Reports of shoshonitic rocks in Precambrian terrains are relatively rare.Pl-Grt amphibolites and Hbl-Bt mafic granulites occurring in the migmatitic gneisses of the Chhotanagpur Gneissic Complex(CGC)show calc-alkaline...Reports of shoshonitic rocks in Precambrian terrains are relatively rare.Pl-Grt amphibolites and Hbl-Bt mafic granulites occurring in the migmatitic gneisses of the Chhotanagpur Gneissic Complex(CGC)show calc-alkaline and shoshonitic characteristics.Relict porphyritic,sub-ophitic and poikilitic textures are noted in these rocks.Their parent magma was emplaced during the waning phase of the regional metamorphism.Geochemically,these metamafics are similar to the GroupⅢpotassic and ultrapotassic rocks of Foley et al.(1987).The magma was derived from the metasomatized subcontinental lithospheric mantle(SCLM).Subduction-related sediment melts metasomatized the SCLM.Compositionally,the SCLM is a metasomatized phlogopite-amphibole-spinel-bearing harzburgite.1%–5%batch melting of the SCLM could produce the parental magma of the mafic granulites.Pressures and temperatures of metamorphic equilibration were carried out by pseudosection modeling.Peak metamorphic assemblage(M_(1):Grt-Cpx-Pl-Qz)in garnetiferous amphibolite equilibrated at 740℃and 8.7 kbar.The Cpx-Pl corona appeared around the garnet during decompression(M_(2):655℃,6 kbar).The Hbl-Pl symplectites around garnet formed during isobaric cooling(M_(3):580℃and 5.9 kbar).The emplacement of shoshonitic magma and subsequent decompression happened at the slab break-off stage of continental collision(~990 Ma).展开更多
Sea surface electric field observations off the coast from Goa (15°25'N, 73°47'E) to Madras (13°04'N, 80°15'E) around Sri Lanka, in a distance range 25-135 km from coast, during 9-2...Sea surface electric field observations off the coast from Goa (15°25'N, 73°47'E) to Madras (13°04'N, 80°15'E) around Sri Lanka, in a distance range 25-135 km from coast, during 9-20 May 1983 were taken. In this paper we have examined the diurnal variation of electric field in the Arabian Sea, Indian Ocean and Bay of Bengal regions covered during the cruise of the research ship ORV Gaveshani. An aspect of electric field dependence on coastal distance and Aitken Nuclei concentration has also been studied. An attempt to examine the latitude dependence of field was also made. Results obtained in the above studies are presented and compared with those obtained elsewhere.展开更多
The sub-continental lithosphere mantle(SCLM)represents a mechanical barrier that separate the convecting mantle from the exosphere,and possesses the ability to sequester and release volatiles,making it an integral par...The sub-continental lithosphere mantle(SCLM)represents a mechanical barrier that separate the convecting mantle from the exosphere,and possesses the ability to sequester and release volatiles,making it an integral part in deep volatile cycling.Here we explore the SCLM in modulating F and Cl cycling through a case study from the Early Cretaceous Dabeigou basalts in the North China Craton(NCC).These lavas are associated with decratonization of the NCC,a process involving significant removal and replacement of the ancient SCLM.The Dabeigou basalts,characterized by arc magma-like trace element signatures,evolved initial^(87)Sr/^(86)Sr(0.70574–0.70578),ε_(Nd)(t)(-11.2–-11.1),andε_(Hf)(t)(-13.7–-13.4)values,along with high Fe/Mn ratios,Ni contents,andδ^(18)O values(5.8‰–6.8‰)of olivine phenocrysts,are compatible with their origination from a veined,pyroxene-rich source formed through the infiltration and interaction of hydrous silicate melts from the subducting Pacific slabs with ambient SCLM.Olivine-hosted melt inclusions of the Dabeigou basalts have compositions similar to the whole rocks,indicating that they record pre-eruptive melt compositions forming the whole rocks.The melt inclusions exhibit high F(1371–2316 ppm)and moderate Cl(688–900 ppm)contents,with high F/Cl ratios of 2.1±0.3(1σ).The F and Cl signatures of the Dabeigou basalts are different from those of typical arc magmas,which are commonly characterized by high Cl contents(median of~1200 ppm)and low F/Cl ratios(<1).The observed differences can be attributed to chromatographic reaction that fractionates elements of different compatibility in the metasomatic agents,with amphibole as the main mineral phase for fractionating and preserving F and Cl.That is,amphibole crystallization prefers to retain F rather than Cl,making the metasomes in vein enriched in F and exhibiting elevated F/Cl ratios compared to original metasomatic agents.It is further suggested that the scenario inferred from the Dabeigou basalts might be common for crystallization of hydrous minerals during chromatographic metasomatism,which causes significant heterogeneity with respect to F and Cl in the SCLM,and might have played a crucial role in regulating F and Cl cycling within the SCLM environments.展开更多
