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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。
The compositional features of melt inclusions in basaltic porphyry can infer the composition of basaltic rocks and reflect the magmatic evolution. The elemental composition of single melt inclusions in basaltic olivine and clinopyroxene phenocrysts in Sihetun Formation was studied by LA-ICPMS. The results show that the melt inclusions in olivine and clinopyroxene phenocrysts exhibit larger ranges of major and trace elements than the whole rock, but the distribution of trace elements is consistent with the whole rock. The contents of CaO, CaO / Al2O3 and Cr2O3 in the clinopyroxene inclusions decrease with the decrease of Mg # value of clinopyroxene, which reflects the influence of the crystal separation of clinopyroxene between the Al2O3 and Sr The relationship between the crystallization of plagioclase and the change of MgO-Ni and MgO-CaO / Al2O3 reflects the crystallization of olivine. Changes in the composition of the inclusions are generally controlled by the crystallization of olivine, clinopyroxene and plagioclase. Combined with the previous research results, it is concluded that the characteristics of shell source components of Sihetun basalt in trace elements and isotopic composition may be partly inherited from the protolith (ie, the olivine pyroxene formed by partial melting of peridotite + eclogite) Rock) instead of being contaminated by crustal rocks during magmatic ascent. A small amount of melt inclusions with high Mg # value, high Si content, low CaO, TiO2, Al2O3 and trace element contents in the high Mg # clinopyroxene crystallites reflect the contamination of Si-bearing rocks during basalt magma ascent , Which is consistent with the underlying geology of the Yixian Formation basalts for the quartzite and quartz sandstone of the Great Hongyu Formation in the Great Wall. Therefore, the melt inclusions in high-Fo olivinic plaques may reflect the original melt composition more faithfully than the original melt calculated by simple addition of olivine to the whole rock.