.Magnetite is identified as the main magnetic carrier in our study.The magnetite shows no evidence of metamorphism or alteration as determined from optical and scanning electron microscope observations.A strong mineral orientation is revealed by anisotropy of magnetic susceptibility analysis both for the intruded dykes and the country rocks.The authors interpret this AMS fabric to have formed during intrusion rather than deformation.Fifteen of 23 sites yield acceptable site mean characteristic remanences with dual polarities.A scatter analysis of the virtual geomagnetic poles suggests that the mean result adequately averaged paleosecular variation.The paleomagnetic pole from the Gangdese dykes yields a paleolatitude of 14.3°N±5.8°N for the southern margin of Asia near Lhasa.The paleolatitude corresponds to an in-between position of the Lhasa terrane during about 130‒60 Ma.Furthermore,the mean declination of the characteristic remanent magnetization reveals a significant counterclockwise rotation of 18°±9°for the sampling location since about 83 Ma.In the light of tectonic setting of the dykes,the strike of the southern margin of Asia near Lhasa is restored to trend approximately about 310°,which is compatible with the hypothesis that the southern margin of Eurasia had a quasi-linear structure prior to its collision with India.
基金Department of Science and Technology,Government of India for financial support to this work(Project Grant No.ESS/16/286/2006)Post-Doctoral,D.S. Kothari Fellowship for financial support
文摘We present new 40Ar-39 Ar plagioclase crystallization ages from the dykes exposed at the northern slope of the Satpura Mountain range near Betul-Jabalpur-Pachmarhi area,~800 km NE of the Western Ghats escarpment.Among the two plateau ages,the first age of 66.56±0.42 Ma from a dyke near Mohpani village represents its crystallization age which is either slightly older or contemporaneous with the nearby Mandla lava flows(63-65 Ma).We suggest that the Mohpani dyke might be one of the feeders for the surrounding lava flows as these lavas are significantly younger than the majority of the main Deccan lavas of the Western Ghats(66.38-65.54 Ma).The second age of 56.95±1.08 Ma comes from a younger dyke near Olini village which cuts across the lava flows of the area.The age correlates well with the Mandla lavas which are chemically similar to the uppermost Poladpur,Ambenali and Mahabaleshwar Formation lavas of SW Deccan.Our study shows that the dyke activities occurred in two phases,with the second one representing the terminal stage.
文摘A precise dating of a mafic dyke of a swarm in shield areas has great advantage to identify Large Igneous Provinces(LIPs;short-lived,mantle-generated magmatic event)(Bryan and Ernst,2008;Ernst et al.,2010).
文摘Geologic, petrographic and petrochemical studies of the lateMesozoic K-rich melanocratic dykes, including lamprophyres, andesite porphyrites and dacite-porphyry in the gold field system in the Jiaodong Peninsula, China, have shown that these dykes are characterized by rich potassium and alkali but poor titanium. They belong to an ultra-high potassic, shoshonitic and high potassic calc-alkaline rock series. The parental magma has relatively high initial strontium ratios ((87Sr/86Sr)i-0.70895-0.71140) and low (143Nd/144Nd)i ratios (varying from 0.51135 to 0.51231); and its δ18OsMow. whole rock values vary from +5.8‰ to +10.6‰ with a mean of +7.1‰. These features suggest that the source region of the magma is an enriched mantle wedge transformed from a continental lithosphere mantle which has experienced me tasomatism by mantle-derived fluids with H2O-dominated fluids that were provided during the underthrusting of an ocean crust. The initial magma was generated by low-degree partial melting of the enriched mantle in its mature stage in the back-arc spreading environment. The evolution of magmas is associated with two trends, i.e., fractional crystallization and mixing with or intensive contamination by palaeo-crust materials or metamorphic rocks. The former process is evident in the gold field system of quartz-vein type, whereas the latter is dominated in the gold field system of the altered-rock type. This conclusion is very important for more detailed study of petrogenesis and mineralization through the crust-mantle interaction (exchange) in the Mesozoic in this region.
基金Financial support for this study was jointly provided by the National Natural Science Foundation of China(Grant Nos.41421002and 41225008)MOST Special Fund from the State Key Laboratory of Continental Dynamics,Northwest UniversityProject for Key Innovative Research Team of Science and Technology in Shaanxi Province(Grant No.2013KCT-18)
文摘The Helanshan tectonic belt(HTB) is a major tectonic divide between the Alxa and Ordos blocks in the North China Craton. The geochronology and petrogenesis of the mafic dykes in the northern HTB are keys to understanding the tectonic evolution of this belt. The mafic dykes, intruded into the Neoarchean-Paleoproterozoic metamorphic basement, are mainly composed of diabase with a mineral assemblage of plagioclase(45%-60%), pyroxene(25%-35%), minor quartz and Fe-Ti oxides. The LA-ICPMS U-Pb analysis of zircon grains from representative dykes yield a weighted mean age of 206 ± 1.9 Ma, which represents the crystallization age of the dyke. The diabases show high contents of Fe_2 O_3~T(11.88-17.55 wt.%), low contents of SiO_2(45.65-50.95 wt.%) and MgO(3.31-5.50 wt.%) with low Mg#(=100×MgO/(MgO + FeO) atomic ration) of 33-44. They are characterized by enrichment of light rare earth elements(LREEs) and large ion lithophile elements(LILEs)(e.g., Rb, Ba and Pb), and slight depletion of high field strength elements(HFSEs). These features suggest that the magma has undergone extensive fractionation of olivine and pyroxene but only minor crustal contamination during its evolution. Their high Sm contents and La/Sm ratios, and low Sm/Yb ratios indicate that magma from which the dykes formed was derived from low degree(about 5%) partial melting of an enriched garnet + spinel lherzolite mantle source. Together with regional geology, these geochemical and geochronological data suggest that the mafic dykes in the HTB were formed in an intracontinental extensional setting during the late Triassic.
