Water Content And Geochemistry Of The Cenozoic Basalts In SE China:Implications For Enrichment In The Mantle Source Of Intra-plate Basalts

The origin and genesis of intra-plate basalts,especially continental basalts,have been studied extensively but still are debated hotly during past several decades.The enriched geochemical characteristics of intra-plate basalts are usually suggested to be closely associated with the recycled crustal materials.The eastern China is an ideal area to decipher the effect and contributions of different recycled components in basalt source,where the Cenozoic basalts are widespread distributed.However,existing evidence for the Cenozoic basalts in eastern China failed to figure out clearly the source of the enriched components and their genesis.Previous studies have proposed several candidates inducing enrichment in the mantle source of eastern China basalts:a)the recycled subducted oceanic crusts and sediments,originated from ancient subduction events(e.g.,Paleo-Tethyan,Paleo-Asian or Izanagi plate subductions)and present subduction event(i.e.,Pacific subduction);b)the delaminated thicken lower continental crust and lithospheric mantle in Mesozoic;and c)the re-enriched continental lithosphere.The eastern China has undergone multiple subduction of oceanic plates since Mesozoic.In addition,geophysical studies have discovered plume structures in the mantle beneath the Changbaishan volcanic area in northeast China and the Hainan volcanic area in the most southern part of eastern China,respectively.Therefore,the complex geological history of eastern China offers an opportunity to conduct comparative studies on basalts from distinct geological backgrounds.In SE China,a stagnant Pacific slab exists in the mantle transition zone(MTZ)beneath the northern part(e.g,Zhejiang and Jiangxi provinces),a plume structure was found in the lower mantle beneath the southern part(e.g.,South Guangdong and Hainan provinces),and neither stagnant slab nor plume has been discovered in the middle part(e.g.,Fujian province).Therefore,the Cenozoic intra-plate basalts in SE China have three kinds of geological backgrounds.H2O and Ce have similar partition coefficients between mantle minerals and melt.Because their ratios remain unchanged through partial melting and crystal fractionation,the H2O/Ce ratios of melts contain information regarding the mantle source.Recycled components have H2O/Ce ratios that are distinguishable from mantle components.Therefore,in combination with other trace elements ratios(e.g.,Ba/Th,Nb/La,Ce/Pb),H2O/Ce ratios in basalts may serve as an index for identifying various components within mantle source.In addition,due to distinct oxygen isotope compositions between normal mantle and recycled crustal materials that acquired abnormal oxygen isotopic ratios by interacting with surface water,?18O values can be used to recognize recycled materials.Several studies have reported H2O content and ?18O values in the Cenozoic basalts from NCC and successfully traced recycled oceanic materials in the mantle source combining with other elemental and isotopic compositions,however,such systematic investigations on the SE China basalts are lacking.To decipher the relationship between the possible recycled components and the geochemical heterogeneities of the SE China basalts that formed on distinct geological backgrounds,the major and trace element contents and Sr-Nd-Pb isotopic compositions of the Cenozoic basalts from Zhejiang,Fujian and Hainan provinces have been measured.In particular,we recovered the H2O contents of the "initial" magmas based on the H2O contents of the clinopyroxene(cpx)phenocrysts and the partition coefficients of H2O between cpx and basaltic melts.Moreover,the oxygen isotope compositions of the cpx phenocrysts have been determined.The results of the Cenozoic basalts from three regions in SE China have given the new insight of the enrichment composition and the genesis of Cenozoic basalts in SE China:1)Zhejiang Cenozoic basalts erupted in two stages,20-27Ma and after 11Ma,respectively,with distinct geochemical features.The H2O contents of the Zhejiang basalts range from 1.3 to 2.6(wt.