南公珠错地幔橄榄岩:雅鲁藏布江缝合带西段一个典型的大洋地幔橄榄岩

发布时间：2018-09-19 06:35

【摘要】：西藏阿里地区的南公珠错蛇绿岩产在公珠错的南侧,空间上属于雅鲁藏布江缝合带西段之南亚带蛇绿岩。该蛇绿岩主要由地幔橄榄岩和辉长岩等基性岩类组成。地幔橄榄岩中约80%为方辉橄榄岩,20%为二辉橄榄岩,纯橄岩较少。南公珠错地幔橄榄岩矿物化学特征表现为橄榄石具有较低的Fo(89.3~91.4)值、辉石具有较高的Al_2O_3含量(1.89%~6.06%)、尖晶石具有较低的Cr~#(12.7~28.3)值。与原始地幔相比南公珠错地幔橄榄岩的全岩地球化学特征具有较高的MgO含量和较低的Al_2O_3、CaO和TiO_2等易熔元素含量;方辉橄榄岩和二辉橄榄岩的稀土元素总含量分别介于0.66×10-6~1.10×10-6和0.90×10~(-6)~3.78×10~(-6)之间,明显低于原始地幔值,其稀土元素配分模式为轻稀土元素轻微富集型;在原始地幔标准化微量元素蜘蛛图中,南公珠错地幔橄榄岩显示出强烈的U正异常、Nd轻微正异常和强不相容元素Zr的负异常;方辉橄榄岩和二辉橄榄岩的铂族元素总量分别介于15.26×10~(-9)~25.23×10~(-9)和18.74×10~(-9)~26.86×10~(-9)之间,二者含量的变化较小,南公珠错地幔橄榄岩PGEs球粒陨石标准化图解显示其为接近于原始地幔的"平坦型"。南公珠错地幔橄榄岩的矿物化学和全岩地球化学特征与深海橄榄岩相似,指示它们可能形成于大洋扩张脊环境。定量模拟估算表明,南公珠错地幔橄榄岩可能来源于地幔中的尖晶石相二辉橄榄岩源区,系经历了至多16%部分熔融的残余。LREE的微富集和较高的Pd/Ir、Rh/Ir比值指示它们还经历了岩石-熔体反应作用。初步结论认为南公珠错地幔橄榄岩形成于大洋脊环境,为尖晶石相二辉橄榄岩地幔源区较低程度部分熔融的残余,但经历了后期岩石-熔体反应作用。
[Abstract]:The Nangongzhu ophiolite in Ali area of Tibet is located on the south side of Gongzhu fault and belongs to the south Asia belt ophiolite in the west section of the Yalu Zangbo River suture belt. The ophiolite is mainly composed of mantle peridotite and gabbro. About 80% of the mantle peridotite consists of lherzolite and 20% of the peridotite is Erlherite, while the pure peridotite is less. The mineralogical characteristics of mantle-peridotite in Nangongzhu are as follows: olivine has a lower Fo (89.3 ~ 91.4) value, pyroxene has a higher Al_2O_3 content (1.896.06%), and spinel has a lower Cr~# (12.7T. 3). Compared with the original mantle, the whole rock geochemical characteristics of the peridotite in the Nangongzhu mantle have higher MgO content and lower Al_2O_3,CaO and TiO_2 contents, the total REE contents of the pyroxenite and the erpyrotite are between 0.66 脳 10-61.10 脳 10-6 and 0.90 脳 10 ~ (-6) 3.78 脳 10 ~ (-6), respectively, and the total REE contents of the peridotite are between 0.66 脳 10-61.10 脳 10 ~ (-6) and 0.90 脳 10 ~ (-6) 3.78 脳 10 ~ (-6), respectively. The REE distribution pattern is light rare earth element slight enrichment type, and in the primitive mantle standardized trace element spider map, The mantle peridotite of Nangongzhu shows strong U positive anomaly and negative anomaly of strongly incompatible element Zr, the total contents of platinum-group elements of pyroxenite and erpyrotite range from 15.26 脳 10 ~ (-9) to 25.23 脳 10 ~ (-9) and 18.74 脳 10 ~ (-9) to 26.86 脳 10 ~ (-9), respectively. The standardized diagram of the PGEs chondrite in the mantle peridotite of Nangongzhu shows that it is a "flat type" close to the primitive mantle. The characteristics of mineralogical chemistry and whole rock geochemistry of mantle peridotite in Nangongzhu are similar to those of deep-sea peridotite indicating that they may be formed in the environment of oceanic extensional ridge. The quantitative simulation results show that the peridotite in the mantle of Nangongzhu may be derived from the spinel source area of Erlherite in the mantle. The microenrichment and high Pd/Ir,Rh/Ir ratio of residual. LREE of up to 16% partial melting indicate that they also undergo rock-melt reaction. The preliminary conclusion is that the mantle peridotite in Nangongzhu is formed in the oceanic ridge environment, which is a remnant of lower degree partial melting of the spinel secondary pyroxenite mantle source area, but has experienced the late rock-melt reaction.
【作者单位】：中国地质科学院地质研究所地幔研究中心;中国地质大学;中国地质科学院地球物理地球化学勘查研究所;中国地质大学地球科学学院;
【基金】：中国地质调查局地质调查项目(12120115027201,121201102000150005,201511022) 国际科技合作专项(2014DFR21270) 国家重点实验室自主课题(Z1301-a20)联合资助
【分类号】：P588.125

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