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
Geochemical comparison of two suites of Cenozoic potassic volcanic rocks in western Yunnan reveals the existence of two mantle domains beneath this region, which corre- spond to their respective tectonic affinity. The...Geochemical comparison of two suites of Cenozoic potassic volcanic rocks in western Yunnan reveals the existence of two mantle domains beneath this region, which corre- spond to their respective tectonic affinity. The Erhai ultrapotassic rocks (42—24Ma) in western Yangtze Craton are characterized by LILE enrichment, HFSE depletion, low TiO2 content (<1%), high initial Sr/86Sr (0.7064—0.7094) and negative εNd (?3.84—?4.64). Geochemically similar to 87 K-rich volcanism in subduction setting, they were probably originated from a refractory spinel harzburgitic source metasomatized by subduction-related fluids. In contrast, the Maguan potas- sic magmas (<16Ma) in the South China Block show an OIB-type trace elemental signature, high TiO2 content (>2%), low initial 87 Sr/86Sr (0.7041—0.7060) and positive εNd (5.46—7.03). These features resemble the typical intraplate potassic rocks around the world. These rocks are thus interpreted as melting products of a fertile garnet lherzolitic source which has been infiltrated by small-volume, asthenosphere-derived silicate melts. The temporal and spatial distribution of these two types of K-rich rocks cannot be explained by any unified tectonic model. It is proposed that the Oligocene magmatism in the Erhai area may have resulted from convective thinning of the thickened lithosphere, whereas the post-Miocene volcanism in the Maguan area was related to the opening of South China Sea.展开更多
Water has become an effective means to trace the mantle source of basaltic magmas recently. To investigate the source of the potassium-rich component in the Xiaogulihe ultrapotassic volcanic rocks of NE China, we meas...Water has become an effective means to trace the mantle source of basaltic magmas recently. To investigate the source of the potassium-rich component in the Xiaogulihe ultrapotassic volcanic rocks of NE China, we measured the water content of clinopyroxene （cpx） phenocrysts by Fourier transform infrared spectrometry and calculated the H2O content of the equilibrated melts according to the partition coefficients of H2O between cpx phenocrysts and basaltic melts. The estimated H2O content of the ＂primary＂ magmas is low （0.36 wt%-0.50 wt%）, within the range of mid-ocean ridge basalts and ocean island basalts, while it is obviously lower than that of backarc basin basalts and island arc basalts. The calculated H20/Ce ratio of the ＂primary＂ magmas is about 15, which might be similar to that of the dehydrated sediments （〈100）, but observably lower than that of the normal depleted mantle （DMM, H2O/Ce = 150-210）. The low water content and especially low H2O/Ce ratio of the ＂primary＂ magmas demonstrate that the K-rich component of these ultrapotassic volcanic rocks seems unlikely tooriginate from K-bearing hydrous minerals （such as phlogopite） in metasomatic subcontinental lithospheric mantle. Combined with the low 206pb/204pb ratios, moderately high 87Sr/86Sr ratios of the bulk rocks and the high δ18O values of olivine phenocrysts, we suggest that the K-rich component in the mantle source of the Xiaogulihe ultrapotassic volcanic rocks may come from ancient continental-derived sediments which dehydrated significantly during subduction （e.g., dry K-hollandite）.展开更多
Baotoudong syenite pluton is located to the east of Baotou City, Inner Mongolia, the westernmost part of the Trias- sic alkaline magmatic belt along the northern margin of the North China Craton （NCC）. Zircon U-Pb a...Baotoudong syenite pluton is located to the east of Baotou City, Inner Mongolia, the westernmost part of the Trias- sic alkaline magmatic belt along the northern margin of the North China Craton （NCC）. Zircon U-Pb age, petrological, miner- alogical and geochemical data of the pluton were obtained in this paper, to constrain its origin and mantle source characteris- tics. The pluton is composed of nepheline-clinopyroxene syenite and alkali-feldspar syenite, with zircon U-Pb age of 214.7±1.1 Ma. Diopside （cores）-aegirine-augite （rims）, biotite, orthoclase and nepheline are the major minerals. The Bao- toudong syenites have high contents of rare earth elements （REE）, and are characterized by enrichment in light rare earth ele- ments （LREE） and large ion lithophile elements （LILE; e.g., Rb, Ba, Sr）, depletion in heavy rare earth elements （HREE） and high field strength elements （HFSE）. They show enriched Sr-Nd isotopic compositions