Petrogenesis Of The Cenozoic High-mg Andesites In The Russian Far East: Geochemical Evidence

This paper reports petrology,mineralogical and geochemical data of the Cenozoic basaltic andesites in the Russian Far East and compares them with the Cenozoic potassic basalts in the adjacent areas,with the aim of revealing petrogenesis of the intracontinental high-Mg andesites and deep crust-mantle interaction process.The previous K-Ar dating indicates that the Cenozoic basaltic andesites in the Russian Far East formed in the early Miocene(17.1-20.6 Ma).The Cenozoic basaltic andesites in the Russian Far East have SiO2?55.42?57.34 wt.%,Al2O3=14.61?15.01 wt.%,MgO=4.92?5.78 wt.%,Mg#[=100Mg/(Mg+Fe2+)]=53.1?56.3,Fe2O3=8.58?8.89 wt.%,CaO=5.82?6.62 wt.%,(Na2O+K2O)?5.15?6.45wt.%,belonging chemically to sub-alkaline series and similar to high-Mg andesites,i.e.,they can be named as the intracontinental high-Mg andesites.These high-Mg andesites are enriched in large ion lithophile elements(LILEs),relatively depleted in high field strength elements(HFSEs),enriched in Sr-Nd-Hf-K isotopic compositions(87Sr/86Sr=0.705629?0.705893,?Nd=-4.9 to-3.4,?Hf=-4.7 to-2.6,and ?41K=-0.832‰?-0.356 ‰)and have low radioactive Pb isotopic compositions(206Pb/204Pb=16.757?19.187,207Pb/204Pb=15.424?15.448,208Pb/204Pb=36.711?37.107),similar to those of EMI,indicating that the recycled ancient crustal material and water could be involved in origin of these high-Mg andesites.The Miocene high-Mg andesites in the Russian Far East,together with the Cenozoic high-Mg andesites in NE China,is located above the stagnant slab front within the mantle transition zone in NE Asia,suggesting that high contents of the recycled continental crust material and water should be responsible for the formation of the Cenozoic intracontinental high-Mg andesites in NE Asia.Compared with the Cenozoic potassic basalts near study area,these intracontinental high-Mg andesites could experience higher degree of partial melting of the mantle materials.