Magmatic oxide deposits in the～260 Ma Emeishan Large Igneous Province（ELIP）,SW China and northern Vietnam,are important sources of Fe,Ti and V.Some giant magmatic Fe-Ti-V oxide deposits, such as the Panzhihua,Hongg...Magmatic oxide deposits in the～260 Ma Emeishan Large Igneous Province（ELIP）,SW China and northern Vietnam,are important sources of Fe,Ti and V.Some giant magmatic Fe-Ti-V oxide deposits, such as the Panzhihua,Hongge,and Baima deposits,are well described in the literature and are hosted in layered mafic-ultramafic intrusions in the Panxi region,the central ELIP.The same type of ELIP- related deposits also occur far to the south and include the Anyi deposit,about 130 km south of Panzhihua,and the Mianhuadi deposit in the Red River fault zone.The Anyi deposit is relatively small but is similarly hosted in a layered mafic intrusion.The Mianhuadi deposit has a zircon U-Pb age of～260 Ma and is thus contemporaneous with the ELIP.This deposit was variably metamorphosed during the Indosinian orogeny and Red River faulting.Compositionally,magnetite of the Mianhuadi deposit contains smaller amounts of Ti and V than that of the other deposits,possibly attributable to the later metamorphism.The distribution of the oxide ore deposits is not related to the domal structure of the ELIP.One major feature of all the oxide deposits in the ELIP is the spatial association of oxide-bearing gabbroic intrusions,syenitic plutons and high-Ti flood basalts.Thus,we propose that magmas from a mantle plume were emplaced into a shallow magma chamber where they were evolved into a field of liquid immiscibility to form two silicate liquids,one with an extremely Fe-Ti-rich gabbroic composition and the other syenitic.An immiscible Fe-Ti-（P） oxide melt may then separate from the mafic magmas to form oxide deposits.The parental magmas from which these deposits formed were likely Fe-Ti-rich picritic in composition and were derived from enriched asthenospheric mantle at a greater depth than the magmas that produced sulfide-bearing intrusions of the ELIP.展开更多
The Emeishan large igneous province(ELIP) in SW China is interpreted to be associated with an ancient mantle plume. Most of the constraints on the role of mantle plume in the generation of the Emeishan flood basalts w...The Emeishan large igneous province(ELIP) in SW China is interpreted to be associated with an ancient mantle plume. Most of the constraints on the role of mantle plume in the generation of the Emeishan flood basalts were provided by geological and geochemical methods, but the geophysical investigation is very limited. In order to better understand the deep structure and features of ELIP, we have studied the crustal velocity structure using the data acquired from the Lijiang-Panzhihua-Qingzhen wide-angle seismic profile. This profile crosses the three sub-zones of the ELIP(the inner, intermediate, and outer zones), divided based on the differential erosion and uplift of the Maokou limestone. The results provided by the active source seismic experiment demonstrate:(1) The average depth of the crystalline basement along the profile is about 2 km.(2) The middle crust in the Inner Zone is characterized by high-velocity anomalies, with the average velocity of 6.2-6.6 km/s, which is about 0.1– 0.2 km/s higher than the normal one. The velocity of the lower crust in the inner zone is 6.9-7.2 km/s, higher than those observed in the intermediate and outer zones(6.7-7.0 km/s). Relatively low velocity anomalies appear in the upper, middle and lower crusts near the junction of the inner zone and intermediate zone, probably due to the effect of the Xiaojiang fault(XJF).(3) The average velocity of the crust is comparatively low on both sides of XJF, especially on the east side, and the average velocity of the consolidated continental crust is also low there. This may suggest that the XJF extends at least down to 40 km deep, even beyond through the crust.(4) The depth to the Moho discontinuity decrease gradually from 47-53 km in the inner zone, via 42-50 km in the intermediate zone to 38-42 km in the outer zone. In the inner zone, the Moho uplifts locally and the(consolidated) crust is characterized by high-velocity anomalies, which are likely related to intensive magma intrusion and underplating associated with melting of plum展开更多