A series of Cenozoic potassium-rich volcanic rocks developed in the Xiaoguli-Keluo-Wudalianchi-Erkeshan districts,northeast China.The source region and potassium-rich mechanism of the potassic rocks remain highly disp...A series of Cenozoic potassium-rich volcanic rocks developed in the Xiaoguli-Keluo-Wudalianchi-Erkeshan districts,northeast China.The source region and potassium-rich mechanism of the potassic rocks remain highly disputed.In this paper,the major elements,trace elements,and Sr-Nd-Pb isotopes of the volcanic rocks in Keluo(KL)and Wudalianchi(WDLC)volcanic districts were analyzed systematically.The results show that the volcanic rocks are characterized by high K2O(4.36wt.%-6.13wt.%),remarkable enrichment in LREEs and LILEs,as well as the strong fractionation of HREEs.The isotopic characteristics with high 87Sr/86Sr(0.704990-0.705272),low 143Nd/144Nd(0.512306-0.512417),low 206Pb/204Pb(16.546-17.135)and 207Pb/204Pb(15.002-15.783)of the volcanic rocks suggest the involvement of EM-I-type mantle.On the basis of the geochemical characteristics,the potassium-rich volcanic magma originated from the new SCLM forming after delamination of the ancient SCLM,with metasomatism of the potassium-rich fluids released from the ancient lower crust during the Late Mesozoic.The proposed genetic model assumes the source which represented by a phlogopite-bearing garnet peridotite(with modal garnet in the range of 2%-10%)experienced very low degrees(i.e.,~0.5)of partial melting.During Cenozoic,the lithosphere in northeast China was affected by the extension and decompression of continental rift,and the metasomatized SCLM underwent low degree partial melting,resulting in the formation of potassium-rich primitive basaltic magma.展开更多
The North China Craton(NCC) represents one of the oldest and largest cratons in the earth with a nearly complete record of Precambrian history. In the northern part of the NCC, the earliest phase of alkaline magmatism...The North China Craton(NCC) represents one of the oldest and largest cratons in the earth with a nearly complete record of Precambrian history. In the northern part of the NCC, the earliest phase of alkaline magmatism occurred in discrete pulses in the Early and Middle Devonian;whereas the next episode of alkaline magmatism took place in the early Mesozoic. The Gucheng pluton is exposed in the northern part of the NCC and forms a composite intrusion, consisting of K-feldspar–bearing clinopyroxenite, clinopyroxene–bearing syenite and alkali-feldspar syenite. Mineral phases in these lithologies include clinopyroxene(Wo43-48En19-35Fs18-38), sanidine(An0 Ab3-11Or89-97), and subordinate titanite, andradite and Na-feldspar. These rocks show homogeneous Sr but variable Nd isotopic compositions, and have relatively high zircon in-situ oxygen isotopes(δ18O=5.2–6.7). The Gucheng plutonic rocks formed through fractional crystallization and accumulation from ultrapotassic magmas, which were originated from partial melting of metasomatic vein systems in the subcontinental lithospheric mantle of the NCC. These vein networks developed as a result of the reactions of fluids derived from subducted pelitic sediments on the downgoing Palaeo-Asian ocean floor with the enriched, subcontinental lithospheric mantle peridotites. SHRIMP U-Pb zircon dating has revealed a crystallization age of 415 Ma for the timing of the emplacement of the Gucheng pluton that marks the early stages of alkaline magmatism associated with the Andean-type continental margin evolution along the northern edge of the NCC facing the Palaeo-Asian Ocean.展开更多
基金partly funded by the UGC-UPE-II given to the University of Calcutta under the Non-focus Area ProgrammeThe Research Fellowship in Sciences for meritorious students(RFSMS)of the University Grants Commission,Government of India,awarded to Susmita Das(No.F.747/2007 BSR)。
