Behavior of Siderophile and Chalcophile Elements in the Subcontinental Lithospheric Mantle beneath the Changbaishan Volcano,NE China

摘要

The mantle xenoliths in the Quaternary ChangbaishanVolcano in southern Jilin Province contain spinel-facies lherzolites.The equilibration temperatures for these samples range from 902oC to 1064oC based on the two-pyroxene thermometer of Brey and K?hler(1990),and using the oxybarometry of Nell and Wood(1991),the oxidation state was estimated from FMQ-1.32 to-0.38 with an average value of FMQ-0.81(n=8),which is comparable to that of abyssal peridotites and the asthenospheric mantle.The fO2 values of peridotites,together with their bulk rock compositions(e.g.,Mg#,Al2O3,CaO,Ni,Co,Cr) and mineral compositions(e.g.,Mg# of olivine and pyroxene,Cr# [=Cr/[Cr+Al]] and Mg# [=Mg/[Mg+Fe2+] of spinel),suggest that the present-day subcontinental lithospheric mantle(SCLM) beneath the Changbaishan Volcano most likely formed from an upwelling asthenosphere at some time after the late Mesozoic and has undergone a low degree of partial melting.The studied lherzolite xenoliths show low concentrations of S,Cu,and platinum group elements(PGE),which plot a flat pattern on primitive-mantle normalized diagram.Very low concentrations in our samples suggest that PGEs occur as alloys or hosted by silicate and oxide minerals.The compositions of the studied samples are similar to those of peridotite xenoliths in the Longgang volcanic field(LVF) in their mineralogy and bulk rock compositions including the abundance of chalcophile and siderophile elements.However,they are distinctly different from those of peridotite xenoliths in other areas of the North China Craton(NCC) in terms of Cu,S and PGE.Our data suggest that the SCLM underlying the northeastern part of the NCC may represent a distinct unit of the newly formed lithospheric mantle.