Based on the temporal-spatial distribution and geochemical characteristics,the Emeishan basalts can be divided into two types: high-P_2O-TiO_2 basalt (HPT) andlow-P_2O_5-TiO_2 basalt (LPT), which differ distinctly in ...Based on the temporal-spatial distribution and geochemical characteristics,the Emeishan basalts can be divided into two types: high-P_2O-TiO_2 basalt (HPT) andlow-P_2O_5-TiO_2 basalt (LPT), which differ distinctly in geochemistry: the LPTs are characterizedby relatively high abundances of MgO, total FeO and P_2O_5 and compatible elements (Cr, Ni, Sc), andrelatively low contents of moderately compatible elements (V, Y, Yb, Co), LREE and otherincompatible elements compared with the HPT. On the diagrams of trace element ratios, they areplotted on an approximately linear mixing line between depleted and enriched mantle sources,suggesting that these two types of basalts resulted from interactions of varying degrees betweenmantle plume and lithospheric mantle containing such volatile-rich minerals as amphibole andapatite. The source region of the LPT involves a smaller proportion of lithospheric components,while that of the HTP has a larger proportion of lithospheric components. Trachyte is generated bypartial melting of the basic igneous rocks at the base of the lower continental crust. Both the twotypes of magmas underwent certain crystal fractionation and contamination of the lower crest athigh-level magma chambers and en route to the surface.展开更多
Lithologically, two kinds of chert can be recognized in the Middle-Upper Permian from the Tieqiao section in Laibin area, Guangxi, i.e., calcic chert occurring mainly in the Maokou Formation and pure chert mainly in t...Lithologically, two kinds of chert can be recognized in the Middle-Upper Permian from the Tieqiao section in Laibin area, Guangxi, i.e., calcic chert occurring mainly in the Maokou Formation and pure chert mainly in the Wujiaping Formation. Geochemical data show that both kinds of chert contain very low A1203 （0-0.23%） and TiO2 （0.001%-0.024%） and low ZREE （0.55-19.94 ppm, averaging 9.97 ppm）, as well as high ratio of Fe2O3/TiO2 （17-443, averaging 111） and low ratio of A1203/（Al2O3＋Fe203） （0-0.26, averaging 0.09）. Given that the average value Ce anomalies in chert deposited in the ridge-proximal environment is about 0.29, and that in the pelagic environment is about 0.60, the low Ce anomalies in the Tieqiao chert （0.24-0.46, averaging 0.35） imply that they were deposited in an ocean basin with influence of submarine hydrothermal fluid and no input of terrigenous materials. The vertical variation of the silica abundance in strata （SAIS） in the Middle-Upper Permian strata, together with the Eu anomalies and the ratios of ∑REE/Fe, indicates a relationship between the hydrothermal activity and the Emeishan basalt eruption, and that submarine hydrothermal activity was stronger in the Upper Permian （the Wujiapingian Stage） than in the Middle Permian （the Maokou Stage）.展开更多
Southeastern Tibet,which has complex topography and strong tectonic activity,is an important area for studying the subsurface deformation of the Tibetan Plateau.Through the two-station method on 10-year teleseismic Ra...Southeastern Tibet,which has complex topography and strong tectonic activity,is an important area for studying the subsurface deformation of the Tibetan Plateau.Through the two-station method on 10-year teleseismic Rayleigh wave data from 132 permanent stations in the southeastern Tibetan Plateau,which incorporates ambient noise data,we obtain the interstation phase velocity dispersion data in the period range of 5–150s.Then,we invert for the shear wave velocity of the crust and upper mantle through the direct 3-D inversion method.We find two low-velocity belts in the mid-lower crust.One belt is mainly in the SongPan-GangZi block and northwestern part of the Chuan-Dian diamond block,whereas the other belt is mainly in the Xiaojiang fault zone and its eastern part,the Yunnan-Guizhou Plateau.The low-velocity belt in the Xiaojiang fault zone is likely caused by plastic deformation or partial melting of felsic rocks due to crustal thickening.Moreover,the significant positive radial anisotropy(VSH>VSV)around the Xiaojiang fault zone further enhances the amplitude of low velocity anomaly in our VSVmodel.This crustal low-velocity zone also extends southward across the Red River fault and farther to northern Vietnam,which may be closely related to heat sources in the upper mantle.The two low-velocity belts are separated by a high-velocity zone near the Anninghe-Zemuhe fault system,which is exactly in the inner and intermediate zones of the Emeishan large igneous province(ELIP).We find an obvious high-velocity body situated in the crust of the inner zone of the ELIP,which may represent maficultramafic material that remained in the crust when the ELIP formed.In the upper mantle,there is a large-scale low-velocity anomaly in the Indochina and South China blocks south of the Red River fault.The low-velocity anomaly gradually extends northward along the Xiaojiang fault zone into the Yangtze Craton as depth increases.Through our velocity model,we think that southeastern Tibet is undergoing three different tectonic modes at t展开更多
