四合屯玄武岩斑晶中单个熔体包裹体元素组成及其对岩浆演化的指示

Chemical compositions of phenocryst-hosted melt inclusions from the Sihetun basalt:Implications for the magma evolution

玄武岩斑晶中熔体包裹体成分特征可以推断玄武岩源区物质组成,反映岩浆形成演化过程。利用LA-ICPMS对四合屯义县组玄武岩橄榄石、单斜辉石斑晶中单个熔体包裹体的元素组成进行了分析测试。研究结果表明,橄榄石、单斜辉石斑晶中的熔体包裹体在主、微量元素含量上表现出了比全岩更大的变化范围,但微量元素分配特征总体和全岩一致。单斜辉石斑晶中包裹体的CaO含量、CaO/Al2O3比值和Cr2O3含量随着单斜辉石Mg#值的降低而降低,反映了单斜辉石结晶分离的影响,Al2O3与Sr之间的显著相关关系则记录了斜长石结晶分离作用的影响,MgO-Ni和MgO-CaO/Al2O3的变化则反映了橄榄石的分离结晶作用。包裹体元素组成变化总体受橄榄石、单斜辉石和斜长石的结晶分离作用控制。结合前人研究成果,认为四合屯玄武岩在微量元素和同位素组成上的壳源组分特征可能部分地继承自原岩(即橄榄岩+榴辉岩部分熔融体反应形成的(橄榄)辉石岩),而不是岩浆上升过程中受地壳岩石混染的结果。高Mg#值单斜辉石斑晶中少量高Mg#值、高Si含量,低CaO、TiO2、Al2O3和微量元素含量的熔体包裹体反映玄武岩浆上升过程中受到了Si质岩石的混染,这与义县组玄武岩下伏地层为长城系大红裕组石英岩、石英砂岩的地质特征一致。因此,高Fo橄榄石斑晶中的熔体包裹体比采用向全岩中简单添加橄榄石方式计算出的原始熔体可能更能真实反映原始熔体组成。

Melt inclusions trapped by phenocrysts in basalt can be used to infer the chemical characteristic of basalt source and the process of magma formation and evolution. Single melt inclusions hosted in olivine and clinopyroxene phenocrysts in the Mesozoic Sihetun basalts were analyzed by LA-ICPMS. The results show that, although melt inclusions show trace element patterns similar to the bulk rocks, melt inclusions have much larger compositional variations than bulk lava in terms of both major and trace elements. Ca~ and Cr2O3 contents and CaO/Al2O3 ratio of melt inclusion in Cpx correlate positively with Mg# of Cpx, reflecting the effect of Cpx crystallization on the magmatic evolution. The positive correlation （R 0.94） of Sr-Al2O3 for melt inclusions indicate plagioclase crystallization during the magmatic evolution. The weak correlations of Ni-MgO and CaO/Al2O3-MgO in melt inclusions suggest that olivine crystallization was probably small. The compositional variations of melt inclusions can be interpreted by O1, Cpx and Pl crystallization, suggesting that the ＂crustal components＂ as revealed by whole rock elemental and isotopic compositions could have inherited from the source rock instead of magmatic mixing. Combining with the previous work, we suggest that these basalts could have been derived from partial melting of （olivine） pyroxenites formed by peridotite ＋ eclogite-derived melt reaction. On the other hand, some high Mg#- and high SiO2-melt inclusions in high-Mg# Cpx suggest assimilation by high Si rocks during the basalt eruption. Therefore, melt inclusions in high-Fo olivine phenocrysts could better reflect the characteristics of primitive melt than ＂primary melt＂ calculated by adding olivine to the bulk lava.