The Geochemical Characteristics Of Mantle Xenoliths From Beiyan,Shandong Province

The North China Craton has been reactivated since the Mesozoic with significant thinning and refertilization of the subcontinental lithospheric mantle.Petrological and geochemical observations both indicate that mantle metasomatism play an important role in the refertilization of lithospheric mantle.However,the mechanism of metasomatism including the nature and source of metasomatic melt/fluid is still difficult to be thoroughly revealed.To better understand the interaction between xenolith and melt/fluid,I present an integrated field,petrographical,whole rock major and trace elements concentrations,minerals major element concentrations,whole rock and clinopyroxene Hf,Nd,Sr and whole rock Mg isotope investigation for the mantle xenoliths and the host alkaline basalts in Beiyan within the Tancheng-Lujiang(Tan-Lu)wrench fault zone,eastern North China Craton.The relationship between mantle metasomatism and coupling Hf-Nd isotopes has also been discussed based on the obtained data.Based on the mineral component,Beiyan xenoliths were subdivided into four types: lherzolite,cpx-rich lherzolite,wehrlite and olivine clinopyroxenite.The lherzolites containing similar major elements to abyssal peridotites represent the fragment of newly accreted oceanic-like lithospheric mantle which experienced subsequent less than 10% melt extraction.The occurrence of hydrous minerals such as phlogopite,amphibole,feldspar and carbonates such as calcite,ankerite and reacted texture indicate the existence of remarkable mantle metasomatism.Compared with the lherzolites,the cpx-rich lherzolites show lower Mg# values(85.0~89.6),lower olivine Fo values(86.5~89.4),LREE-enriched patterns,higher ?REE(9.19~26.3 ppm),more fertile Sr-Nd-Hf isotopic compositions.Particularly,the Mg isotopic compositions of cpx-rich lherzolites BY13-08(?26Mg=-0.03±0.07‰)and BY13-16(?26Mg=-0.12±0.09‰)are heavier than lherzolites and mantle normal value(?26Mg=-0.25±0.07‰).Combining to the high Sr/Hf ratio,enrichment of Sr,depletion of Zr,the cpx-rich lherzolites may be the result of interaction between lherzolite and fluid inheriting the heavy Mg isotopic compositions of subducted Pacific slab.Similarly,the wehrlites also show lower Mg# values(81.2~87.3)of whole rock,lower Fo value(81.1~86.2)of olivine,LREE-enriched patterns,higher ?REE(9.04~16.1 ppm)and lower Nd-Hf isotopic values than lherzolites.But the most distinct difference between cpx-rich lherzolite and wehrlite is that the latter show variable Mg isotopic compoisiton(?26Mg =-0.12‰ to-0.43‰).Part of wehrlites BY13-29 and BY13-33 contain higher Mg isotopic compositions,while sample BY13-03,BY13-30,BY13-31 and BY13-32 have lower Mg isotopic compositions than normal mantle value.The petrographical observations indicated that wehrlites record the reaction between orthopyroxene and low-SiO2 basalt to form second clinopyroxene,olivine and high-SiO2 melt.Coincidentally,we observed that the host basalt also contain fertile Sr-Nd-Hf isotopes and light Mg isotope values.Hence,the wehrlites with variable Mg isotopic compositions are formed by lherzolites or cpx-rich lherzolites and basalts which have similar low Mg isotopic compositions to host magma.And part of wehrlites inherite the heavy Mg isotopic compositions from cpx-rich lherzolitesThe Hf-Nd isotopic compositions of Beiyan lherzolites(whole rock: ??Hf=-2.57~7.08,clinopyroxene: ??Hf=-0.72~6.69)and cpx-rich lherzolites(whole rock: ??Hf=-1.91~2.66,clinopyroxene: ??Hf=0.47)locates near the mantle array,which indicate fluid containing heavy Mg isotopic compositions does not induce the Hf-Nd isotopic compositions of Beiyan mantle xenoliths to decouple.While Beiyan wehrlites(whole rock: ??Hf=-3.51~-0.90,clinopyroxene: ??Hf=-5.39~-2.22)and olivine clinopyroxenites(whole rock: ??Hf=-2.91~-2.19,clinopyroxene: ??Hf=-4.27~-2.81)show weakly negative Hf-Nd decoupling,which may be the results of reaction between basaltic melts containing low Hf isotopes and peridotites.