There existed intense Cu anomaly on the northeastern side of the geochemical bound-ary with NW strike in the border area between the Yunnan and Guizhou provinces. Through field observation, ore bodies of high-grade na...There existed intense Cu anomaly on the northeastern side of the geochemical bound-ary with NW strike in the border area between the Yunnan and Guizhou provinces. Through field observation, ore bodies of high-grade native coppers have been found. The copper mineralization was constrained by the ancient volcanic vents of Permian basalt eruption and the overlain strata of carbonaceous argillites. Native coppers with flaky, net veined and impregnated occurrences, fine-grained tenorites and massive chalcocites widely occur in volcanic breccias, tuffs, carbona-ceous-siliceous argillites and siliceous bitumen rocks with bed thickness of about 15—80 m. Cu contents vary from 0.5% to 20%. The copper mineralization was tightly related to actino-lite-tremolitization, zeolitization and bituminization and involved in extensive reduction environ-ments. Continental flood basalts erupted in mantle plume environments usually have high Cu concentrations (~170 ?0-6 in the Emeishan basalts), which provided a copper source of minerali-zation. Thus, metallogenesis of the native copper deposits in the Yunnan-Guizhou border area is tightly associated with intensive crust-mantle and organic-inorganic interactions. The tremolitiza-tion and chalcocitization indicate that the metallogenic temperatures are in a range of 400—100℃. The geologic background and characteristics of ore and alteration for the native copper deposits in this area are somewhat similar to those of the Keweenawan native copper deposit in Michigan, USA.展开更多
The Late Permian Emeishan Large Igneous Province (ELIP) is commonly regarded as being located in the western part of the Yangtze craton, SW China, with an asymmetrical shape and a small area. This area, however, is ju...The Late Permian Emeishan Large Igneous Province (ELIP) is commonly regarded as being located in the western part of the Yangtze craton, SW China, with an asymmetrical shape and a small area. This area, however, is just a maximum estimation because some parts of the ELIP were not recognized or dismembered and destroyed during the Triassic to Cenozoic tectonism. In this paper, the chemostratigraphical data of the Zongza block, the Garze-Litang belt and the Songpan-Garze block suggest that the Late Permian basalts in these areas have remarkable similarities to the ELIP basalts in petrography and geochemistry. Flood basalts in the Sanjiangkou area are composed of the lower part of the low-Ti (LT) tholeiite and the upper part of the high-Ti (HT) tholeiite, which is the same as the flood basalts on the western margin of the Yangtze craton. Flood basalts in the Zongza and Songpan-Garze areas, which are far from the Yangtze craton, consist of HT tholeiite only. This is the same as the flood basalts within the Yangtze craton. Therefore we argue that these contemporary basalts all originated from the Emeishan mantle plume, and the ELIP could have a significant westward extension with an outcropped area of over 500,000 km2. This new scenario shows that the LT tholeiite occurs on the western margin of the Yangtze craton, while the HT tholeiite overlying the LT basalts occupies the whole area of the ELIP.展开更多
