Objective The giant Dongping gold deposit, situated in northern Hebei Province, is the first recognized deposit among the alkaline complex-hosted gold system in China. The gold deposits in the Dongping district are u...Objective The giant Dongping gold deposit, situated in northern Hebei Province, is the first recognized deposit among the alkaline complex-hosted gold system in China. The gold deposits in the Dongping district are usually defined as alkaline rock-related gold deposits because they are hosted by the Middle Devonian Shuiquangou alkaline complex （SAC）. However, geochronological data show that these gold deposits were formed in Late Jurassic （Jiang Sihong and Nie Fengjun, 2000; Zhang Guorui et al., 2012）.展开更多
Various petrographic features and geochemical characteristics indicative of disequilibrium are preserved in plagioclase phenocrysts from basaltic to andesitic lavas in East Junggar, northwest China. These characterist...Various petrographic features and geochemical characteristics indicative of disequilibrium are preserved in plagioclase phenocrysts from basaltic to andesitic lavas in East Junggar, northwest China. These characteristics indicate that they crystallized in a magma chamber, which was replenished by less differentiated and high-temperature magmas. The petrographic and geochemical features of the plagioclase phenocrysts are interpreted to record responses to changes in temperature, composition and mechanical effect during magma replenishment. Distinct rare earth element(REE) patterns between cores and rims of the same plagioclase crystal suggest derivation from two end-member magmas. From core to rim, plagioclase phenocrysts commonly display sharp fluctuations of anorthite(An) content up to 20, which either correspond to reverse zoning associated with ovoidal cores and resorption surface(PI), or normal zoning with euhedral form and no resorption surface(P2). Plagioclase crystals with diverse textures and remarkably different An content coexist on the scale of a thin-section. Cores of these plagioclases in each sample display a bimodal distribution of An content. From core to rim in PI, concentrations o f FeOT and Sr increase remarkably as An content increases. During magma replenishment, pre-existing plagioclase phenocrysts in the andesitic magma, which were immersed into hotter and less differentiated magmas, were heated and resorbed to form ovoidal cores, and then were overgrown by a thin rim with much higher contents of An, FeO^T and Sr. However, pre-existing plagioclase phenocrysts in the basaltic magma were injected into cooler and more evolved magmas, and were remained as euhedral cores, which were later enclosed by oscillatory zoned rims with much lower contents of An, Sr and Ba.展开更多
Intracrystalline distortions(like undulose extinction,dislocations,and subgrain boundaries)in olivine from naturally-deformed peridotites are generally taken as signs of dislocation creep.However,similar features in o...Intracrystalline distortions(like undulose extinction,dislocations,and subgrain boundaries)in olivine from naturally-deformed peridotites are generally taken as signs of dislocation creep.However,similar features in olivine phenocrysts that have been found in basaltic magmas are still not well understood.In particular,whether subgrain boundaries in olivine phenocrysts arise from plastic deformation or grain growth is still debated(in the latter case,they are essentially grain boundaries but not subgrain boundaries.Therefore,we used hereinafter subgrain-boundary-like structures instead of subgrain boundaries to name this kind of intracrystalline distortion).Here we carried out a detailed study on dislocations and subgrainboundary-like(SG-like)structures in olivine phenocrysts from two Hawaiian basaltic lavas by means of petrographic microscopy,scanning electron microscopy,and transmission electron microscopy(TEM).Abundant and complex dislocation substructures(free dislocations,dislocation walls,and dislocation tangles)were observed in the decorated olivine grains,similar to those in olivine from peridotite xenoliths entrained by the Hawaiian basalts.The measured average dislocation density is(2.9±1.3)×1011 m-2,and is three to five orders of magnitude higher than that in laboratory-synthesized,undeformed olivine.TEM observations on samples cut across the SG-like structures by FIB(focused ion beam)demonstrated that this kind of structures is made of an array of dislocations.These observations clearly indicate that these structures are real subgrain boundaries rather than grain boundaries.These facts suggest that the observed high dislocation densities and subgrain boundaries cannot result from crystal crystallization/growth,but can be formed by plastic deformation.These deformation features do not prove that the olivine phenocrysts(and implicitly mantle xenoliths)were deformed after their capture by the basaltic magmas,but can be ascribed to a former deformation event in a dunitic cumulate,which was formed by magmatic fractionation,then plastically deformed,and finally disaggregated and captured by the basaltic magma that brought them to the surface.展开更多
