内蒙古贺根山蛇绿岩中方辉橄榄岩岩石地球化学特征及构造环境分析

Geochemical Characteristics and Tectonic Setting of Harzburgite from Hegenshan Ophiolitic Block in Inner Mongolia

贺根山蛇绿岩块位于二连浩特-贺根山缝合带中段,其橄榄岩端元主体为方辉橄榄岩,少量纯橄榄岩,均发生了较强的蛇纹石化。对方辉橄榄岩进行岩石学和岩石地球化学分析研究,结果显示:方辉橄榄岩中主要造岩矿物橄榄石为镁橄榄石(Fo=90.6~90.7),斜方辉石为顽火辉石(En=88.9~90.0),单斜辉石为透辉石,以低铝(1.82%~2.13%)和高镁(Mg^#=94.2~94.7)为特征。橄榄岩主量元素表现出高Mg(MgO=34.98%~40.86%)、高Cr(Cr2O3=0.18%~0.27%)、高Ni(NiO=0.27%~0.35%)、低Si(SiO2=36.10%~41.41%)、贫Al(Al2O3=0.89%~1.58%)、低Fe(FeOT=7.58%~9.63%)特征,Mg^#=91.59~92.36,m/f=9.16~10.13,属典型镁质超基性岩。稀土元素总含量较低,ΣREE为1.42×10^-6~2.94×10^-6,球粒陨石标准化稀土元素分布模式为轻稀土富集的弱右倾型,其(La/Yb)N=2.15~10.47,Eu异常不明显,Ce显示负异常。富集Ba,U,Pb,Sr,亏损Nb,Ta,Ti。贺根山地幔橄榄岩属变质橄榄岩,为SSZ型蛇绿岩底部组成单元,形成于俯冲带环境中洋内弧后盆地环境。

The Hegenshan ophiolite block is located in middle segment of Erlian- Hengenshan suture zone in Inner Mongolia,whose peridotites are dominated by harzburgite and dunite which have subjected to strong serpentinization.In this paper,field investigation,petrological and geochemical study on harzburgite,shows that the Fo values of olivine in harzburgite vary in the range of 90.6~90.7.The orthopyroxenes in the rocks are enstatite (En=88.9~90.0),and the clinopyroxenes are endiopside and diopside with low Al2O3(Al2O3=0.89%~1.58%)contents and high Mg^#(94.2~94.7) values. Geochemical analysis shows that the main elements of the peridotites are characteristic of high Mg (MgO=34.98%- 40.86%),Cr(Cr2O3=0.18%~0.27%) and Ni(NiO=0.27%~0.35%),low Si(SiO2=36.10%~41.41%),poor Al(Al2O3= 1.04%~3.16%)and low Fe(FeOT=5.04%~10.66%),with Mg# value of 91.59~92.36 and m/f value of 9.16~10.13,belong to typical magnesium ultra basic rock. Rare earth element analysis indicates that the ΣREE is extremely low(ΣREE= 1.42×10^-6~2.94×10^-6)and the normalized rare earth element partition pattern is characterized by enrichment of weak right dip type pattern.(La/Yb)N is 2.15~10.47,with not obvious Eu anomalies and slightly negative Ce anomalies. Trace elements are enriched in Ba,U,Pb,Sr,and other elements and depleted in Nb,Ta,Ti. It is considered that the Hggenshan peridotites are metamorphic peridotites and belong to the bottom unit of SSZ type ophiolite. These peridotites formed in the intra-oceanic back-arc basin setting in the subduction zone.