A suite of continental flood basalts sampled over a vast exposure and stratigraphic thickness in the Emeishan large igneous province （LIP）, SW China was investigated for laser microprobe ^40Ar/^39Ar dating. There ar...A suite of continental flood basalts sampled over a vast exposure and stratigraphic thickness in the Emeishan large igneous province （LIP）, SW China was investigated for laser microprobe ^40Ar/^39Ar dating. There are two ^40Ar/^39Ar age groups for these basalts, corresponding to 259-246 Ma and 177-137 Ma, respectively. A well-defined isochron gives an eruption age of huge quantities of mafic magmas at 258.9±3.4 Ma, which is identical to previous dating and paleontological data. Much younger ^40Ar/^39Ar ages for some basalts with Iow-greenschist metamorphic facies probably recorded a late thermo-tectonic event caused by collision between the Yangtze and Qiangtang continental blocks during the Mesozoic, which resulted in the reset of argon isotope system. The ^40Ar/^39Ar age data, we present here, combined with previous dating and paleontological data, suggest relatively short duration （about 3 Ma） of mafic volcanism, which have important implication on mantle plume genesis of the Emeishan continental flood basalts in the LIP.展开更多
The Xinjie layered intrusion in the Panxi region,SW China,hosts both Fe-Ti oxide and platinum-group element（PGE） sulfide mineralization.The intrusion can be divided,from the base upward,into UnitsⅠ,ⅡandⅢ,in terms...The Xinjie layered intrusion in the Panxi region,SW China,hosts both Fe-Ti oxide and platinum-group element（PGE） sulfide mineralization.The intrusion can be divided,from the base upward,into UnitsⅠ,ⅡandⅢ,in terms of mineral assemblages.UnitsⅠandⅡare mainly composed of wehrlite and clino-pyroxenite, whereas UnitⅢis mainly composed of gabbro.PGE sulfide-rich layers mainly occur in Unit I, whereas thick Fe-Ti oxide-rich layers mainly occur in UnitⅢ.An ilmenite-rich layer occurs at the top of UnitⅠ.Fe-Ti oxides include magnetite and ilmenite.Small amounts of cumulus and intercumulus magnetite occur in UnitsⅠandⅡ.Cumulus magnetite grains are commonly euhedral and enclosed within olivine and clinopyroxene.They have high Cr2O3 contents ranging from 6.02 to 22.5 wt.%,indicating that they are likely an early crystallized phase from magmas.Intercumulus magnetite that usually displays ilmenite exsolution occupies the interstices between cumulus olivine crystals and coexists with interstitial clinopyroxene and plagioclase.Intercumulus magnetite has Cr2O3 ranging from 1.65 to 6.18 wt.%, lower than cumulus magnetite.The intercumulus magnetite may have crystallized from the trapped liquid.Large amounts of magnetite in UnitⅢcontains Cr2O3（＆lt;0.28 wt.%） much lower than magnetite in UnitsⅠandⅡ.The magnetite in UnitⅢis proposed to be accumulated from a Fe-Ti-rich melt.The Fe-Ti-rich melt is estimated to contain 35.9 wt.%of SiO2,26.9 wt.%of FeOt,8.2 wt.%of TiO2,13.2 wt.%of CaO, 8.3 wt.%of MgO,5.5 wt.%of Al2O3 and 1.0 wt.%of P2O5.The composition is comparable with the Fe-rich melts in the Skaergaard and Sept Iles intrusions.Paired non-reactive microstructures,granophyre pockets and ilmenite-rich intergrowths,are representative of Si-rich melt and Fe-Ti-rich melt,and are the direct evidence for the existence of an immiscible Fe-Ti-rich melt that formed from an evolved ferro-basaltic magma.展开更多
The Sichuan Basin, located in the western margin of Yangtze Plate, is one of the important oil-gas-bearing basins in China. During the Early Permian-Middle Triassic, the Sichuan Basin experienced regional lithospheric...The Sichuan Basin, located in the western margin of Yangtze Plate, is one of the important oil-gas-bearing basins in China. During the Early Permian-Middle Triassic, the Sichuan Basin experienced regional lithospheric extension and Emeishan basalt activities, both of which influenced the basin development and thermal evolution. Here we simulated the thermal effects of lithospheric extension and the Emeishan mantle plume based on different geodynamical models. Modeling results indicated that the lithospheric temperature together with the basement heat flow was generally increasing with time due to extension. As the stretching factor was relatively small, the thinning of lithosphere, and consequently the thermal disturbance, was not great. The lithospheric extension yielded about 20% increase of the basement heat flow, with maximum value of 60?62 mW m?2 in the Early Triassic. Mantle plume model shows that the thermal evolution of the inner zone above the plume head was influenced greatly by plume activity. But the outer zone and its outside area where the Sichuan Basin is located were affected only slightly. The basalts that had erupted in the southwestern basin might disturb the basin temperature significantly, although shortly and locally. Generally, the thermal history of the Sichuan basin during the Early Permian-Middle Triassic was controlled by the lithospheric extension, but locally it superimposed thermal effects of basalt activities in its southwestern area.展开更多