Textural and compositional zoning within plagioclase phenocrysts records the magma chamber processes,such as magma differentiation,magma recharge and mixing,and crustal contamination.The plagioclase phenocrysts in the...Textural and compositional zoning within plagioclase phenocrysts records the magma chamber processes,such as magma differentiation,magma recharge and mixing,and crustal contamination.The plagioclase phenocrysts in the Daqiao and Qiaojia plagioclase-phyric basalts from the Emeishan Large Igneous Province(LIP)show complex textural and compositional zoning patterns,e.g.,normal,reverse,oscillatory,and patchy zoning patterns.Most plagioclase phenocrysts exhibit a core–rim normal zoning pattern(Pl-A)with euhedral high-An cores(An=76–78%,in mole fraction)and low-An rims(An=68–72%),indicative of the crystal regrowth processes caused by recharge of relatively evolved magmas after the formation of high-An cores.Some phenocrysts have a core–rim reverse zoning pattern(Pl-B)with irregular ovaloid cores,characterized by extremely low An(60–61 mol%)and Ba(84–88 ppm)contents and extremely high87Sr/86Sr ratios(0.7120–0.7130).The rims of the Pl-B have relatively high An(69–72%),Ba(~160 ppm)contents,and low87Sr/86Sri(~0.7056).These Pl-B plagioclase phenocrysts preserve the information about the interaction between the crustal xenocrysts and the transporting magmas.Some plagioclase phenocrysts show a core–mantle–rim oscillatory zoning pattern(Pl-C)with multiple oscillations of An(70–80%),Ba(88–147ppm)from core to rim,revealing replenishment and mixing of multiple batches of basaltic melts with diverse compositions.87Sr/86Sr ratios of the Pl-C do not vary significantly(0.7050–0.7054).A small portion of phenocrysts has patchy patterns in the cores(Pl-D),where the low-An patches(72–75%)in form of elliptical or irregular elongated shapes were enclosed by the high-An domains(80–87%).These features can be attributed to crystal dissolution and regrowth processes during the reaction between earlyformed low-Cumulates and recharged hot primitive melts.The cores,mantles,and rims of different types of plagioclase phenocrysts(except the core of Pl-B)commonly display nearly constant Sr isotopic compositions,implying insignificant wall-rock assimilation at shallow-level magma reservoir(s)during the growth of these plagioclase phenocrysts.In conclusion,the massive crystallization of plagioclase in the late stage was an important controlling factor for the formation of iron-rich basalts in the Emeishan LIP.展开更多
Electron microprobe analysis was conducted on plagioclase from the plagioclase ultraphyric basalts(PUBs)erupted on the Southwest Indian Ridge(SWIR)(51°E)to investigate the geochemical changes in order to better u...Electron microprobe analysis was conducted on plagioclase from the plagioclase ultraphyric basalts(PUBs)erupted on the Southwest Indian Ridge(SWIR)(51°E)to investigate the geochemical changes in order to better understand the magmatic processes occurring under ultraslow spreading ridges and to provide insights into the thermal and dynamic regimes of the magmatic reservoirs and conduit systems.The phenocryst cores are generally calcic(An74–82)and are depleted in FeO and MgO.Whereas the phenocryst rims(An67–71)and the plagioclase in the groundmass(An58–63)are more sodic and have higher FeO and MgO contents than the phenocryst cores.The crystallization temperatures of the phenocryst cores and the calculation of the equilibrium between the phenocrysts and the matrix suggest that the plagioclase cores are unlikely to have crystallized from the host basaltic melt,but are likely to have crystallized from a more calcic melt.The enrichment in incompatible elements(FeO and MgO),as well as the higher FeO/MgO ratios of the outermost phenocryst rims and the groundmass,are the result of plagioclase-melt disequilibrium diffusion during the short residence time in which the plagioclase crystallized.Our results indicate that an evolved melt replenishing under the SWIR(51°E)drives the eruption over a short period of time.展开更多