文摘Reports of shoshonitic rocks in Precambrian terrains are relatively rare.Pl-Grt amphibolites and Hbl-Bt mafic granulites occurring in the migmatitic gneisses of the Chhotanagpur Gneissic Complex(CGC)show calc-alkaline and shoshonitic characteristics.Relict porphyritic,sub-ophitic and poikilitic textures are noted in these rocks.Their parent magma was emplaced during the waning phase of the regional metamorphism.Geochemically,these metamafics are similar to the GroupⅢpotassic and ultrapotassic rocks of Foley et al.(1987).The magma was derived from the metasomatized subcontinental lithospheric mantle(SCLM).Subduction-related sediment melts metasomatized the SCLM.Compositionally,the SCLM is a metasomatized phlogopite-amphibole-spinel-bearing harzburgite.1%–5%batch melting of the SCLM could produce the parental magma of the mafic granulites.Pressures and temperatures of metamorphic equilibration were carried out by pseudosection modeling.Peak metamorphic assemblage(M_(1):Grt-Cpx-Pl-Qz)in garnetiferous amphibolite equilibrated at 740℃and 8.7 kbar.The Cpx-Pl corona appeared around the garnet during decompression(M_(2):655℃,6 kbar).The Hbl-Pl symplectites around garnet formed during isobaric cooling(M_(3):580℃and 5.9 kbar).The emplacement of shoshonitic magma and subsequent decompression happened at the slab break-off stage of continental collision(~990 Ma).
文摘Sea surface electric field observations off the coast from Goa (15°25'N, 73°47'E) to Madras (13°04'N, 80°15'E) around Sri Lanka, in a distance range 25-135 km from coast, during 9-20 May 1983 were taken. In this paper we have examined the diurnal variation of electric field in the Arabian Sea, Indian Ocean and Bay of Bengal regions covered during the cruise of the research ship ORV Gaveshani. An aspect of electric field dependence on coastal distance and Aitken Nuclei concentration has also been studied. An attempt to examine the latitude dependence of field was also made. Results obtained in the above studies are presented and compared with those obtained elsewhere.
基金supported by the National Natural Science Foundation of China(Grant Nos.42288201,42003029,42322203,42303027,and 42073034)。
文摘The sub-continental lithosphere mantle(SCLM)represents a mechanical barrier that separate the convecting mantle from the exosphere,and possesses the ability to sequester and release volatiles,making it an integral part in deep volatile cycling.Here we explore the SCLM in modulating F and Cl cycling through a case study from the Early Cretaceous Dabeigou basalts in the North China Craton(NCC).These lavas are associated with decratonization of the NCC,a process involving significant removal and replacement of the ancient SCLM.The Dabeigou basalts,characterized by arc magma-like trace element signatures,evolved initial^(87)Sr/^(86)Sr(0.70574–0.70578),ε_(Nd)(t)(-11.2–-11.1),andε_(Hf)(t)(-13.7–-13.4)values,along with high Fe/Mn ratios,Ni contents,andδ^(18)O values(5.8‰–6.8‰)of olivine phenocrysts,are compatible with their origination from a veined,pyroxene-rich source formed through the infiltration and interaction of hydrous silicate melts from the subducting Pacific slabs with ambient SCLM.Olivine-hosted melt inclusions of the Dabeigou basalts have compositions similar to the whole rocks,indicating that they record pre-eruptive melt compositions forming the whole rocks.The melt inclusions exhibit high F(1371–2316 ppm)and moderate Cl(688–900 ppm)contents,with high F/Cl ratios of 2.1±0.3(1σ).The F and Cl signatures of the Dabeigou basalts are different from those of typical arc magmas,which are commonly characterized by high Cl contents(median of~1200 ppm)and low F/Cl ratios(<1).The observed differences can be attributed to chromatographic reaction that fractionates elements of different compatibility in the metasomatic agents,with amphibole as the main mineral phase for fractionating and preserving F and Cl.That is,amphibole crystallization prefers to retain F rather than Cl,making the metasomes in vein enriched in F and exhibiting elevated F/Cl ratios compared to original metasomatic agents.It is further suggested that the scenario inferred from the Dabeigou basalts might be common for crystallization of hydrous minerals during chromatographic metasomatism,which causes significant heterogeneity with respect to F and Cl in the SCLM,and might have played a crucial role in regulating F and Cl cycling within the SCLM environments.