基金supported by the Research Grant Council of Hong Kong(HKU707012P)to MFZfrom a Chinese National "973" project (2011CB808903)+1 种基金a "CAS Hundred Talents" project under Chinese Academy of Sciences to CYWSouth African National Science Foundation Grant SA/China Project 67220 to SP and MFZ
文摘Magmatic oxide deposits in the～260 Ma Emeishan Large Igneous Province（ELIP）,SW China and northern Vietnam,are important sources of Fe,Ti and V.Some giant magmatic Fe-Ti-V oxide deposits, such as the Panzhihua,Hongge,and Baima deposits,are well described in the literature and are hosted in layered mafic-ultramafic intrusions in the Panxi region,the central ELIP.The same type of ELIP- related deposits also occur far to the south and include the Anyi deposit,about 130 km south of Panzhihua,and the Mianhuadi deposit in the Red River fault zone.The Anyi deposit is relatively small but is similarly hosted in a layered mafic intrusion.The Mianhuadi deposit has a zircon U-Pb age of～260 Ma and is thus contemporaneous with the ELIP.This deposit was variably metamorphosed during the Indosinian orogeny and Red River faulting.Compositionally,magnetite of the Mianhuadi deposit contains smaller amounts of Ti and V than that of the other deposits,possibly attributable to the later metamorphism.The distribution of the oxide ore deposits is not related to the domal structure of the ELIP.One major feature of all the oxide deposits in the ELIP is the spatial association of oxide-bearing gabbroic intrusions,syenitic plutons and high-Ti flood basalts.Thus,we propose that magmas from a mantle plume were emplaced into a shallow magma chamber where they were evolved into a field of liquid immiscibility to form two silicate liquids,one with an extremely Fe-Ti-rich gabbroic composition and the other syenitic.An immiscible Fe-Ti-（P） oxide melt may then separate from the mafic magmas to form oxide deposits.The parental magmas from which these deposits formed were likely Fe-Ti-rich picritic in composition and were derived from enriched asthenospheric mantle at a greater depth than the magmas that produced sulfide-bearing intrusions of the ELIP.
基金supported by the National Basic Research Program of China(Grant No.2011CB808904)the National Natural Science Foundation of China(Grants Nos.41274070,41474068)
文摘The Emeishan large igneous province(ELIP) in SW China is interpreted to be associated with an ancient mantle plume. Most of the constraints on the role of mantle plume in the generation of the Emeishan flood basalts were provided by geological and geochemical methods, but the geophysical investigation is very limited. In order to better understand the deep structure and features of ELIP, we have studied the crustal velocity structure using the data acquired from the Lijiang-Panzhihua-Qingzhen wide-angle seismic profile. This profile crosses the three sub-zones of the ELIP(the inner, intermediate, and outer zones), divided based on the differential erosion and uplift of the Maokou limestone. The results provided by the active source seismic experiment demonstrate:(1) The average depth of the crystalline basement along the profile is about 2 km.(2) The middle crust in the Inner Zone is characterized by high-velocity anomalies, with the average velocity of 6.2-6.6 km/s, which is about 0.1– 0.2 km/s higher than the normal one. The velocity of the lower crust in the inner zone is 6.9-7.2 km/s, higher than those observed in the intermediate and outer zones(6.7-7.0 km/s). Relatively low velocity anomalies appear in the upper, middle and lower crusts near the junction of the inner zone and intermediate zone, probably due to the effect of the Xiaojiang fault(XJF).(3) The average velocity of the crust is comparatively low on both sides of XJF, especially on the east side, and the average velocity of the consolidated continental crust is also low there. This may suggest that the XJF extends at least down to 40 km deep, even beyond through the crust.(4) The depth to the Moho discontinuity decrease gradually from 47-53 km in the inner zone, via 42-50 km in the intermediate zone to 38-42 km in the outer zone. In the inner zone, the Moho uplifts locally and the(consolidated) crust is characterized by high-velocity anomalies, which are likely related to intensive magma intrusion and underplating associated with melting of plum
