Genesis of Small-scale Flood Basalts–以赤峰与新昌—嵊州溢流玄武岩为例

Author:Yu Xun

Supervisor:Chen Lihui, lee Cin-Ty, xie Liewen

Database:Doctor

Degree Year:2016

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Pages:151

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As one of the largest volcanisms on the Earth,continental flood basalts received a lot of attention.In previous studies,Large Igneous Provinces(LIPs)are regarded as the main target to understand the continental flood basalts.It is because LIPs are always characterized by large scale eruption of magmas(>105 km3),and close relationship with extinctions in earth history.However,besides LIPs,there is another kind of continental flood basalts on the earth which are smaller in size than LIPs.These continental flood basalts are widely distributed but have not received enough attention.Here we define them as small-scale flood basalts.Small-scale flood basalts are characterized by layered flood basalts in field with a much smaller volume than LIPs,which are usually smaller than 105 km3.In terms of time scale,rather than finishing eruption within one or two million years,small-scale flood basalts are characterized by a long eruption duration(~5 Myrs)which are even longer than 10 million years.In terms of lithology,small-scale flood basalts are mainly composed by tholeiitic basalts and trachybasalts,showing obviously evolved geochemical features.In this dissertation,we try to understand the formation of small-scale flood basalts by case studies of Chifeng flood basalts from Inner Mongolia and Xinchang-Shengzhou flood basalts from Zhejiang Province.In the research work of Baichahe basalt section from Chifeng flood basaltic field,we found that geochemical features of these basalts show regular variations with the younging of eruption.For example,the content of MgO and CaO increase,the content of incompatible elements like Nb and La decrease,and the Pb isotopic ratios increase with the younging of eruption.In short,the Baichahe basalt magmas become less evolved and less crustal contaminated.K-Ar geochronology work shows that Baichahe section erupted continuously and shortly.According to REAFC modeling(Recharge,Eruption,Assimilation,Fractional Crystallization),magma chamber processes like magmatic recharge,crustal assimilation,and fractional crystallization are needed to work simultaneously to explain the formation of Baichahe section.In comparison with LIPs worldwide,we found that magmatic recharge is very important and universal for continental flood basalts.Therefore,our study suggests that magmatic recharge during magma chamber processes is inevitable for the formation of small-scale flood basalts.In the research work of Xinchang-Shengzhou flood basalts,we found that the spatial and temporal distributions of small-scale flood basalts have close relationship with magma chamber processes.Eruption of Xinchang-Shengzhou flood basalts were controlled by Lishui-Yuyao fault and showed younging trend northward.Based on the elemental geochemistry,Xinchang-Shengzhou flood basalts could be divided into low-SiO2 alkaline basalts,Xinchang high-SiO2 basalts,and Shengzhou high-SiO2 basalts.Among them,high-SiO2 basalts are the results of magma chamber processes,which show obvious signal of crustal assimilation and fractional crystallization.The evolution trend of high-SiO2 basalts can further be classified into alkaline series and tholeiitic series,where Shengzhou high-SiO2 basalts are always more evolved than Xinchang high-SiO2 basalts.Combined with geochronology,we found the evolution of high-SiO2 magmas was stagnant for about 6 Myrs((Na2O + K2O)/MgO = 0.40-0.66;TiO2/MgO = 0.23-0.35),so that the magmatic recharge in magma chamber processes is inevitable.By using REAFC modeling,we found the signal of magmatic recharge could be verified by geochemical modeling.Based on the structure of translithospheric fault,we proposed that magmatic recharge and magma chamber growth model could explain the spatial and temporal distributions of small-scale flood basalts.According to isotopic geochemistry,we further found that the magmas experienced magmatic recharge recorded the isotopic signal from mantle sources,showing higher 207Pb/204Pb and 208Pb/204Pb ratios with similar 206Pb/204Pb ratios,and higher 176Hf/177Hf ratios with similar 143Nd/144Nd ratios than low-SiO2 alkaline basalts.In addition,these basalts also show moderate Ba/Th and low Th/La ratios.All these elemental or isotopic features suggest that recycled sediments could exist in mantle sources and the mantle is heterogeneous in geochemistry.In conclusion,on one hand,magmatic recharge provides the basis for using isotope systematics of small-scale flood basalts to trace the mantle sources.On the other hand,recycled crustal materials in mantle sources could be the reason why mantle keeps on melting and the magma chamber keeps on recharging.In summary,magma chamber processes,especially the magmatic recharge,play an important role in formation of small-scale flood basalts.Magmatic recharge not only contributes to the flood basalts by materials,but also leads to the geochemical variations of the flood basalts.The growth model of magma chambers then could explain the spatial and temporal distributions of small-scale flood basalts.In addition,mantle heterogeneity exists in the source of small-scale flood basalts which indicates that recycled crustal materials in mantle sources could be the prime reason to explain the formation of small-scale flood basalts.