Genesis Of Cenozoic Intraplate High Magnesium Andesite In Great Xing'an Range

High-magnesium andesites(HMAs),which refer to the magnesium-rich,intermediate volcanic rocks(SiO2=54-65 wt.%,Mg#?45),are generally distributed in the island-arc setting,related to subduction.The research area of this study is located in the Wuchagou Town,Inner Mongolia Province,which is in the central Greater Xing'an Range.The Wuchagou andesites compose a 700 km2 Cenozoic volcanic field.In general,the Cenozoic volcanic rocks in eastern China are mainly basaltic,whereas the andesites are rare in this area.Both of the Cenozoic andesites and basalts are produced in intraplate setting.However,the chemical compositions of these andesites are similar to the arc-related HMAs.Their SiO2 and Mg#range from 54.10 wt.%to 57.97 wt.%and from 53 to 60,respectively.Their trace elements are characterized in enrichment of LILEs and depletion of HFSEs.Therefore,we defined those Cenozoic andesites as intraplate HMAs.To discuss those intraplate HMA's genesis and their characteristics of magma source,we choose a lava cross section from Wuchagou Feijibao for study and present major,trace-element,Sr-Nd-Pb isotopes,and also mineral's electron microprobe data.Wuchagou HMAs have relatively heterogeneous characteristics in major elements,fractional crystallization seems not quite apparent in them.From the trace element's spider gram,the Wuchagou HMAs have Ba,Sr,Pb positive anomalies,accompanied with Nb,Ta,Zr,Hf and also,Ti negative anomalies.The pattern looks similar with the average of continental crust.There is also a strong Ce negative anomaly can be found in REE pattern,the Ce/Ce*ratio is about 0.74-0.93.The Wuchagou HMAs have the richest Sr isotope and the lowest ?Nd in Great Xing'an Range,they show more similar to the relative old sedimentary end-member in Pb-Pb isotopes.Comparing to the high-pressure experimental melts,the Wuchagou HMAs have similar chemical compositions to the eclogite-derived melts,except for the high MgO contents.The eclogite-derived melts could be reacted with the mantle peridotite and elevated the MgO and Mg#of the melts,implied that Wuchagou HMAs might be the product of those reactions.Wuchagou HMAs present similar patterns with the lower continental crust,like Ce/Pb,Th/La etc ratios,also the spider gram and isotopes.However,it's difficult to-use lower continental crust to explain the appearance of Ce negative anomaly.Furthermore,the Ce negative anomaly and the lowest ?Nd pointing out the existence of the ancient sediments.Therefore,we suggest that the Wuchagou HMAs might be derived from the melting of recycled crustal materials(ancient sediments)in the mantle,and then underwent interactions with the surrounding peridotites during their ascent.