The Changning-Menglian suture in SW Yunnan has been accepted as the Paleotethyan main ocean. However, it has been a matter of debate as to its southerly extension in NW Thailand（the Chiang Mai-Chiang Rai vs. Nan-Utta...The Changning-Menglian suture in SW Yunnan has been accepted as the Paleotethyan main ocean. However, it has been a matter of debate as to its southerly extension in NW Thailand（the Chiang Mai-Chiang Rai vs. Nan-Uttaradit zone）. Our field investigation identified the high-iron basaltic rocks in the Chiang Dao Permian standard profile in NW Thailand. The high-iron rocks provide crucial records for understanding the controversy on the location of Paleotethyan main ocean in NW Thailand. The Early Permian high-iron samples show extremely high Fe Ot（20.96 wt.%–25.56 wt.%） and TiO2（6.07 wt.%–6.34 wt.%） and low SiO2（38.54 wt.%–43.46 wt.%） and Mg O（1.61 wt.%–2.40 wt.%） contents. Such characteristics are similar to those of the Fenner differentiation trend rarely observed in the natural system, distinct from those of the ＂normal＂ Bowen trend. Their chondrite- normalized REE and primitive mantle-normalized patterns are generally similar to those of typical OIB. The initial 87Sr/86 Sr ratios and εNd（t） values range from 0.704 677 to 0.705 103 and 3.16 to 3.48, respectively, falling near the field of typical OIB（oceanic-island basalt）. These data synthetically suggest that the Chiang Dao high-iron rocks are the products of high-degree partial melting of peridotite with Fe-rich eclogitic blobs/streaks in response to a seamount setting. In comparison with the Permian tectonic setting in SW Yunnan and NW Thailand, it is inferred that the Paleotethyan Ocean was located between the Shan-Thai terrane of Sibumasu and Sukhothai arc along the Inthanon zone of the Chiang Mai-Chiang Rai rather than Nan-Uttaradit zones.展开更多
The Cenozoic basalts with OIB-affinity in northern marginal region of the North China Craton are thought to experience minor even no crustal contamination during the magma evolution.The whole-rock Sr-Nd-Pb-Hf isotopes...The Cenozoic basalts with OIB-affinity in northern marginal region of the North China Craton are thought to experience minor even no crustal contamination during the magma evolution.The whole-rock Sr-Nd-Pb-Hf isotopes are attributed to a two-component mixing between depleted and enriched mantle sources,while the major element variations are controlled by the fractional crystallization of olivine and clinopyroxene.However,in this study,the new Os isotopic data proposes an opposite model for the Cenozoic basalts in northern marginal region of the North China Craton.In this model,the Jining basalts were contaminated by the Archean mafic rocks during the magma storage and ascent.The crustal contamination process is supported by(1)the highly radiogenic Os isotopic compositions,and(2)the positive correlation between 187Os/188Os and 1/Os of the Jining basalts.By modeling the Os isotopic composition of the basalts,an incorporation of<10%mafic granulites/amphibolites to the parental magma can successfully explain the initial values of highly radiogenic Os.In contrast,the unradiogenic and uniform Os isotopic compositions of the Chifeng basalts suggest negligible crustal contamination.Os isotopic data acts as an indicator of crustal contamination during magma evolution,providing us a novel insight into the evolution of the intra-continental OIB-like basalts worldwide.展开更多
Located southeast of the city of Ngaoundere(Adamawa Plateau)within the continental section of the Cameroon Volcanic Line(CVL),the village of Dibi has been the scene of numerous volcanic manifestations,dominated by thr...Located southeast of the city of Ngaoundere(Adamawa Plateau)within the continental section of the Cameroon Volcanic Line(CVL),the village of Dibi has been the scene of numerous volcanic manifestations,dominated by three types of dynamism(explosive phreatomagmatic,effusive,and Strombolian eruptions);whose rocks,however,are poorly studied.Here,the lavas emitted during those eruptions are analyzed to constrain their origin.Whole rock geochemistry revealed that these rocks consist of basanites and foidites with Mg#varying from 67.3%to 71.2%.They are porphyritic and mainly composed of olivine[(Fo77.28-87.28 in foidites)and(Fo78.92-87.04 in basanites)],diopside[(Wo51En35Fs14 in foidites)and(Wo50-46.18En34.8-41.29Fs10.41-15.92 in basanites)],plagioclase[(labradorite:Ab37.31-39.67An57.43-60.70Or1.99-2.28)in basanites],and alkali feldspar[(anorthoclase:Ab64.45-65.13An6.81-9.15Or26.87-27.08 in foidites and Ab60.92-64.11An8-8.47Or28.45-32.27 in basanite)].Accessory minerals include Al-chromite,Ferrian chromite,titanomagnetite,and apatite.The lavas are silica-undersaturated(SiO_(2):38.9-40.3 wt.%in foidites and SiO_(2):42.5-43.6 wt.%in basanites).They are within-plate basalts with similar features to Oceanic Island Basalt(OIB)magmas,affinity to High-μ(HIMU),Enriched type I(EM1),and recycled component.Their primary melts derived from low degrees of partial melting(3%-5%)of a garnet peridotite mantle source,as shown by the Nb/La(1.30-1.51),Zr/La(4.91-5.85),and Zr/Ce(2.49-2.88)ratios that are consistent with an OIB-like asthenospheric mantle source.Equilibration temperatures have been estimated from the clinopyroxene-liquid thermometer and range between 981.6˚C and 1051.6˚C,at pressures of 0.9-1.6 kbar.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Financial supports from the National Natural Science Foundation of China(Nos.41190073 and 41402165the Fundamental Research Funds for the Central Universities to SYSU are gratefully acknowledged
