To unveil formation mechanism of key sequence boundaries of inland faulting basin and coal accumulation charac- teristics of coal seams in isochrohal stratigraphic framework, sequence stratigraphy, palaeogeographic re...To unveil formation mechanism of key sequence boundaries of inland faulting basin and coal accumulation charac- teristics of coal seams in isochrohal stratigraphic framework, sequence stratigraphy, palaeogeographic recovery and other methods were used to research the sequence stratigraphy and coal accumulation in the example of Banding Basn in the west margin of Yangtze Platform, and the authors advanced a coal accumulation model of Faulting basin on the basis of accommo- dation space changes in the background of palaeogeography of sequence framework. The results show that： normal lacustrine regression and forced lacustrine regression are the main driving forces for the formation of sequence boundaries of Faulting ba- sin; basement subsidence is the main source of accommodation space of Faulting basin; and subsidence disequilibrium is the main cause for the difference in generation rate of accommodation space of Faulting basin. Coal accumulation in Faulting basin is obviously controlled by accommodation space changes in sequence framework and basin evolution. As Faulting basin evolves into depression basin, both subsidence rate of basin basement and generation rate of accommodation space decrease vertically, it appears as a progradational reverse-graded sedimentary sequence, coal accumulation in secondary sequence framework intensifies first and then weakens, and coal accumulation at the middle stage of highstand system tract is the best. During steady rifting period, minable coal seams were mostly developed in initial lacustrine flooding surface of fourth-order sequence and at the middle-late period of highstand system tract, and coal accumulation center lay in palaeogeographic unit of delta plain with moderate available accommodation space. During shrinking period, minable coal seams gradually migrated to the maximum lacustrine flooding surface and coal accumulation center lay in palaeographical unit of lacustrine with large available accommodation space.展开更多
The North China Craton(NCC) has been thinned from >200 km to <100 km in its eastern part. The ancient subcontinental lithospheric mantle(SCLM) has been replaced by the juvenile SCLM in the Meoszoic. During this ...The North China Craton(NCC) has been thinned from >200 km to <100 km in its eastern part. The ancient subcontinental lithospheric mantle(SCLM) has been replaced by the juvenile SCLM in the Meoszoic. During this period, the NCC was destructed as indicated by extensive magmatism in the Early Cretaceous. While there is a consensus on the thinning and destruction of cratonic lithosphere in North China, it has been hotly debated about the mechanism of cartonic destruction.This study attempts to provide a resolution to current debates in the view of Mesozoic mafic magmatism in North China. We made a compilation of geochemical data available for Mesozoic mafic igneous rocks in the NCC. The results indicate that these mafic igneous rocks can be categorized into two series,manifesting a dramatic change in the nature of mantle sources at ~121 Ma. Mafic igneous rocks emplaced at this age start to show both oceanic island basalts(OIB)-like trace element distribution patterns and depleted to weakly enriched Sr-Nd isotope compositions. In contrast,mafic igneous rocks emplaced before and after this age exhibit both island arc basalts(IAB)-like trace element distribution patterrs and enriched Sr-Nd isotope compositions.This difference indicates a geochemical mutation in the SCLM of North China at^121 Ma. Although mafic magmatism also took place in the Late Triassic, it was related to exhumation of the deeply subducted South China continental crust because the subduction of Paleo-Pacific slab was not operated at that time. Paleo-Pacific slab started to subduct beneath the eastern margin of Eruasian continent since the Jurrasic. The subducting slab and its overlying SCLM wedge were coupled in the Jurassic, and slab dehydration resulted in hydration and weakening of the cratonic mantle. The mantle sources of ancient IAB-like mafic igneous rocks are a kind of ultramafic metasomatites that were generated by reaction of the cratonic mantle wedge peridotite notonly with aqueous solutions derived from dehydration of the subducting Paleo-Pacific oceanic crust in the Jurassic but also with hydrous melts