with initial 87Sr/86Sr ranging from 0.7061 to 0.7067 and eNd（t） values from -9.0 to -11.2. Mineralogy, petrology and geochemical studies show that the parental magma of the syenites is SiO2-undersaturated potassic-ultrapotassic, and is characterized by high contents of CaO, Fe2O3, K2O, Na2O and fluid compositions （H2O）, and by high temperature and high oxygen fugacity. The syenites were originated from a phlogopite-rich, enriched lithospheric mantle source in garnet-stable area （〉80 km）. The occurrence of the Baotoudong sye- nites, together with many other ultrapotassic, alkaline complexes of similar ages on the northern margin of the NCC in Late Triassic implies that the lithospheric mantle beneath the northern margin of the NCC was previously metasomatized by melts/fluids from the subducted, altered paleo-Mongolian oceanic crust, and the northern margin of the craton has entered into an extensively extensional regime as a destructive continental margin in Late Triassic.展开更多
The Yaojiazhuang ultramafic-syenitic complex is one of the representative Triassic alkaline plutons on the northern margin of the North China Craton （NCC）. Based on detailed study of the zircon U-Pb age, petrologica...The Yaojiazhuang ultramafic-syenitic complex is one of the representative Triassic alkaline plutons on the northern margin of the North China Craton （NCC）. Based on detailed study of the zircon U-Pb age, petrological, mineralogical, and geochemical data of the complex, the characteristics of the magmas system, the petrogenesis of different rock types, and the nature of the mantle source were discussed to provide new constraints on the origin and tectonic setting of the Triassic alkaline belt. Cumu- lus ultramafic rocks, clinopyroxene-syenites and syenites are the main rock types of the complex. The zircons from the sye- nites yielded a U-Pb age of 209 Ma. Diopside-augite, biotite, and sanidine-orthoclase are the major minerals, with subordinate apatite and magnetite. Rocks from the complex are enriched in large ion lithophile elements （LILE） and light rare earth ele- ments （LREE）, depleted in high field strength elements （HFSE） and heavy rare earth elements （HREE）, and the initial 878r]86Sr ranges from 0.7057 to 0.7061 and eNd（t） from -9.4 to -11.4. Mineralogy and geochemical data demonstrate that the parent magma of the complex is SiO2-undersaturated ultrapotassic alkaline-peralkaline, and is characterized by high CaO content and fluid compositions （P205, CO2, H20）, and by high oxygen fugacity and high temperature. The complex was originated from a phlogopite-clinopyroxenite-rich lithospheric mantle source in the garnet-stable area （〉 80 km） that had previously been meta- somatized by melts/fluids from altered oceanic crust. The parent magma has been contaminated by little ancient TTG gneisses during magma emplacement. The development of the Yaojiazhuang complex indicates that the northern margin of the NCC has entered into an extensively extensional regime in the Late Triassic.展开更多
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.
基金supported by the National Natural Science Foundation of China(Grant No.49925308)the Chinese Academy of Sciences(Gramt Nos.KZCX2-SW-117,KZCX2-101 and KZCX2-209).
文摘Geochemical comparison of two suites of Cenozoic potassic volcanic rocks in western Yunnan reveals the existence of two mantle domains beneath this region, which corre- spond to their respective tectonic affinity. The Erhai ultrapotassic rocks (42—24Ma) in western Yangtze Craton are characterized by LILE enrichment, HFSE depletion, low TiO2 content (<1%), high initial Sr/86Sr (0.7064—0.7094) and negative εNd (?3.84—?4.64). Geochemically similar to 87 K-rich volcanism in subduction setting, they were probably originated from a refractory spinel harzburgitic source metasomatized by subduction-related fluids. In contrast, the Maguan potas- sic magmas (<16Ma) in the South China Block show an OIB-type trace elemental signature, high TiO2 content (>2%), low initial 87 Sr/86Sr (0.7041—0.7060) and positive εNd (5.46—7.03). These features resemble the typical intraplate potassic rocks around the world. These rocks are thus interpreted as melting products of a fertile garnet lherzolitic source which has been infiltrated by small-volume, asthenosphere-derived silicate melts. The temporal and spatial distribution of these two types of K-rich rocks cannot be explained by any unified tectonic model. It is proposed that the Oligocene magmatism in the Erhai area may have resulted from convective thinning of the thickened lithosphere, whereas the post-Miocene volcanism in the Maguan area was related to the opening of South China Sea.