The Southwest Indian Ridge（SWIR） is an ultraslow spreading end-member of mid-ocean ridge system and is characterized by weak or even an absence of magmatism. The segment between Indomed（ITF） and Gallieni（GTF） tr...The Southwest Indian Ridge（SWIR） is an ultraslow spreading end-member of mid-ocean ridge system and is characterized by weak or even an absence of magmatism. The segment between Indomed（ITF） and Gallieni（GTF） transform faults in the SWIR, however, displays extremely magmatic accretion with an unusual thick crust（up to 9.5 km）. Although H_2O is present in trace amounts in the mantle, it has a strong influence on mantle melting and magmatism in the shallow crust. The mid-ocean ridge basalts（MORB） worldwide show strong variation in H_2O contents, but with a nearly uniform H_2O/Ce ratio. Regionally distinctive H_2O contents and H_2O/Ce ratios are inferred to be related to the H_2O variation in the source and can be used to constrain the mantle heterogenity. In this study, we measured the H_2O and trace elements of clinopyroxene phenocrysts from one basalt dredged from the ITF-GTF segment, SWIR（51.56 o E）. The estimated H_2O content（1.3 wt.%± 0.3 wt.%） in the primitive ITF-GTF basaltic melt is much higher than that in typical MORB samples, but similar to oceanic island basalts（OIB） and back-arc basalts（BABB）. In addition, the calculated H_2O/Ce ratio（1 672–4 990） are extremely high, bearing ＂arc-like＂ signature. This study provides evidence that arc-related hydrous components are involved in the mantle source beneath the ITF-GTF ridge segment. It further lends support to the hypothesis that the mantle beneath the central SWIR may have experienced an ancient hydrous melting event in an arc terrain prior to or during the closure of the Mozambique Ocean in the Neoproterozoic.展开更多
基金funded by National Nature Science Foundation of China(grants No.41672070 and41372096)
文摘Objective The giant Dongping gold deposit, situated in northern Hebei Province, is the first recognized deposit among the alkaline complex-hosted gold system in China. The gold deposits in the Dongping district are usually defined as alkaline rock-related gold deposits because they are hosted by the Middle Devonian Shuiquangou alkaline complex （SAC）. However, geochronological data show that these gold deposits were formed in Late Jurassic （Jiang Sihong and Nie Fengjun, 2000; Zhang Guorui et al., 2012）.
基金financial support by the Natural Science Foundation of China (41573030, 41503024)the Geological Survey Program from China Geological Survey (No. DD20190518)
文摘Various petrographic features and geochemical characteristics indicative of disequilibrium are preserved in plagioclase phenocrysts from basaltic to andesitic lavas in East Junggar, northwest China. These characteristics indicate that they crystallized in a magma chamber, which was replenished by less differentiated and high-temperature magmas. The petrographic and geochemical features of the plagioclase phenocrysts are interpreted to record responses to changes in temperature, composition and mechanical effect during magma replenishment. Distinct rare earth element(REE) patterns between cores and rims of the same plagioclase crystal suggest derivation from two end-member magmas. From core to rim, plagioclase phenocrysts commonly display sharp fluctuations of anorthite(An) content up to 20, which either correspond to reverse zoning associated with ovoidal cores and resorption surface(PI), or normal zoning with euhedral form and no resorption surface(P2). Plagioclase crystals with diverse textures and remarkably different An content coexist on the scale of a thin-section. Cores of these plagioclases in each sample display a bimodal distribution of An content. From core to rim in PI, concentrations o f FeOT and Sr increase remarkably as An content increases. During magma replenishment, pre-existing plagioclase phenocrysts in the andesitic magma, which were immersed into hotter and less differentiated magmas, were heated and resorbed to form ovoidal cores, and then were overgrown by a thin rim with much higher contents of An, FeO^T and Sr. However, pre-existing plagioclase phenocrysts in the basaltic magma were injected into cooler and more evolved magmas, and were remained as euhedral cores, which were later enclosed by oscillatory zoned rims with much lower contents of An, Sr and Ba.