derived from partial melting of the subducting South China continental crust in the Triassic. On the other hand, the mantle sources of juvenile OIB-like mafic igneous rocks are also a kind of ultramafic metasomatites that were generated by reaction of the asthenospheric mantle underneath the North China lithosphere with hydrous felsic melts derived from partial melting of the subducting Paleo-Pacific oceanic crust. The subducting Paleo-Pacific slab became rollback at^144 Ma. Afterwards the SCLM base was heated by laterally filled asthenospheric mantle, leading to thinning of the hydrated and weakened cratonic mantle. There was extensive bimodal magmatism at 130 to 120 Ma, marking intensive destruction of the cratonic lithosphere. Not only the ultramafic metasomatites in the lower part of the cratonic mantle wedge underwent partial melting to produce mafic igneous rocks showing negative ε_(Nd)(t) values, depletion in Nb and Ta but enrichment in Pb, but also the lower continent crust overlying the cratonic mantle wedge was heated for extensive felsic magmatism. At the same time, the rollback slab surface was heated by the laterally filled astheno spheric mantle, resulting in partial melting of the previously dehydrated rocks beyond rutile stability on the slab surface. This produce still hydrous felsic melts, which metasomatized the overlying astheno spheric mantle peridotite to generate the ultramafic metasomatites that show positive ε_(Nd)(t) values, no depletion or even enrichment in Nb and Ta but depletion in Pb. Partial melting of such metasomatites started at^121 Ma, giving rise to the mafic igneous rocks with juvenile OIB-like geochemical signatures. In this context, the age of ~121 Ma may terminate replacement of the ancient SCLM by the juvenile SCLM in North China. Paleo-Pacific slab was not subducted to the mantle transition zone in the Mesozoic as revealed by moder seismic tomography, and it was subducted at a low angle since the Jurassic, like the subduction of Nazca Plate beneath American continent. This flat subduction would not only chemically metasomatize the cratonic mantle but also physically erode the cratonic mantle. Therefore, the interaction between Paleo-Pacific slab and the cratonic mantle is the first-order geodynamic mechanism for the thinning and destruction of cratonic lithosphere in North China.展开更多
基金Supported by the State Key Scientific and Technological Program （2011 ZX05009-002） the National Natural Science Foundation （41002049） the Central University Basic Research Fund （2009QD12）
文摘To unveil formation mechanism of key sequence boundaries of inland faulting basin and coal accumulation charac- teristics of coal seams in isochrohal stratigraphic framework, sequence stratigraphy, palaeogeographic recovery and other methods were used to research the sequence stratigraphy and coal accumulation in the example of Banding Basn in the west margin of Yangtze Platform, and the authors advanced a coal accumulation model of Faulting basin on the basis of accommo- dation space changes in the background of palaeogeography of sequence framework. The results show that： normal lacustrine regression and forced lacustrine regression are the main driving forces for the formation of sequence boundaries of Faulting ba- sin; basement subsidence is the main source of accommodation space of Faulting basin; and subsidence disequilibrium is the main cause for the difference in generation rate of accommodation space of Faulting basin. Coal accumulation in Faulting basin is obviously controlled by accommodation space changes in sequence framework and basin evolution. As Faulting basin evolves into depression basin, both subsidence rate of basin basement and generation rate of accommodation space decrease vertically, it appears as a progradational reverse-graded sedimentary sequence, coal accumulation in secondary sequence framework intensifies first and then weakens, and coal accumulation at the middle stage of highstand system tract is the best. During steady rifting period, minable coal seams were mostly developed in initial lacustrine flooding surface of fourth-order sequence and at the middle-late period of highstand system tract, and coal accumulation center lay in palaeogeographic unit of delta plain with moderate available accommodation space. During shrinking period, minable coal seams gradually migrated to the maximum lacustrine flooding surface and coal accumulation center lay in palaeographical unit of lacustrine with large available accommodation space.