文摘The Changning-Menglian suture in SW Yunnan has been accepted as the Paleotethyan main ocean. However, it has been a matter of debate as to its southerly extension in NW Thailand（the Chiang Mai-Chiang Rai vs. Nan-Uttaradit zone）. Our field investigation identified the high-iron basaltic rocks in the Chiang Dao Permian standard profile in NW Thailand. The high-iron rocks provide crucial records for understanding the controversy on the location of Paleotethyan main ocean in NW Thailand. The Early Permian high-iron samples show extremely high Fe Ot（20.96 wt.%–25.56 wt.%） and TiO2（6.07 wt.%–6.34 wt.%） and low SiO2（38.54 wt.%–43.46 wt.%） and Mg O（1.61 wt.%–2.40 wt.%） contents. Such characteristics are similar to those of the Fenner differentiation trend rarely observed in the natural system, distinct from those of the ＂normal＂ Bowen trend. Their chondrite- normalized REE and primitive mantle-normalized patterns are generally similar to those of typical OIB. The initial 87Sr/86 Sr ratios and εNd（t） values range from 0.704 677 to 0.705 103 and 3.16 to 3.48, respectively, falling near the field of typical OIB（oceanic-island basalt）. These data synthetically suggest that the Chiang Dao high-iron rocks are the products of high-degree partial melting of peridotite with Fe-rich eclogitic blobs/streaks in response to a seamount setting. In comparison with the Permian tectonic setting in SW Yunnan and NW Thailand, it is inferred that the Paleotethyan Ocean was located between the Shan-Thai terrane of Sibumasu and Sukhothai arc along the Inthanon zone of the Chiang Mai-Chiang Rai rather than Nan-Uttaradit zones.
基金This work was supported financially by Beijing Natural Science Foundation(8194073)the Science Foundation of China University of Petroleum,Beijing(2462017YJRC032 and 2462021YXZZ004)+1 种基金the Science Foundation of State Key Laboratory of Petroleum Resources and Prospecting,China University of Petroleum,Beijing(PRP/indep-4-1702)the National Natural Science Foundation of China(41872057 and 42002238).
文摘The Cenozoic basalts with OIB-affinity in northern marginal region of the North China Craton are thought to experience minor even no crustal contamination during the magma evolution.The whole-rock Sr-Nd-Pb-Hf isotopes are attributed to a two-component mixing between depleted and enriched mantle sources,while the major element variations are controlled by the fractional crystallization of olivine and clinopyroxene.However,in this study,the new Os isotopic data proposes an opposite model for the Cenozoic basalts in northern marginal region of the North China Craton.In this model,the Jining basalts were contaminated by the Archean mafic rocks during the magma storage and ascent.The crustal contamination process is supported by(1)the highly radiogenic Os isotopic compositions,and(2)the positive correlation between 187Os/188Os and 1/Os of the Jining basalts.By modeling the Os isotopic composition of the basalts,an incorporation of<10%mafic granulites/amphibolites to the parental magma can successfully explain the initial values of highly radiogenic Os.In contrast,the unradiogenic and uniform Os isotopic compositions of the Chifeng basalts suggest negligible crustal contamination.Os isotopic data acts as an indicator of crustal contamination during magma evolution,providing us a novel insight into the evolution of the intra-continental OIB-like basalts worldwide.
文摘Located southeast of the city of Ngaoundere(Adamawa Plateau)within the continental section of the Cameroon Volcanic Line(CVL),the village of Dibi has been the scene of numerous volcanic manifestations,dominated by three types of dynamism(explosive phreatomagmatic,effusive,and Strombolian eruptions);whose rocks,however,are poorly studied.Here,the lavas emitted during those eruptions are analyzed to constrain their origin.Whole rock geochemistry revealed that these rocks consist of basanites and foidites with Mg#varying from 67.3%to 71.2%.They are porphyritic and mainly composed of olivine[(Fo77.28-87.28 in foidites)and(Fo78.92-87.04 in basanites)],diopside[(Wo51En35Fs14 in foidites)and(Wo50-46.18En34.8-41.29Fs10.41-15.92 in basanites)],plagioclase[(labradorite:Ab37.31-39.67An57.43-60.70Or1.99-2.28)in basanites],and alkali feldspar[(anorthoclase:Ab64.45-65.13An6.81-9.15Or26.87-27.08 in foidites and Ab60.92-64.11An8-8.47Or28.45-32.27 in basanite)].Accessory minerals include Al-chromite,Ferrian chromite,titanomagnetite,and apatite.The lavas are silica-undersaturated(SiO_(2):38.9-40.3 wt.%in foidites and SiO_(2):42.5-43.6 wt.%in basanites).They are within-plate basalts with similar features to Oceanic Island Basalt(OIB)magmas,affinity to High-μ(HIMU),Enriched type I(EM1),and recycled component.Their primary melts derived from low degrees of partial melting(3%-5%)of a garnet peridotite mantle source,as shown by the Nb/La(1.30-1.51),Zr/La(4.91-5.85),and Zr/Ce(2.49-2.88)ratios that are consistent with an OIB-like asthenospheric mantle source.Equilibration temperatures have been estimated from the clinopyroxene-liquid thermometer and range between 981.6˚C and 1051.6˚C,at pressures of 0.9-1.6 kbar.
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.