文摘Based on the temporal-spatial distribution and geochemical characteristics,the Emeishan basalts can be divided into two types: high-P_2O-TiO_2 basalt (HPT) andlow-P_2O_5-TiO_2 basalt (LPT), which differ distinctly in geochemistry: the LPTs are characterizedby relatively high abundances of MgO, total FeO and P_2O_5 and compatible elements (Cr, Ni, Sc), andrelatively low contents of moderately compatible elements (V, Y, Yb, Co), LREE and otherincompatible elements compared with the HPT. On the diagrams of trace element ratios, they areplotted on an approximately linear mixing line between depleted and enriched mantle sources,suggesting that these two types of basalts resulted from interactions of varying degrees betweenmantle plume and lithospheric mantle containing such volatile-rich minerals as amphibole andapatite. The source region of the LPT involves a smaller proportion of lithospheric components,while that of the HTP has a larger proportion of lithospheric components. Trachyte is generated bypartial melting of the basic igneous rocks at the base of the lower continental crust. Both the twotypes of magmas underwent certain crystal fractionation and contamination of the lower crest athigh-level magma chambers and en route to the surface.
基金supported by National Basic Research Program of China (Grant No. 2005CB422101)
文摘Lithologically, two kinds of chert can be recognized in the Middle-Upper Permian from the Tieqiao section in Laibin area, Guangxi, i.e., calcic chert occurring mainly in the Maokou Formation and pure chert mainly in the Wujiaping Formation. Geochemical data show that both kinds of chert contain very low A1203 （0-0.23%） and TiO2 （0.001%-0.024%） and low ZREE （0.55-19.94 ppm, averaging 9.97 ppm）, as well as high ratio of Fe2O3/TiO2 （17-443, averaging 111） and low ratio of A1203/（Al2O3＋Fe203） （0-0.26, averaging 0.09）. Given that the average value Ce anomalies in chert deposited in the ridge-proximal environment is about 0.29, and that in the pelagic environment is about 0.60, the low Ce anomalies in the Tieqiao chert （0.24-0.46, averaging 0.35） imply that they were deposited in an ocean basin with influence of submarine hydrothermal fluid and no input of terrigenous materials. The vertical variation of the silica abundance in strata （SAIS） in the Middle-Upper Permian strata, together with the Eu anomalies and the ratios of ∑REE/Fe, indicates a relationship between the hydrothermal activity and the Emeishan basalt eruption, and that submarine hydrothermal activity was stronger in the Upper Permian （the Wujiapingian Stage） than in the Middle Permian （the Maokou Stage）.
基金supported by Key Research and Development Project of the Ministry of Science and Technology(Grant No.2018YFC1503400)China Earthquake Science Experiment Project,China Earthquake Administration(Grant No.2018CSES0101)。
文摘Southeastern Tibet,which has complex topography and strong tectonic activity,is an important area for studying the subsurface deformation of the Tibetan Plateau.Through the two-station method on 10-year teleseismic Rayleigh wave data from 132 permanent stations in the southeastern Tibetan Plateau,which incorporates ambient noise data,we obtain the interstation phase velocity dispersion data in the period range of 5–150s.Then,we invert for the shear wave velocity of the crust and upper mantle through the direct 3-D inversion method.We find two low-velocity belts in the mid-lower crust.One belt is mainly in the SongPan-GangZi block and northwestern part of the Chuan-Dian diamond block,whereas the other belt is mainly in the Xiaojiang fault zone and its eastern part,the Yunnan-Guizhou Plateau.The low-velocity belt in the Xiaojiang fault zone is likely caused by plastic deformation or partial melting of felsic rocks due to crustal thickening.Moreover,the significant positive radial anisotropy(VSH>VSV)around the Xiaojiang fault zone further enhances the amplitude of low velocity anomaly in our VSVmodel.This crustal low-velocity zone also extends southward across the Red River fault and farther to northern Vietnam,which may be closely related to heat sources in the upper mantle.The two low-velocity belts are separated by a high-velocity zone near the Anninghe-Zemuhe fault system,which is exactly in the inner and intermediate zones of the Emeishan large igneous province(ELIP).We find an obvious high-velocity body situated in the crust of the inner zone of the ELIP,which may represent maficultramafic material that remained in the crust when the ELIP formed.In the upper mantle,there is a large-scale low-velocity anomaly in the Indochina and South China blocks south of the Red River fault.The low-velocity anomaly gradually extends northward along the Xiaojiang fault zone into the Yangtze Craton as depth increases.Through our velocity model,we think that southeastern Tibet is undergoing three different tectonic modes at t
