| The genesis of the continental basalts generated within the plate is of great interest for several decades.The widespread Cenozoic continental basalts in eastern China provide a noteworthy opportunity to investigate the continental intraplate magmatism.Recently,novel geochemical approaches have been developed to trace different components that are involved in the mantle source,such as H2O content of magma,O isotopic compositions of phenocryst,Mg isotopic compositions of bulk rock and compositions of melt inclusion.The signal of recycled oceanic components have been identified in the mantle source of these basalts and has been linked with the presence of the stagnant Pacific slab in the MTZ(Mantle transition zone).However,the recycled oceanic materials could also result from ancient subduction events(e.g.,Paleo-Tethyan,Paleo-Asian or Izanagi plate subduction).The subduction event introducing the recycled materials in the mantle source of the Cenozoic basalts in eastern China(and eastern Asia),therefore,remains unresolved.This work is focused on the role of the Pacific subduction on the genesis of the Cenozoic continental basalts in eastern China,based on the approaches of the water content(analyzed by Fourier transform infrared spectroscopy(FTIR)),oxygen isotope composition(analyzed by Secondary ion mass spectrometry(SIMS))and the temporal and spatial heterogeneity in the mantles source.In this study,the Shuangliao basalts,Wulanhada basalts,Chaihe-aershan basalts,Xiaogulihe ultrapotassic volcanic rocks in Northeast China were chosen.Major and trace element compositions of bulk rocks and the O isotope compositions,water contents and major element compositions of clinopyroxene(cpx)phenocrysts were investigated in detail.The water contents of the "primary" magmas were estimated by averaging the melts equilibrated with all the earliest crystallized cpx phenocrysts(Mg#>75).The ?180 values of cpx phenocrysts of the Shuangliao basalts range from 4.10‰ to 6.73‰,overlapping with the typical oxygen isotope profile of the altered oceanic crust,which provides a strong evidence for the contribution of recycled oceanic crust(ROC)in the mantle source.The calculated H2O contents of the "primary" magma vary from 0.90 to 3.06 wt.%,which are higher than the values of MORBs(mid-ocean ridge basalts)and OIBs(ocean island basalts).The significant K positive anomaly and the correlations of H2O/Ce with Ba/Th and Ce/Pb in the Shuangliao basalts suggest the contribution of water-rich sediment component to the mantle source.Compared with the Shuangliao basalts,the Wulanhada basalts located above the front edge of subduted slab are characterized by low H2O contents(0.21-0.69 wt.%),which fall in the range of MORBs and OIBs,and high ?18O values of cpx phenocrysts(5.49‰-7.38‰),which are apparently higher than the values of cpx in typical MORBs and mantle peridotites.Based on the high oxygen isotope compositions and the correlations between H2O/Ce and 87Sr/86Sr,Ba/Th,Nb/La,Nb/U and Ce/Pb,the signal of the significantly dehydrated sediments and ROC in the mantle source can be identified.The Chaihe-aershan volcanic field includes more than 35 small-volume Quaternary basaltic volcanoes.The compositional heterogeneity of the mantle source was indicated by the large variations in the ?18O values of cpx phenocrysts(4.27 to 8.57‰)and the H2O contents of "primary" magmas(0.23-2.70 wt.%).The ?18O values and H2O contents within individual samples also vary widely,suggesting the mixing of magmas and that the magma mixing occurred shortly before the eruption.The correlations between the ?18O values of cpx phenocrysts and the H2O/Ce ratios,Ba/Th ratios and Eu anomalies of whole rocks demonstrate the contributions of three components to the mantle source(hydrothermally altered upper oceanic crust and marine sediments,altered lower gabbroic oceanic crust,and ambient mantle).For the Xiaogulihe ultrapotassic volcanic rocks,the estimated H2O contents and H2O/Ce ratios are low(0.36-0.50 wt%,?15),which means that K-rich component in the mantle source is "dry".Combined with the high ?18O values of olivine phenocrysts,the low 206Pb/204Pb ratios and moderately high 87Sr/86Sr ratios of the bulk rock,we concluded that highly dehydrated ancient sediments could be a most likely candidate for the K-rich component in the mantle source.Because the water contents in different layers of the mantle vary significantly,the water content in the source of continental basalts can be a direct way to constrain the source region of basalts.The water contents of the mantle source for the basalts in eastern China were estimated by the H2O contents of "primary" magma,degrees of partial melting and partition coefficients of H2O.The conservatively estimated water contents of mantle source are 150-4500 ppm(mostly higher than 500 ppm),still significantly higher than those of lithospheric mantle from the same region.Moreover,the spatial variations of water content in the source of these basalts displayed a different variation trend with those of peridotite xenoliths,which clearly demonstrated that the lithospheric mantle was not the source of the continental basalts.In addition,the stagnated slabs in the MTZ beneath the eastern China would hinder the rising of deeper mantle plume and no signal of deeper mantle plume was recognized by geophysical observations(except for the Hainan Island).Overall,the basalts with high water content in the source are probably derived from the hydrous MTZ.In combination with the ages of the Chaihe-aershan and Shuangliao basalts,we found that the contributions of different recycled oceanic components(marine sediments,altered upper oceanic crust and altered lower gabbroic oceanic crust)to the mantle source of the basalts changed significantly within a limited time.Liu et al.(2016)also observed that the compositions of recycled oceanic components varied with time in the mantle source of the Zhejiang basalts.The Pacific slab is constantly subducted under the eastern China and continuously transports recycled materials to the deep mantle.Thus,the temporal heterogeneity of the source components for these basalts is most likely triggered by the ongoing Pacific slab subduction,which means that the Pacific slab located in the MTZ may be a primary source for the enriched components.Based on statistics of the water content of the basalts,the H2O contents and H2O/Ce ratios vary widely(0.19 to 3.89 wt.%,12 to 737),and are relatively higher than those of typical OIBs.Interestingly,the H2O contents and H2O/Ce ratios of the basalts close to the Pacific trench(Shuangliao,Shandong and Zhejiang)are higher than those of the basalts far away from the Pacific trench(Wulanhada,Xiaogulihe and Taihang),except for the Chaihe-aershan basalts.This situation maybe induced by the dehydration process of the subducted Pacific plate.Combined the results of basalts from different areas of eastern China(Shuangliao,Wulanhada,Xiaogulihe,Shandong,Taihang and Zhejiang),the contributions of recycled components in their mantle source vary widely.The proportion and composition of the recycled components in different mantle sources are expected to be variable due to varying degrees of alteration and dehydration during the entire recycling process,which could account for the heterogeneous mantle source of the intraplate basalts in eastern China. |
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