Genesis Of Cenozoic Basalts From Halaha-Chaihe And Megacrysts From Shuangyashan

As the direct products of mantle partial melting,basalts and megacrysts which were captured by these basalts are significant windows to explore the deep geologic process.However,the genesis of such basalts and megacrysts still remained unsolved.Here we present major,trace element,and Sr-Nd-Pb-Hf isotopic compositions for the Cenozoic basalts from Halaha-Chaihe,as well as the major element and in-situ trace element compositions for clinopyroxene and garnet megacrysts from Shuangyashan,to constrain the origin and formation process of intra-plate sodic basalts and megacrysts.In order to further explore the nature of mantle source beneath the Northeast China,we present major,trace element,and Sr-Nd-Pb-Hf isotopic compositions for the Cenozoic intra-plate volcanic rocks from the Halaha-Chaihe field in the Greater Khingan Range.These volcanic rocks are mainly alkaline(sodic)basalts,and generally exhibit OIB-like incompatible trace element characteristics,e.g.enrichment in large Iithophile elements(LILEs)and positive Nb-Ta anomalies.They show moderate depleted Sr-Nd-Hf isotopic compositions(87Sr/86Sr=0.7035?0.7039??Nd=5.21?6.55??Hf=10.00?11.25)and almost represent the most depleted mantle end-member among the Cenozoic basalts in Eastern China.Their Pb isotopic compositions(206Pb/204Pb=18.374?18.572,207Pb/204Pb=15.521?15.549,208Pb/204Pb=38.244?38.435)lie between 4.42 Ga and 4.45 Ga geochrons on the 207Pb/204Pb versus 206Pb/204Pb diagram.They also show similar Sr-Nd-Pb isotopic compositions with those mantle plume-derived ocean island basalts(3He/4He>30 Ra),which implies a deep mantle source.The high MgO(8.49%?11.58 wt.%),Ni(174?362 ppm)contents and high Mg#values(59.1?66.9)of these basalts imply that their compositions are close to those of the primary magmas.The calculated primitive compositions of Halaha-Chaihe basalts show moderate SiO2,low Al2O3 contents and high CaO/Al2O3 ratios,which are accordant with the compositions of experimental melts of garnet peridotite under high pressure(>2.5 GPa)conditions,suggesting a garnet peridotitic mantle source.Moreover,trace-element modeling result suggests low-degree melts from a primitive mantle(rather than a depleted mantle)are consistent with these basalts.In summary,we suggest that the mantle source of the Halaha-Chaihe basalts from the Greater Khingan Range contains ancient,primordial and peridotitic component from the deep mantle.We also present the major element and in-situ trace element compositions for megacrysts from Shuangyashan field in east of the Songliao Basin.These megacrysts are mainly augites and pyropes,and generally exhibit the same geochemical features as the megacrysts from other locations where the megacrysts were captured in Cenozoic basalts,eastern China,e.g.low Mg#(Cpx:79.5?88.2;Gt:61.1?76.0)and depleted in large lithophile elements(LILEs).They show slight positive linear relations between Na2O(Cpx:1.62?2.06 wt.%;Gt:0.02?0.05 wt.%),CaO(Cpx:16.3?17.0 wt.%;Gt:5.3?6.0 wt.%),Ti02(Cpx:1.07?1.62 wt.%;Gt:0.39?0.55 wt.%),Sm/Yb and Gd/Yb versus Mg#,which implies the megacrysts were directly crystallization from basaltic magma rather than the relic of mantle peridotite or kimberlite.Compare with Cpx,Gt from Shuangyashan are more enriched in HREE and Zr,suggests that the Cpx and Gt were simultaneously crystallized from primordial melts.The value of Yb/Er remains relatively same under different Mg#,which implies no obvious magma-magma reaction nor mixing happened in the source.Additionally,the primordial melts with the same trace element compositions of host magma may generate 70%Cpx and 30%Gt when F=?90%according to the back calculation based on equilibrium fractionation crystallization.As a result,the primordial melts and the host magma may share the same source.We estimate the temperature and pressure crystallization(P=2.4?3.3 GPa;T=1164?1331 ?)for megacrysts from Shuangyashan,thus the megacrysts may origin from the base of lithosphere(Depth=78-105 km).In summary,we suggest that the megacrysts from Shuangyashan originated at the base of lithosphere and were captured by the late co-magma,which transported the megacrysts to the near-surface.