文摘There existed intense Cu anomaly on the northeastern side of the geochemical bound-ary with NW strike in the border area between the Yunnan and Guizhou provinces. Through field observation, ore bodies of high-grade native coppers have been found. The copper mineralization was constrained by the ancient volcanic vents of Permian basalt eruption and the overlain strata of carbonaceous argillites. Native coppers with flaky, net veined and impregnated occurrences, fine-grained tenorites and massive chalcocites widely occur in volcanic breccias, tuffs, carbona-ceous-siliceous argillites and siliceous bitumen rocks with bed thickness of about 15—80 m. Cu contents vary from 0.5% to 20%. The copper mineralization was tightly related to actino-lite-tremolitization, zeolitization and bituminization and involved in extensive reduction environ-ments. Continental flood basalts erupted in mantle plume environments usually have high Cu concentrations (~170 ?0-6 in the Emeishan basalts), which provided a copper source of minerali-zation. Thus, metallogenesis of the native copper deposits in the Yunnan-Guizhou border area is tightly associated with intensive crust-mantle and organic-inorganic interactions. The tremolitiza-tion and chalcocitization indicate that the metallogenic temperatures are in a range of 400—100℃. The geologic background and characteristics of ore and alteration for the native copper deposits in this area are somewhat similar to those of the Keweenawan native copper deposit in Michigan, USA.
文摘The Late Permian Emeishan Large Igneous Province (ELIP) is commonly regarded as being located in the western part of the Yangtze craton, SW China, with an asymmetrical shape and a small area. This area, however, is just a maximum estimation because some parts of the ELIP were not recognized or dismembered and destroyed during the Triassic to Cenozoic tectonism. In this paper, the chemostratigraphical data of the Zongza block, the Garze-Litang belt and the Songpan-Garze block suggest that the Late Permian basalts in these areas have remarkable similarities to the ELIP basalts in petrography and geochemistry. Flood basalts in the Sanjiangkou area are composed of the lower part of the low-Ti (LT) tholeiite and the upper part of the high-Ti (HT) tholeiite, which is the same as the flood basalts on the western margin of the Yangtze craton. Flood basalts in the Zongza and Songpan-Garze areas, which are far from the Yangtze craton, consist of HT tholeiite only. This is the same as the flood basalts within the Yangtze craton. Therefore we argue that these contemporary basalts all originated from the Emeishan mantle plume, and the ELIP could have a significant westward extension with an outcropped area of over 500,000 km2. This new scenario shows that the LT tholeiite occurs on the western margin of the Yangtze craton, while the HT tholeiite overlying the LT basalts occupies the whole area of the ELIP.
基金supported by a Youth Foundation from the former Ministry of GeologyMineral Resources and an 0utstanding Youth Foundation from the National Natural Science Foundation of China(grant 40425014).
文摘A suite of continental flood basalts sampled over a vast exposure and stratigraphic thickness in the Emeishan large igneous province （LIP）, SW China was investigated for laser microprobe ^40Ar/^39Ar dating. There are two ^40Ar/^39Ar age groups for these basalts, corresponding to 259-246 Ma and 177-137 Ma, respectively. A well-defined isochron gives an eruption age of huge quantities of mafic magmas at 258.9±3.4 Ma, which is identical to previous dating and paleontological data. Much younger ^40Ar/^39Ar ages for some basalts with Iow-greenschist metamorphic facies probably recorded a late thermo-tectonic event caused by collision between the Yangtze and Qiangtang continental blocks during the Mesozoic, which resulted in the reset of argon isotope system. The ^40Ar/^39Ar age data, we present here, combined with previous dating and paleontological data, suggest relatively short duration （about 3 Ma） of mafic volcanism, which have important implication on mantle plume genesis of the Emeishan continental flood basalts in the LIP.
