Origin of different dunites in the Luobusa ophiolite,Tibet
Origin of different dunites in the Luobusa ophiolite,Tibet
复制标题
作者：
Li Jin
罗布莎蛇绿岩中的纯橄榄岩有三种类型：过渡带纯橄榄岩厚约700 ~ 1000 m，主要由高镁橄榄石（Fo 93 -95）组成，过渡带纯橄榄岩主要由高镁橄榄石（Fo 93 -95）组成。低铝高镁单斜辉石（Al_2O_3 = 0.47% ~ 0.85%，Mg# = 95 ~ 97）和高Chrome低镁铬尖晶石（Cr# = ~ 77; Mg# = ~ 51）。这些纯橄榄岩中的浸染状铬铁矿层中的铬尖晶石与寄主岩石的成分相似，但镁含量略高（Mg# = ~ 59）豆荚状铬铁矿周围的纯橄榄岩包体成分与过渡带纯橄榄岩相似（橄榄石Fo 92 -94; Al_2O_3在单斜辉石中含量为1%，Mg# = 95 ~ 97;铬尖晶石中的Cr#= 71，Mg# = 52）。哈茨布尔诺纯橄榄岩透镜体中有较低Fo（91 ~ 92）的橄榄石和较低Cr# s的铬尖晶石（约60），但单斜辉石具有较高的Al 2 O3（1.41% ~ 1.71%），这些透镜体的矿物成分与寄主方斑玄武岩的矿物成分重叠，过渡带纯橄榄岩与经典的超镁铁质堆晶岩有明显的区别，很可能是拉斑玄武岩熔体与地幔橄榄岩反应形成的。豆荚状铬铁矿周围的纯橄榄岩包层被认为是以与过渡带纯橄榄岩相同的方式形成的，但块状铬铁矿的成因尚不清楚。
Three types of dunite occur in the Luobusa ophiolite,a thick layer of transition zone dunite,thin envelopes of dunite around podiform chromitites and small bands and lenses of dunite in the harzburgites,respectively.The transition zone dunite is about 700 ～ 1000m thick,and consists chiefly of high magnesium olivine(Fo93-95),low alumina and high magnesia clinopyroxene(Al2O3 = 0.47% ～ 0.85% ;Mg# = 95 ～ 97),and high chrome and low magnesia Cr-spinel(Cr# = ～ 77;Mg# = ～ 51).Cr spinels in layers of disseminated chromite hosted in these dunites are similar in composition to those of the host rock but are slightly more magnesian(Mg# = ～ 59) The dunite envelopes around the podiform chromitites are similar in composition to the transition zone dunite(olivine Fo92-94;Al2O3 in clinopyroxene 1% and Mg# = 95 ～ 97;Cr# in Cr-spinel = 71 and Mg# = 52).The massive chromitites associated with the dunite envelopes have Cr#s averaging 79 and Mg#s averaging 68.The dunite lenses in harzburgite have olivine with somewhat lower Fo(91 ～ 92) and Cr-spinel with lower Cr# s(about 60),but clinopyroxene with higher Al2O3(1.41% ～ 1.71%).The mineral compositions in these lenses overlap those in the host harzburgite,with a gradual transition between them.The transition zone dunite in the Luobusa ophiolite is distinctly different from classic ultramafic cumulates,and most likely formed by reaction between tholeiitic basalt melt and mantle peridotite.Dunite lenses in the harzburgite may reflect either inhomogeneous partial melting of the host peridotites or in-situ melt-rock reaction.The dunite envelopes around the podiform chromitites are thought to have formed in the same way as the transition zone dunite,but the origin of the massive chromitites is not clear.