Genesis Of Cenozoic Strongly Alkaline Basalts In Eastern China

Cenozoic strongly alkaline basalts?sodic?are widely distributed in eastern China and these mantle-derived magmas provide an ideal setting to study the genesis of intraplate strongly alkaline basalts in eastern China and the characteristics of the mantle source.In this study,we represent major element,trace element and Sr-Nd-Hf-Pb-Mg isotopic compositions for the Cenezoic strongly alkaline basalts from Huihe volcanic field?Northeast China?and Jiande,Zhejiang Province?South China?,to discuss the genesis of intraplate strongly alkaline basalts and further,find out the genetic links between strongly alkaline basalts and weakly alkaline basalts.Cenozoic Huihe nephelinites,which distributed to the west of the DTGL,have low contents of SiO2?37.68-41.28 wt.%?and Al2O3?10.04-12.00 wt.%?,medium to high content of MgO?7.51-10.95 wt.%?,high contents of CaO?10.57-13.06wt.%?,TiO2?3.50-3.72 wt.%?.These alkaline rocks share typical elemental signatures of carbonatites,e.g.negative K,Zr,Hf,and Ti anomalies in a primitive mantle-normalized gram?with low Hf/Hfy*and Ti/Ti*ratios,Hf/Hf*=0.58-0.69,Ti/Ti*=0₆2-0.83?,high Zr/Hf?43.5-46.6?and Ca/Al?1.19-1.72?ratios.Their Mg isotopic compositions?S26Mg=-0.43‰ to-0.35%??are obviously lighter than those of the peridotitic mantle?average 826Mg=-0.23±0.040‰?,but are comparable to those of Cenozoic basalts in eastern China?526Mg=-0.60‰ to-0.30‰?.The elemental signatures and light Mg isotopic compositions suggest that Huihe nephelinites are derived from a carbonated mantle source.If Huihe nephelinites and Cenozoic basalts in the nearby DTGL area?Chaihe basalts?are plotted together,positive correlation of 826Mg versus Ti/Ti*ratios and negative correlations of 826Mg versus Ca/Al,La/Sm and La/Yb ratios can be observed.Such correlations can be generated by preferential melting of enriched component in a heterogeneous mantle.Moreover,vow-?26Mg basalts are interpreted to represent melts derived from carbonated mantle source.The mantle domain feeding low-?26Mg Cenozoic basalts in eastern China overlaps the so-called Big Mantle Wedge?BMW?above the stagnant Pacific slab in the mantle transition zone,which indicates that the BMW is an important carbon reservoir generated by the slab.However,the Mg isotopic compositions of Huihe nephelinites demonstrate that low 826Mg mantle sources exist at significant distances from the present-day BMW,which indicates that the genetic relationship between the carbonated components and the BMW is still in doubt for the mantle beneath eastern Asia.The distribution of Cenozoic continental basalts from Zhejiang Province,southeast China are controlled by three northeast-southwest trending faults.According to their outcropping locations and eruption ages,these basalts are divide into three groups:western part,central part and eastern part basalts in Zhejiang province.Here,we carried out a geochemical study on Cenozoic Jiande basalts,the oldest basalts?-40 Ma?crop out at Jiangshan-Shaoxing fault in the west of Zhejiang,to further explore the nature of mantle source beneath Zhejiang province.These volcanic rocks are mainly alkaline?sodic?basanites with low contents of SiO2?41.3-42.3 wt.%?and Al2O3?9.70-12.6 wt.%?,high contents of MgO?8.90-15.6 wt.%?,CaO?8.92-12.1wt.%?,TiO2?2.78-3.18 wt.%?and Fe2O3T?14.1-16.2wt.%?.They are characterized by negative K,Zr,Hf,and Ti anomalies?Hf/Hf*=0.74-0.77,Ti/Ti*=0.70-0.74?,and high Zr/Hf?48.5-50.1?,Ca/Al?1.02-1.16?ratios,which suggest the contribution of recycled carbonated materials in the mantle source.They show depleted Sr,Nd,Hf isotopic compositions?87Sr/86Sr=0.7032-0.7034,sNd=5.85-5.95,?Hf=7.78-8.56?and have 206Pb/204 Pb ratios of 18.491-18.554,207Pb/204Pb ratios of 15.488-15.518 and 208Pb/204 Pb ratios of 38.387-38.523.When compared to weakly alkaline basalts in central and eastern part of Zhejiang,the strongly alkaline basalts in western part of Zhejiang show higher contents of trace element and more pronounced negative Zr,Hf,and Ti anomalies,which suggest that the strongly alkaline basalts in western part of Zhejiang are generated by low-degree partial melting of a carbonated mantle source.The good correlations between Ti/Ti*versus Ba/Th and K/La of Zhejiang Cenozoic basalts suggest a mixing process of two components.The carbonated melts derived from partial melting of carbonate-bearing oceanic crust reacted with the depleted asthenosphere mantle.This carbonated mantle component is characterized with low Ba/Th,K/La,Ti/Ti*ratios and 87Sr/86Sr?basalts in western part of Zhejiang as a representative?.Another enriched component is melting residual of recycled oceanic crust with high Ba/Th,K/La and Ti/Ti*ratios and 87Sr/86Sr?basalts in eastern part of Zhejiang as a representative?.Therefore,the mantle source beneath Cenozoic Zhejiang basalts,which is mainly controlled by two components with genetic relationship,is heterogeneous.In summary,the strongly alkaline basalts in eastern China share typical elemental signatures of carbonatites,e.g.low content of SiO2,high content of CaO,high Zr/Hf,Ca/Al ratios and negative K,Zr,Hf,and Ti anomalies in a primitive mantle-normalized gram,which suggest recycled carbonated materials play an important role in the mantle source of strongly alkaline basalts in eastern China.When compared to weakly alkaline basalts,the higher contents of trace element and more pronounced negative Zr,Hf,and Ti anomalies of strongly alkaline basalts suggest that the strongly alkaline basalts in eastern China are generated by low-degree partial melting of a carbonated mantle source.