柴北缘绿梁山蛇纹石化橄榄岩成因:来自蛇纹石B同位素的证据OA

The genetic mechanism of serpentinized peridotite in Lüliangshan area of the North Qaidam UHPM belt is controversial.It was proposed as product of subducted slabs,refertilised lithospheric mantle or mantle wedge.In this paper,two types of peridotite(garnet lherzolite and dunite)were collected from Lüliangshan area,and the major and trace elements,and in-situ boron isotope of serpentine were analyzed.The two types of peri-dotite have experienced different types of serpentinization respectively.The dunite is dominated by the serpen-tinization of olivine,while the garnet lherzolite is dominated by the serpentinization of pyroxene.The serpentine produced by the serpentinization of olivine has higher content of MgO and As but lower content of Al2O3,Cr2O3,FeO,Li,Yb,Y,Ti,Sc and Zn;while the serpentine produced by the serpentinization of pyroxene is opposite.Serpentine in two types of peridotite showed significant differences in boron isotope.The boron content of ser-pentine in dunite is from 13.3×10-6 to 38.5×10-6,and its δ11B value is from-9.5‰ to+5.0‰,which is close to or slight higher than the δ11B value of the mantle.The boron content of serpentine in garnet lherzolite is from 10.5×10-6 to 49.9×10-6,and its δ11B value is from 6.3‰ to 23.0‰,indicating that they are significantly enriched in heavy boron isotope.The difference of boron isotope composition of serpentines indicates that the Lüliangshan peridotite was formed in two different tectonic settings.The garnet lherzolite is formed in litho-spheric mantle and has been altered by seawater,thus with enrichment of heavy boron isotope.While the dunite is of mantle-wedge origin and had undergone serpentinization interacted with subduction fluids,thus with en-richment of light boron isotope.