基金the National Natural Science Foundation of China(Nos.41590623,41972231,41872230)。
文摘Intracrystalline distortions(like undulose extinction,dislocations,and subgrain boundaries)in olivine from naturally-deformed peridotites are generally taken as signs of dislocation creep.However,similar features in olivine phenocrysts that have been found in basaltic magmas are still not well understood.In particular,whether subgrain boundaries in olivine phenocrysts arise from plastic deformation or grain growth is still debated(in the latter case,they are essentially grain boundaries but not subgrain boundaries.Therefore,we used hereinafter subgrain-boundary-like structures instead of subgrain boundaries to name this kind of intracrystalline distortion).Here we carried out a detailed study on dislocations and subgrainboundary-like(SG-like)structures in olivine phenocrysts from two Hawaiian basaltic lavas by means of petrographic microscopy,scanning electron microscopy,and transmission electron microscopy(TEM).Abundant and complex dislocation substructures(free dislocations,dislocation walls,and dislocation tangles)were observed in the decorated olivine grains,similar to those in olivine from peridotite xenoliths entrained by the Hawaiian basalts.The measured average dislocation density is(2.9±1.3)×1011 m-2,and is three to five orders of magnitude higher than that in laboratory-synthesized,undeformed olivine.TEM observations on samples cut across the SG-like structures by FIB(focused ion beam)demonstrated that this kind of structures is made of an array of dislocations.These observations clearly indicate that these structures are real subgrain boundaries rather than grain boundaries.These facts suggest that the observed high dislocation densities and subgrain boundaries cannot result from crystal crystallization/growth,but can be formed by plastic deformation.These deformation features do not prove that the olivine phenocrysts(and implicitly mantle xenoliths)were deformed after their capture by the basaltic magmas,but can be ascribed to a former deformation event in a dunitic cumulate,which was formed by magmatic fractionation,then plastically deformed,and finally disaggregated and captured by the basaltic magma that brought them to the surface.
基金funded by the Strategic Priority Research Program(B)of the Chinese Academy of Sciences(XDB41000000)the NSFC(41573009,41873026,and 42173020)the‘‘Western Light Young scholar’’program of the Chinese Academy of Sciences to Prof.Lie-Meng Chen。
文摘Textural and compositional zoning within plagioclase phenocrysts records the magma chamber processes,such as magma differentiation,magma recharge and mixing,and crustal contamination.The plagioclase phenocrysts in the Daqiao and Qiaojia plagioclase-phyric basalts from the Emeishan Large Igneous Province(LIP)show complex textural and compositional zoning patterns,e.g.,normal,reverse,oscillatory,and patchy zoning patterns.Most plagioclase phenocrysts exhibit a core–rim normal zoning pattern(Pl-A)with euhedral high-An cores(An=76–78%,in mole fraction)and low-An rims(An=68–72%),indicative of the crystal regrowth processes caused by recharge of relatively evolved magmas after the formation of high-An cores.Some phenocrysts have a core–rim reverse zoning pattern(Pl-B)with irregular ovaloid cores,characterized by extremely low An(60–61 mol%)and Ba(84–88 ppm)contents and extremely high87Sr/86Sr ratios(0.7120–0.7130).The rims of the Pl-B have relatively high An(69–72%),Ba(~160 ppm)contents,and low87Sr/86Sri(~0.7056).These Pl-B plagioclase phenocrysts preserve the information about the interaction between the crustal xenocrysts and the transporting magmas.Some plagioclase phenocrysts show a core–mantle–rim oscillatory zoning pattern(Pl-C)with multiple oscillations of An(70–80%),Ba(88–147ppm)from core to rim,revealing replenishment and mixing of multiple batches of basaltic melts with diverse compositions.87Sr/86Sr ratios of the Pl-C do not vary significantly(0.7050–0.7054).A small portion of phenocrysts has patchy patterns in the cores(Pl-D),where the low-An patches(72–75%)in form of elliptical or irregular elongated shapes were enclosed by the high-An domains(80–87%).These features can be attributed to crystal dissolution and regrowth processes during the reaction between earlyformed low-Cumulates and recharged hot primitive melts.The cores,mantles,and rims of different types of plagioclase phenocrysts(except the core of Pl-B)commonly display nearly constant Sr isotopic compositions,implying insignificant wall-rock assimilation at shallow-level magma reservoir(s)during the growth of these plagioclase phenocrysts.In conclusion,the massive crystallization of plagioclase in the late stage was an important controlling factor for the formation of iron-rich basalts in the Emeishan LIP.
