内蒙古崇根山地区超基性岩岩石地球化学特征及构造环境分析

针对出露在二连浩特-贺根山缝合带东段的崇根山超基性岩进行了野外地质调查、岩石学和岩石地球化学方面的分析研究,结果显示:崇根山超基性岩已发生了强烈蛇纹石化,基本已经蚀变成蛇纹岩;主量元素表现出高Mg(w(MgO)=36.00%~38.25%)、高Cr(w(Cr2O3)=0.17%~0.46%)、高Ni(w(NiO)=0.21%~0.24%)、低Si(w(SiO2)=39.79%~42.35%)、贫Al(w(Al2O3)=1.04%~1.96%)、低Fe(w(FeOT)=6.79%~9.47%)的特征,其Mg#值为90.05~93.04,m/f值为8.72~11.60,属典型的镁质超基性岩,原岩可能为方辉橄榄岩;稀土元素总量很低,w(∑REE)为1.28×10^-6~2.94×10^-6,稀土元素配分模式为轻稀土元素微弱富集的近平坦型,其(La/Yb)N为2.25~5.95,具一定程度的负Eu异常(δEu=0.35~1.16)和负Ce异常(δCe=0.48~0.68);微量元素富集Ba,U,Ta,亏损Nb,Ti。崇根山超基性岩属变质橄榄岩,是SSZ型蛇绿岩的底部组成单元,为原始地幔经5%~15%部分熔融形成的亏损地幔岩,形成于俯冲带环境中的洋内弧后盆地环境。

Geochemical Characteristics and Tectonic Setting of Ultra Basic Rocks in the Chonggenshan Area,Inner Mongolia

Field investigation, petrological and geochemical study on the ultra basic rocks in the Chonggenshan area, western side of the Erlianhot-Hegenshan suture zone, shows that the ultra basic rocks have subjected to strong serpentinization and turned into serpentinite. Geochemical analysis shows that the main elements of the rock are characteristically of high Mg (MgO=36.00%-38.25%),Cr(Cr 2O 3=0.17%- 0.46%) and Ni(NiO=0.21%-0.24%),low Si(SiO 2=39.79%-42.35%), poor Al(Al 2O 3=1.04%- 1.96%) and low Fe(FeO T=6.79%-9.47%), with Mg # value of 90.05-93.04 and m/f value of 8.72-11.60. This belongs to typical magnesium ultra basic rock, whose primary rock may be harzburgite. Rare earth element analysis indicates that the ∑REE is extremely low (∑REE=1.28×10^-6 -2.94×10^-6 ) and the normalized rare earth element partition pattern is characterized by a weak enrichment of near flat type pattern.(La/Yb) N is 2.25-5.95, with slight negative Eu anomalies (δ Eu=0.35-1.16) and significant negative Ce anomalies (δ Ce=0.48-0.68). Trace elements are enriched in Ba,U,Ta,and other elements and depleted in Nb and Ti. This paper suggests that the Chonggenshan ultra basic rocks are metamorphic peridotites and belong to the bottom unit of SSZ type ophiolite and to the lossy mantle rocks resulting from the partial melting (5%-15%) of original mantle. These ultra basic rocks are formed in the intra-oceanic back-arc basin setting in the subduction zone.