峨眉山大火成岩省平川苦橄岩地幔源区性质--来自橄榄石微量元素的制约

The Nature of Mantle Source of the Pingchuan Picrites in the Emeishan Large Igneous Province——Constrains from the Trace Element Composition of Olivines

橄榄石是基性岩浆中最早期结晶的硅酸盐矿物之一,其主量、微量元素特征可以反映出岩浆演化环境、岩浆源区岩性和再循环组分性质等重要信息。本次研究通过对峨眉山大火成岩省平川苦橄岩中橄榄石主量和微量元素分析,以及橄榄石内尖晶石包裹体分析,并与大理苦橄岩中橄榄石和尖晶石成分进行对比,来探讨不同苦橄岩母岩浆氧逸度及源区性质的异同。橄榄石-尖晶石-斜方辉石氧逸度计计算结果表明,平川苦橄岩母岩浆比大理苦橄岩母岩浆具有更高的氧逸度(分别为ΔQFM+0.24和ΔQFM−0.51),这与平川橄榄石较大理橄榄石具有更高的V/Sc值相一致。平川橄榄石具有较高Ni/Co值,暗示苦橄岩源区中存在再循环物质的参与,橄榄石较低的Li含量则进一步表明这种再循环物质更可能是洋壳而不是陆壳物质。在地幔源区判别图中,平川橄榄石主要位于橄榄岩起源熔体结晶的橄榄石成分区域。然而,平川橄榄石比大理橄榄石具有更高Ni、Co含量和Zn/Fe、Fe/Mn值,低Ca、Mn、Sc含量和Mn/Zn值,暗示平川苦橄岩源区中存在辉石岩组分。少量的再循环洋壳与橄榄岩地幔混合,会形成富橄榄石的混合源区,且这种地幔产生的熔体结晶的橄榄石与平川橄榄石一样,成分仍近似于橄榄岩熔体中结晶的橄榄石。但平川橄榄石主量、微量元素特征表明,其成分受源区中辉石岩组分的控制,结合平川橄榄石较大的成分变化范围,进一步表明平川苦橄岩地幔源区中是含有辉石岩组分的,而不是均一的橄榄岩源区。

Olivine is one of the earliest crystallized silicates from basaltic magmas.The major and trace element compositions of olivine phenocrysts can provide important information about the thermodynamic condition of the magma,source lithology,and source component,including the nature of the recycled component.The major and trace element compositions of the olivines and Cr-spinel inclusions in the olivines from the Pingchuan picrites in the Emeishan large igneous province were analyzed in this study,and compared with the compositions of the olivines and spinels in the Dali picrites,and the differences of source component and oxygen fugacity of parental magmas of different picrites were discussed.The Pingchuan olivines have higher V/Sc ratio than the Dali olivines,which indicates a more oxidized condition for the Pingchuan magma.The olivine-spinel oxybarometer shows that the logfo2 values of the Pingchuan and the Dali magmas,which are saturated with olivine and spinel,are ΔQFM+0.24 andΔQFM−0.51,respectively.The high Fo olivines in the Pingchuan picrites have Ni/Co>20,which implies the presence of recycled component in the source of the Pingchuan picrites.The lower Li content of the Pingchuan olivines indicates that the component that recycled into the source is oceanic crust.In the mantle source discrimination diagrams of olivine Ni vs.Mn/Zn and 100×Mn/Fe vs.10000×Zn/Fe,the Pingchuan olivines are mainly plotted within the fields of peridotite sources.However,the Pingchuan olivines exhibit higher Ni,Co,Zn/Fe,Fe/Mn,and lower Ca,Mn,Sc,Mn/Zn than the Dali olivines,which suggest the presence of pyroxenite component in the source of the Pingchuan picrites.A small amount of recycled oceanic crust adding into the mantle peridotite would produce an olivine relatively enriched sources.The olivines crystallized from melts derived from such a mantle source would have characteristics similar to the olivines from the Pingchuan picrites,which still have compositions similar to the olivines crystallized from a peridotite partial melting melts.However,the compositions of the olivines in the Pingchuan picrites are also controlled by the pyroxenite component in the source.Combined with the wide compositional ranges of the olivines,we suggest that the Pingchuan picrites were derived from a source containing pyroxenite components rather than a homogeneous peridotite source.