基金supported by the Marine S&T Fund of Shandong Province for Pilot National Laboratory for Marine Science and Technology(Qingdao)(No.2021QNLM020001-1)Natural Science Foundation of Shandong Province(No.ZR2021MD083)+1 种基金National Natural Science Foundation of China(Grant Nos.41302102 and 41772190)Graduate Innovation Project of China University of Petroleum(East China)(No.YCX2021020).
文摘A series of Cenozoic potassium-rich volcanic rocks developed in the Xiaoguli-Keluo-Wudalianchi-Erkeshan districts,northeast China.The source region and potassium-rich mechanism of the potassic rocks remain highly disputed.In this paper,the major elements,trace elements,and Sr-Nd-Pb isotopes of the volcanic rocks in Keluo(KL)and Wudalianchi(WDLC)volcanic districts were analyzed systematically.The results show that the volcanic rocks are characterized by high K2O(4.36wt.%-6.13wt.%),remarkable enrichment in LREEs and LILEs,as well as the strong fractionation of HREEs.The isotopic characteristics with high 87Sr/86Sr(0.704990-0.705272),low 143Nd/144Nd(0.512306-0.512417),low 206Pb/204Pb(16.546-17.135)and 207Pb/204Pb(15.002-15.783)of the volcanic rocks suggest the involvement of EM-I-type mantle.On the basis of the geochemical characteristics,the potassium-rich volcanic magma originated from the new SCLM forming after delamination of the ancient SCLM,with metasomatism of the potassium-rich fluids released from the ancient lower crust during the Late Mesozoic.The proposed genetic model assumes the source which represented by a phlogopite-bearing garnet peridotite(with modal garnet in the range of 2%-10%)experienced very low degrees(i.e.,~0.5)of partial melting.During Cenozoic,the lithosphere in northeast China was affected by the extension and decompression of continental rift,and the metasomatized SCLM underwent low degree partial melting,resulting in the formation of potassium-rich primitive basaltic magma.
基金financially supported by grants from the Nature Science Foundation of China (Grant Nos. 416720634177302941302038 and 41720104009)
文摘The North China Craton(NCC) represents one of the oldest and largest cratons in the earth with a nearly complete record of Precambrian history. In the northern part of the NCC, the earliest phase of alkaline magmatism occurred in discrete pulses in the Early and Middle Devonian;whereas the next episode of alkaline magmatism took place in the early Mesozoic. The Gucheng pluton is exposed in the northern part of the NCC and forms a composite intrusion, consisting of K-feldspar–bearing clinopyroxenite, clinopyroxene–bearing syenite and alkali-feldspar syenite. Mineral phases in these lithologies include clinopyroxene(Wo43-48En19-35Fs18-38), sanidine(An0 Ab3-11Or89-97), and subordinate titanite, andradite and Na-feldspar. These rocks show homogeneous Sr but variable Nd isotopic compositions, and have relatively high zircon in-situ oxygen isotopes(δ18O=5.2–6.7). The Gucheng plutonic rocks formed through fractional crystallization and accumulation from ultrapotassic magmas, which were originated from partial melting of metasomatic vein systems in the subcontinental lithospheric mantle of the NCC. These vein networks developed as a result of the reactions of fluids derived from subducted pelitic sediments on the downgoing Palaeo-Asian ocean floor with the enriched, subcontinental lithospheric mantle peridotites. SHRIMP U-Pb zircon dating has revealed a crystallization age of 415 Ma for the timing of the emplacement of the Gucheng pluton that marks the early stages of alkaline magmatism associated with the Andean-type continental margin evolution along the northern edge of the NCC facing the Palaeo-Asian Ocean.