基金supported by the National Key Basic Research Program of China(Grant No.2015CB856100)the National Natural Science Foundation of China(Grant No.41690620)
文摘The North China Craton(NCC) has been thinned from >200 km to <100 km in its eastern part. The ancient subcontinental lithospheric mantle(SCLM) has been replaced by the juvenile SCLM in the Meoszoic. During this period, the NCC was destructed as indicated by extensive magmatism in the Early Cretaceous. While there is a consensus on the thinning and destruction of cratonic lithosphere in North China, it has been hotly debated about the mechanism of cartonic destruction.This study attempts to provide a resolution to current debates in the view of Mesozoic mafic magmatism in North China. We made a compilation of geochemical data available for Mesozoic mafic igneous rocks in the NCC. The results indicate that these mafic igneous rocks can be categorized into two series,manifesting a dramatic change in the nature of mantle sources at ~121 Ma. Mafic igneous rocks emplaced at this age start to show both oceanic island basalts(OIB)-like trace element distribution patterns and depleted to weakly enriched Sr-Nd isotope compositions. In contrast,mafic igneous rocks emplaced before and after this age exhibit both island arc basalts(IAB)-like trace element distribution patterrs and enriched Sr-Nd isotope compositions.This difference indicates a geochemical mutation in the SCLM of North China at^121 Ma. Although mafic magmatism also took place in the Late Triassic, it was related to exhumation of the deeply subducted South China continental crust because the subduction of Paleo-Pacific slab was not operated at that time. Paleo-Pacific slab started to subduct beneath the eastern margin of Eruasian continent since the Jurrasic. The subducting slab and its overlying SCLM wedge were coupled in the Jurassic, and slab dehydration resulted in hydration and weakening of the cratonic mantle. The mantle sources of ancient IAB-like mafic igneous rocks are a kind of ultramafic metasomatites that were generated by reaction of the cratonic mantle wedge peridotite notonly with aqueous solutions derived from dehydration of the subducting Paleo-Pacific oceanic crust in the Jurassic but also with hydrous melts derived from partial melting of the subducting South China continental crust in the Triassic. On the other hand, the mantle sources of juvenile OIB-like mafic igneous rocks are also a kind of ultramafic metasomatites that were generated by reaction of the asthenospheric mantle underneath the North China lithosphere with hydrous felsic melts derived from partial melting of the subducting Paleo-Pacific oceanic crust. The subducting Paleo-Pacific slab became rollback at^144 Ma. Afterwards the SCLM base was heated by laterally filled asthenospheric mantle, leading to thinning of the hydrated and weakened cratonic mantle. There was extensive bimodal magmatism at 130 to 120 Ma, marking intensive destruction of the cratonic lithosphere. Not only the ultramafic metasomatites in the lower part of the cratonic mantle wedge underwent partial melting to produce mafic igneous rocks showing negative ε_(Nd)(t) values, depletion in Nb and Ta but enrichment in Pb, but also the lower continent crust overlying the cratonic mantle wedge was heated for extensive felsic magmatism. At the same time, the rollback slab surface was heated by the laterally filled astheno spheric mantle, resulting in partial melting of the previously dehydrated rocks beyond rutile stability on the slab surface. This produce still hydrous felsic melts, which metasomatized the overlying astheno spheric mantle peridotite to generate the ultramafic metasomatites that show positive ε_(Nd)(t) values, no depletion or even enrichment in Nb and Ta but depletion in Pb. Partial melting of such metasomatites started at^121 Ma, giving rise to the mafic igneous rocks with juvenile OIB-like geochemical signatures. In this context, the age of ~121 Ma may terminate replacement of the ancient SCLM by the juvenile SCLM in North China. Paleo-Pacific slab was not subducted to the mantle transition zone in the Mesozoic as revealed by moder seismic tomography, and it was subducted at a low angle since the Jurassic, like the subduction of Nazca Plate beneath American continent. This flat subduction would not only chemically metasomatize the cratonic mantle but also physically erode the cratonic mantle. Therefore, the interaction between Paleo-Pacific slab and the cratonic mantle is the first-order geodynamic mechanism for the thinning and destruction of cratonic lithosphere in North China.