基金This study was supported by the State"Ninth Five-Year Plan"Key Project No.96-915-05-02.
文摘The mafic dykes are composed of a series of northwest-trending diabase in Kuokesu of the Kuruktag region, Xinjiang. Four whole-rock samples of the mafic dykes in the region give an 40Ar / 36Ar-40K/ 36Ar isochron age of 282 Ma with the initial 40Ar/ 36Ar = 507. The characteristics of the major, trace and rare earth elements of the dyke rocks indicate that they belong to the calc-alkaline series and were emplaced in extensional structures at an intermediate depth of the crust. This shows that there was a large-scale extension in the Permian Period, which might be associated with the post-collisional extension of the ancient Tianshan Mountains.
基金supported by the NGRI MLP 6513INDEX project funds
文摘Anisotropy of magnetic susceptibility (AMS) studies were carried out on a precisely dated (2216.0 ± 0.9 Ma),450 km long NeS striking dyke in the Dharwar Craton,to determine the magma flow direction along the dyke length.In order to use the imbrication of the magnetic foliation,forty eight samples were collected from 13 locations along the length of the dyke.Magnetogranulometry studies show that AMS fabric is dominated by medium grained interstitial Ti-poor multidomain magnetite.The corrected anisotropy degree (Pj) of the samples was found to be low to moderate,between 1.007 and 1.072,which indicates primary magnetic fabric.The magnetic ellipsoid is either triaxial,prolate or oblate and clearly defines normal,intermediate and inverse magnetic fabrics related to magma flow during the dyke emplacement.The maximum susceptibility axes (Kmax) of the AMS tensor of the dyke is predominantly inclined at low angles (<30°),with no systematic variation in depth along the NeS profile,indicating sub-horizontal flow even at mid crustal levels which could probably be governed by location of the focal region of the magma source (mantle plume?),flow dynamics together with the compressive stresses exerted by the overlying crust.
文摘The work is a vivid description of the structural relationship between brittle deformation of the Precambrian basement in the southern continental part of the Cameroon Line and intrusive Paleozoic and Mesozoic basalt dykes swarms. A multidisciplinary approach that involves a combination of remote sensing techniques and field studies show that the major trend of brittle structures correspond to well-known regional structures: N70°E (Adamawa Shear Zone), N135°E (upper Benue trend) and N30°E (Cameroon Volcanic Line) corresponding to E-W and N-S directions respectively. Basalt dykes are associated to NE-SW, E-W and NW-SE oriented fractures. An integration of the available information on brittle structures and basalt dykes directions suggest an emplacement of the Mesozoic and Paleozoic basalt dykes structurally controlled by Precambrian structures that were originated through Riedel’s fracture kinematic model with dextral strike-slip Adamawa Shear Zone as the main shear zone during late stage of the Pan-African collision. Spatially, the restriction of the basalt dykes to the corridor of the Adamawa Shear Zone indicate that a rejuvenation of Precambrian faults may very well be the origin of the dykes with possibility that they may have been reworked several times during the Phanerozoic eon.
文摘Main factors, which should be considered in the classification of dyke foundation, are discussed in this paper. Engineering conditions should be taken into account when the levee safety is appraised based on engineering geologic appraisement and classification. On the basis of safety appraisement, dyke foundation may be classified with regard of suitable reinforcement measures. Examples are presented to illustrate the instructive significance of dyke foundation classification to dyke reinforcement design.