基金supported by a National Program on Key Basic Research Project(973 Program,Grant No.2011CB808903)National Natural Science Foundation of China(Grant Nos.41073030 and 41121002)a 'CAS Hundred Talents' project under Chinese Academy of Sciences to CYW and a GIGCAS 135 project Y234041001
文摘The Xinjie layered intrusion in the Panxi region,SW China,hosts both Fe-Ti oxide and platinum-group element（PGE） sulfide mineralization.The intrusion can be divided,from the base upward,into UnitsⅠ,ⅡandⅢ,in terms of mineral assemblages.UnitsⅠandⅡare mainly composed of wehrlite and clino-pyroxenite, whereas UnitⅢis mainly composed of gabbro.PGE sulfide-rich layers mainly occur in Unit I, whereas thick Fe-Ti oxide-rich layers mainly occur in UnitⅢ.An ilmenite-rich layer occurs at the top of UnitⅠ.Fe-Ti oxides include magnetite and ilmenite.Small amounts of cumulus and intercumulus magnetite occur in UnitsⅠandⅡ.Cumulus magnetite grains are commonly euhedral and enclosed within olivine and clinopyroxene.They have high Cr2O3 contents ranging from 6.02 to 22.5 wt.%,indicating that they are likely an early crystallized phase from magmas.Intercumulus magnetite that usually displays ilmenite exsolution occupies the interstices between cumulus olivine crystals and coexists with interstitial clinopyroxene and plagioclase.Intercumulus magnetite has Cr2O3 ranging from 1.65 to 6.18 wt.%, lower than cumulus magnetite.The intercumulus magnetite may have crystallized from the trapped liquid.Large amounts of magnetite in UnitⅢcontains Cr2O3（＆lt;0.28 wt.%） much lower than magnetite in UnitsⅠandⅡ.The magnetite in UnitⅢis proposed to be accumulated from a Fe-Ti-rich melt.The Fe-Ti-rich melt is estimated to contain 35.9 wt.%of SiO2,26.9 wt.%of FeOt,8.2 wt.%of TiO2,13.2 wt.%of CaO, 8.3 wt.%of MgO,5.5 wt.%of Al2O3 and 1.0 wt.%of P2O5.The composition is comparable with the Fe-rich melts in the Skaergaard and Sept Iles intrusions.Paired non-reactive microstructures,granophyre pockets and ilmenite-rich intergrowths,are representative of Si-rich melt and Fe-Ti-rich melt,and are the direct evidence for the existence of an immiscible Fe-Ti-rich melt that formed from an evolved ferro-basaltic magma.
基金supported by Sinopec Marine Forward-looking Projects (Grant No. YPH08101)
文摘The Sichuan Basin, located in the western margin of Yangtze Plate, is one of the important oil-gas-bearing basins in China. During the Early Permian-Middle Triassic, the Sichuan Basin experienced regional lithospheric extension and Emeishan basalt activities, both of which influenced the basin development and thermal evolution. Here we simulated the thermal effects of lithospheric extension and the Emeishan mantle plume based on different geodynamical models. Modeling results indicated that the lithospheric temperature together with the basement heat flow was generally increasing with time due to extension. As the stretching factor was relatively small, the thinning of lithosphere, and consequently the thermal disturbance, was not great. The lithospheric extension yielded about 20% increase of the basement heat flow, with maximum value of 60?62 mW m?2 in the Early Triassic. Mantle plume model shows that the thermal evolution of the inner zone above the plume head was influenced greatly by plume activity. But the outer zone and its outside area where the Sichuan Basin is located were affected only slightly. The basalts that had erupted in the southwestern basin might disturb the basin temperature significantly, although shortly and locally. Generally, the thermal history of the Sichuan basin during the Early Permian-Middle Triassic was controlled by the lithospheric extension, but locally it superimposed thermal effects of basalt activities in its southwestern area.