%),falling within the range of the H2O contents of back-arc basin(BABs)or island arc basalts(IABs).The older basalts have more alkaline,lower Si and Al contents,higher trace element concentrations,La/Yb,Ce/Pb and Nb/La ratios,lower H2O/Ce and Ba/Th ratios,and stronger negative K,Pb,Zr,Hf and Ti anomalies than the younger ones.The younger basalts are less alkaline and have higher Si and Al contents;lower trace element concentrations;lower La/Yb,Ce/Pb and Nb/La ratios;higher H2O/Ce and Ba/Th ratios.The co-relationships between Ba/La,H2O/Ce,Nb/La,Ce/Pb and Ba/Th ratios in two groups of Zhejiang Cenozoic basalts indicate that a component of recycled dehydrated oceanic basalt is needed in the source of the older rocks,along with the depleted mantle component.Meanwhile,additional recycled dehydrated sediments together with recycled dehydrated oceanic basalts are required to serve as the enrichment components in the source of the younger rocks.The change of the recycled oceanic materials over time in the mantle source of Zhejiang Cenozoic basalts require a continuous supply of the recycled materials,demonstrating the recycled oceanic components originated from the Pacific slab,which is the only subducted plate since 100 Ma in the Zhejiang region.Therefore,the recycled oceanic materials are likely to be provided by the stagnant Pacific slab,which has been revealed in the Zhejiang region by the geophysical evidence of the seismic image in the MTZ.2)Fujian Cenozoic basaltic rocks erupted after 10Ma and have similar trace element patterns to Zhejiang Cenozoic basalts.All the Fujian samples have strong negative Th,U,and Pb anomalies in the extended trace element pattern.Unlike the basalts,the diabases have very weak or no negative K,Zr,Hf,and Ti anomalies in the extended trace element pattern.Fujian Cenozoic basalts have more radiogenic Sr,Nd,and Pb isotopic data than Zhejiang Cenozoic basalts.Unsimilar to Zhejiang,Fujian samples from three areas have heterogeneous H2O contents.The estimated H2O contents of initial basaltic melts for the Shiheng(SH)basalts range from 1.3 to 2.4(wt.%)that is within the H2O contents range of BABBs and IABs;the Bailin(BL)diabases have "fake" basaltic melt water contents from 1.9 to 2.1(wt.%),due to the later H re-equilibrium of the cpx phenocrysts;and the Mingxi(MX)basalts have lower initial H2O contents(0.3-0.5 wt.%),when the H-loss are excluded.The higher ?18O values of the cpx phenocryst from Fujian Cenozoic basalts than the nonnal mantle are observed,suggesting the recycled materials from the surface of earth in their mantle source.The highest ?180 values are found in the cpx phenocrysts with the intermediate ratios of Si/Al and Ca/Al and the relationship between Mg#and ?18O,suggesting a mixture of two kinds of magma.Fujian Cenozoic basalts display similar relationships between the ratios of Ba/La,Nb/La,Ce/Pb,and Ba/Th to Zhejiang Cenozoic basalts,indicating that the components of recycled dehydrated oceanic crusts and sediments are also needed in their mantle source along with the depleted mantle component.The heterogeneity of H2O contents in Fujian Cenozoic basalts may be the result of different degree of recycled materials dehydration or the heterogeneity of the mantle.Because of the fuzzy existence of the subducted slab beneath the Fujian region in the seismic image,the pronounced recycled oceanic materials are likely distributed in the upper mantle,most likely the residue of the previous subducted slab sunk to the lower mantle.3)Relative to Fujian and Zhejiang Cenozoic basalts,Hainan Cenozoic basalts have complex trace element patterns,including HIMUs-like and EM-like characters.These differences indicate Hainan Cenozoic basalts have more heterogeneous mantle source than Zhejiang and Fujian Cenozoic basalts.Considering the more enriched Sr,Nd,and Pb isotopic ratios,and higher ?18O values of the cpx phenocrysts in Hainan Cenozoic basalts than those of the normal mantle,the recycled materials are also required in the mantle source.The Li isotopic data of the some basalts suggests the cpx phenocryst have undergone Li diffusion after crystallization.Some of Hainan Cenozoic basalts are almost dry,due to intense H-loss.H2O content the basalts from Leihuling(LH),Dayang(DY)and Fujitian(FJT)areas are 2.7 wt.%,1.3 wt.%,and 2.6 wt.%,respectively.The coexistence of high and low ?18O cpx phenocrysts observed in the LH basalts,combined with their trace element composition,the recycled upper oceanic crust components are dominantly required in the LH basalts,while limited recycled lower oceanic gabbro components were also involved.The mixture of DMM,recycled oceanic sediments,and crusts most likely resulted in the DY and FJT basalts.These recycled materials should not be very old and are most likely caught by upwelling plume from the deep mantle.Overall,the SE China Cenozoic basalts with different geological backgrounds have involved the recycled oceanic materials in their mantle source.The H2O contents of the Cenozoic basalts in SE China are generally as enriched as in BABBs and IABs,although the basalts have various range of Sr-Nd-Pb radiogenic ratios,and discrepant trace element characteristics,implying various proportions of different compositions and the mantle heterogeneity.The recycled oceanic materials have different existential state and locations,when the geological history of SE China are considered.The subducted Pacific slab most likely effect the composition of mantle in SE China.