基金funded by the Fundamental Research Funds for the Central Universities in China(Grant No.19lgpy99)the National Natural Science Foundation of China(Grant Nos.41776056,41706055,41602231)the Natural Science Foundation of Guangdong Province(Grant Nos.2020A1515010501,2018B030311030)。
文摘In this study we present new mineral chemistry,whole-rock geochemical and zircon U-Pb geochronological data for 12 metamafic dykes in the mantle sequence of the Sangsang ophiolite in South Tibet(China).Modal analyses of these dykes gave averages of^40%-65%plagioclase and^35%-60%amphibole and small amounts of(igneous)clinopyroxene,epidote and opaque minerals.This mineral assemblage resembles that of typical orthoamphibolites.Nevertheless,due to the absence of foliation the investigated rocks are described as metamafic lithologies.These rocks have primitive mantle(PM)-normalized multi-element patterns with negative Nb and Ta anomalies as well as weak,negative Ti anomalies.In addition,they have initial 87Sr/86Sr ratios[(87Sr/86Sr)1]of0.702844-0.703581,initial 143Nd/144Nd ratios[(143Nd/144Nd)i]of 0.512891-0.512959 and high εNd(t)values(+7.9 to+9.3).Uranium-Pb ages of magmatic zircons separated from the investigated metamafic dykes indicate that the parental melts of their protoliths intruded the Sangsang mantle at^119.0-118.5 Ma.The metamorphic mineral assemblages recognized in the investigated dykes are suggestive of a retrograde metamorphic process,from(epidote-)amphibolite facies(~470-610℃,-1.9-4.3 kbar)and to prehnitepumpellyite facies(≤280℃,<3 kbar),active within a rift-produced oceanic lithosphere.Microtextural and geochemical data suggest that the protoliths of the dykes were most likely massive gabbros.Compositional data show that the parental magmas of the gabbroic protoliths were generated by melting of a depleted mantle(DM)source that had been weakly modified by fluids emanating from a subducted oceanic lithospheric slab.The age of the gabbroic protoliths is slightly younger than the existing ages for ophiolites from the central Yarlung-Zangbo Suture Zone(YZSZ)in the literature(~129-123 Ma).We,therefore,suggest that the gabbroic protoliths of the Sangsang metamafic dykes were formed in an incipient forearc setting during Neo-Tethyan subduction reinitiation(Aptian).Our tectonomagmatic model provides insights into the igneous accretion and postsolidification evolution of the oceanic lithosphere in South Tibet.
文摘It is yet unclear whether large-scale segregation of immiscibile liquids and eruption of high-Si lavas exist in nature(Charlier et al.,2013).We present a possible case of segregation of immscible liquids in the 1780 Ma
文摘About 300 mafic dykes are intruded Eocene volcanic and pyroclastic rocks,north east of Meshkinshahr city in the NW of Iran.Most of dyke swarms display NNE-SSE trend.Their composition varies from basalt to tephrite
基金financially supported by the Key Program of the Ministry of Land and Resources of China(DD20190370,DD20190009,DD20190358,DD20190003)。
文摘Komatiites are presented as direct evidence for higher mantle temperatures during the Archean. In the North China Craton, komatiites with spinifex structure have been identified only at one locality, i.e. the Sujiagou area, western Shandong. They were considered as formed during the early Neoarchean mainly based on their association with supracrustal rocks considered to be that age. This study carried out SHRIMP U-Pb zircon dating on metamorphosed trondhjemitic and monzogranitic dykes intruding the Sujiagou komatiites, and they have magmatic zircon ages of 2592 ± 12 Ma and 2586 ± 13 Ma respectively. This provides direct evidence that the komatiites formed during the early Neoarchean.
基金supported by the National Key R&D 445 Program of China(2017YFC0601206)the Strategic Priority Research Program(B)of the Chinese Academy of Sciences(XDB18020203)+1 种基金the Opening Foundation of State Key Laboratory of Continental Dynamics,Northwest University(17LCD04)National Science Foundation of China(Grant nos.41473053,41573045,41611530698,41873060 and 41830216)
文摘Late Silurian-early Devonian mafic--intermediate dykes occur in the Xiemisitai mountain of the northern West Junggar. However, their ages, petrogenesis and geodynamic process remain to be unraveled. We report firstly in situ zircon U–Pb and hornblende 40Ar-39Ar ages, geochemical and in situ zircon Hf isotopic data for the Xiemisitai dykes. The Xiemisitai dykes give zircon U–Pb ages of 416.7±2.1 Ma and 405.1±2.5 Ma and hornblende 40Ar-39Ar age of 405.9±4.9 Ma, respectively. They are characterized by high εHf(t)values(+11.9 to +15.5), suggesting a depleted mantle features. The Xiemisitai dykes show low Mg#(46–59) and low Cr(11.3–197 ppm) and Ni(19.9–102 ppm) abundances indicating that they have experienced significant fractional crystallization. These dykes contain hornblende and biotite and display negative Nb-Ta-Ti anomalies, enrichment of LREEs, LILEs and depletion of HREEs and HFSEs with high Ba/La ratios, similar with an origin from a depleted lithospheric mantle metasomatized by subducted slabderived fluids. In addition, the Xiemisitai dykes are plotted within melting trends with little to no garnet(Cpx:Grt=6:1) in their source. The La/Yb vs. Tb/Yb plot also indicates the presence of less than 1% residual garnet in the source region for the Xiemisitai dykes. Therefore, it can be inferred that the Xiemisitai dykes were generated at a correspondingly shallow depth, mostly within the spinel stability field. Finally, the Xiemisitai dykes were most probably generated by partial melting of metasomatized lithospheric mantle in relatively shallow level(<80 km). They could be possibly triggered by the asthenospheric upwelling as a result of the rollback of the subducted Irtysh-Zaysan oceanic lithosphere.