基金The National Natural Science Foundation of China under contract Nos 41606041 and 41903046the Scientific Research Fund of the Second Institute of Oceanography,MNR under contract Nos JG1604 and JT1504China Ocean Mineral R&D Association(COMRA)Project under contract Nos DY135-G2-1-03 and DY135-N2-1-04。
文摘Electron microprobe analysis was conducted on plagioclase from the plagioclase ultraphyric basalts(PUBs)erupted on the Southwest Indian Ridge(SWIR)(51°E)to investigate the geochemical changes in order to better understand the magmatic processes occurring under ultraslow spreading ridges and to provide insights into the thermal and dynamic regimes of the magmatic reservoirs and conduit systems.The phenocryst cores are generally calcic(An74–82)and are depleted in FeO and MgO.Whereas the phenocryst rims(An67–71)and the plagioclase in the groundmass(An58–63)are more sodic and have higher FeO and MgO contents than the phenocryst cores.The crystallization temperatures of the phenocryst cores and the calculation of the equilibrium between the phenocrysts and the matrix suggest that the plagioclase cores are unlikely to have crystallized from the host basaltic melt,but are likely to have crystallized from a more calcic melt.The enrichment in incompatible elements(FeO and MgO),as well as the higher FeO/MgO ratios of the outermost phenocryst rims and the groundmass,are the result of plagioclase-melt disequilibrium diffusion during the short residence time in which the plagioclase crystallized.Our results indicate that an evolved melt replenishing under the SWIR(51°E)drives the eruption over a short period of time.
基金funded by the China Ocean Mineral R&D Association(COMRA)Project(No.DY125-11-R-05)
文摘The Southwest Indian Ridge（SWIR） is an ultraslow spreading end-member of mid-ocean ridge system and is characterized by weak or even an absence of magmatism. The segment between Indomed（ITF） and Gallieni（GTF） transform faults in the SWIR, however, displays extremely magmatic accretion with an unusual thick crust（up to 9.5 km）. Although H_2O is present in trace amounts in the mantle, it has a strong influence on mantle melting and magmatism in the shallow crust. The mid-ocean ridge basalts（MORB） worldwide show strong variation in H_2O contents, but with a nearly uniform H_2O/Ce ratio. Regionally distinctive H_2O contents and H_2O/Ce ratios are inferred to be related to the H_2O variation in the source and can be used to constrain the mantle heterogenity. In this study, we measured the H_2O and trace elements of clinopyroxene phenocrysts from one basalt dredged from the ITF-GTF segment, SWIR（51.56 o E）. The estimated H_2O content（1.3 wt.%± 0.3 wt.%） in the primitive ITF-GTF basaltic melt is much higher than that in typical MORB samples, but similar to oceanic island basalts（OIB） and back-arc basalts（BABB）. In addition, the calculated H_2O/Ce ratio（1 672–4 990） are extremely high, bearing ＂arc-like＂ signature. This study provides evidence that arc-related hydrous components are involved in the mantle source beneath the ITF-GTF ridge segment. It further lends support to the hypothesis that the mantle beneath the central SWIR may have experienced an ancient hydrous melting event in an arc terrain prior to or during the closure of the Mozambique Ocean in the Neoproterozoic.