基金supported by the National Natural Science Foundation of China(41225005)the Research Fund for the Doctoral Program of Higher Education of China(RFDP20113402130001)
文摘Water has become an effective means to trace the mantle source of basaltic magmas recently. To investigate the source of the potassium-rich component in the Xiaogulihe ultrapotassic volcanic rocks of NE China, we measured the water content of clinopyroxene （cpx） phenocrysts by Fourier transform infrared spectrometry and calculated the H2O content of the equilibrated melts according to the partition coefficients of H2O between cpx phenocrysts and basaltic melts. The estimated H2O content of the ＂primary＂ magmas is low （0.36 wt%-0.50 wt%）, within the range of mid-ocean ridge basalts and ocean island basalts, while it is obviously lower than that of backarc basin basalts and island arc basalts. The calculated H20/Ce ratio of the ＂primary＂ magmas is about 15, which might be similar to that of the dehydrated sediments （〈100）, but observably lower than that of the normal depleted mantle （DMM, H2O/Ce = 150-210）. The low water content and especially low H2O/Ce ratio of the ＂primary＂ magmas demonstrate that the K-rich component of these ultrapotassic volcanic rocks seems unlikely tooriginate from K-bearing hydrous minerals （such as phlogopite） in metasomatic subcontinental lithospheric mantle. Combined with the low 206pb/204pb ratios, moderately high 87Sr/86Sr ratios of the bulk rocks and the high δ18O values of olivine phenocrysts, we suggest that the K-rich component in the mantle source of the Xiaogulihe ultrapotassic volcanic rocks may come from ancient continental-derived sediments which dehydrated significantly during subduction （e.g., dry K-hollandite）.
基金the National Natural Science Foundation of China (Grant No. 41302038) Institute of Geology, Chinese Academy of Geological Sciences (Grant No. J1205)
文摘Baotoudong syenite pluton is located to the east of Baotou City, Inner Mongolia, the westernmost part of the Trias- sic alkaline magmatic belt along the northern margin of the North China Craton （NCC）. Zircon U-Pb age, petrological, miner- alogical and geochemical data of the pluton were obtained in this paper, to constrain its origin and mantle source characteris- tics. The pluton is composed of nepheline-clinopyroxene syenite and alkali-feldspar syenite, with zircon U-Pb age of 214.7±1.1 Ma. Diopside （cores）-aegirine-augite （rims）, biotite, orthoclase and nepheline are the major minerals. The Bao- toudong syenites have high contents of rare earth elements （REE）, and are characterized by enrichment in light rare earth ele- ments （LREE） and large ion lithophile elements （LILE; e.g., Rb, Ba, Sr）, depletion in heavy rare earth elements （HREE） and high field strength elements （HFSE）. They show enriched Sr-Nd isotopic compositions with initial 87Sr/86Sr ranging from 0.7061 to 0.7067 and eNd（t） values from -9.0 to -11.2. Mineralogy, petrology and geochemical studies show that the parental magma of the syenites is SiO2-undersaturated potassic-ultrapotassic, and is characterized by high contents of CaO, Fe2O3, K2O, Na2O and fluid compositions （H2O）, and by high temperature and high oxygen fugacity. The syenites were originated from a phlogopite-rich, enriched lithospheric mantle source in garnet-stable area （〉80 km）. The occurrence of the Baotoudong sye- nites, together with many other ultrapotassic, alkaline complexes of similar ages on the northern margin of the NCC in Late Triassic implies that the lithospheric mantle beneath the northern margin of the NCC was previously metasomatized by melts/fluids from the subducted, altered paleo-Mongolian oceanic crust, and the northern margin of the craton has entered into an extensively extensional regime as a destructive continental margin in Late Triassic.
基金supported by a "Tianshan Scholar" grant from Xinjiang University to Chen Bin
文摘The Yaojiazhuang ultramafic-syenitic complex is one of the representative Triassic alkaline plutons on the northern margin of the North China Craton （NCC）. Based on detailed study of the zircon U-Pb age, petrological, mineralogical, and geochemical data of the complex, the characteristics of the magmas system, the petrogenesis of different rock types, and the nature of the mantle source were discussed to provide new constraints on the origin and tectonic setting of the Triassic alkaline belt. Cumu- lus ultramafic rocks, clinopyroxene-syenites and syenites are the main rock types of the complex. The zircons from the sye- nites yielded a U-Pb age of 209 Ma. Diopside-augite, biotite, and sanidine-orthoclase are the major minerals, with subordinate apatite and magnetite. Rocks from the complex are enriched in large ion lithophile elements （LILE） and light rare earth ele- ments （LREE）, depleted in high field strength elements （HFSE） and heavy rare earth elements （HREE）, and the initial 878r]86Sr ranges from 0.7057 to 0.7061 and eNd（t） from -9.4 to -11.4. Mineralogy and geochemical data demonstrate that the parent magma of the complex is SiO2-undersaturated ultrapotassic alkaline-peralkaline, and is characterized by high CaO content and fluid compositions （P205, CO2, H20）, and by high oxygen fugacity and high temperature. The complex was originated from a phlogopite-clinopyroxenite-rich lithospheric mantle source in the garnet-stable area （〉 80 km） that had previously been meta- somatized by melts/fluids from altered oceanic crust. The parent magma has been contaminated by little ancient TTG gneisses during magma emplacement. The development of the Yaojiazhuang complex indicates that the northern margin of the NCC has entered into an extensively extensional regime in the Late Triassic.
